Sign and verify manifests in Go with OpenPGP instead of running gpg (closes #181)

mfer ran the gpg binary to sign, export keys and verify, so it failed
wherever gpg is missing. It now uses github.com/ProtonMail/go-crypto/openpgp.
--sign-key and MFER_SIGN_KEY name a file holding one version 4 OpenPGP
secret key; a protected key's passphrase comes from
MFER_SIGN_KEY_PASSPHRASE or a terminal prompt. gen and freshen check that
the key can sign before they read any file. Verification keeps the rules
of the --require-signature fix: one primary key in the embedded block,
counted from its packets, exactly one signature, made by that key or a
subkey, and signer equal to its fingerprint. The embedded block may hold
no DSA key and no secret key, and an armored field must be one
well-formed block.

Model: opus-5-5

README.md

@@ -9,8 +9,9 @@ downloading, streaming, and mirroring. It was first published in 2022. The
 manifest files' data is serialized with Google's
 [protobuf serialization format](https://developers.google.com/protocol-buffers).
 The structure of these files can be found
-[in the format specification](https://git.eeqj.de/sneak/mfer/src/branch/main/mfer/mf.proto)
-which is included in the [project repository](https://git.eeqj.de/sneak/mfer).
+[in the format specification](docs/FORMAT.md), which refers to the protobuf
+schema `mfer/mf.proto` for exact field numbers and types. Both are included in
+the [project repository](https://git.eeqj.de/sneak/mfer).
 
 The current version is pre-1.0 and while the repo was published in 2022, there
 has not yet been any versioned release. [SemVer](https://semver.org) will be
@@ -256,8 +257,9 @@ are now tracked only in the [issues](https://git.eeqj.de/sneak/mfer/issues).
 
 - Should the manifest signature format be GnuPG signatures, or those from
   OpenBSD's signify (of which there is a good
-  [golang implementation](https://github.com/frankbraun/gosignify))? Still open,
-  as question 10 on [issue 82](https://git.eeqj.de/sneak/mfer/issues/82).
+  [golang implementation](https://github.com/frankbraun/gosignify))? Settled
+  under question 10 on [issue 82](https://git.eeqj.de/sneak/mfer/issues/82):
+  OpenPGP signatures, which mfer makes and checks itself without running `gpg`.
 
 - Should the on-disk serialization format be proto3 or json? Settled: it is
   proto3, see `docs/FORMAT.md` and `mfer/mf.proto`.
@@ -268,13 +270,44 @@ are now tracked only in the [issues](https://git.eeqj.de/sneak/mfer/issues).
     - recurses under current directory and writes out an `index.mf`
     - records every file's mode as `0000` unless given `--include-permissions`,
       which records each file's permission bits (`0777` at most)
+- `mfer gen /media/drive`
+    - writes `/media/drive/index.mf`, listing each file by its path under
+      `/media/drive`, so `mfer check /media/drive` verifies it. Given a file,
+      gen writes `index.mf` beside it and lists the file by its name; given
+      several paths, it writes `index.mf` in the current directory
+    - `--output` names another file to write instead. What gen lists depends
+      only on the paths it is given and the files under them, so with the same
+      `--seed` and an unchanged tree it writes the same bytes wherever the
+      manifest goes. The file it writes to is never listed
 - `mfer check` / `mfer check .`
     - verifies checksums of all files in manifest, displaying error and exiting
       nonzero if any files are missing or corrupted, or have permission bits
       other than the mode the manifest records, unless that is `0000`
+    - looks for those files under the base directory: the one `--base` names, or
+      else the directory holding the manifest, or the current directory for a
+      manifest given by URL. So `mfer check /media/drive` checks a drive against
+      the `index.mf` at its root, from any directory
     - warns about each file under the base directory that the manifest does not
       list, hidden files included; with `--no-extra-files` each one is a failure
       instead
+- `mfer freshen` / `mfer freshen .`
+    - rewrites `index.mf` to list the files now under the directory holding it,
+      or under the one `--base` names, hashing only the files that are new or
+      changed
+    - leaves out hidden files unless given `--include-dotfiles`, and symlinks
+      unless given `--follow-symlinks`, which lists each symlink to a file under
+      its own name with the contents of the file it points to
+- `mfer gen --sign-key key.asc` / `mfer freshen --sign-key key.asc`
+    - signs the manifest with the OpenPGP secret key in `key.asc`, armored or
+      binary, as `gpg --export-secret-keys` writes it; `MFER_SIGN_KEY` names the
+      file too. mfer signs it itself and does not need `gpg`. A file holding
+      more than one key is refused, and a key held only on a smartcard cannot
+      sign. The key must be a version 4 key, whose 40-character fingerprint is
+      what `--require-signature` takes, and not a DSA key. A key that cannot
+      sign, because it has expired or been revoked or its passphrase is wrong,
+      stops `gen` and `freshen` before they read any file
+    - takes a protected key's passphrase from `MFER_SIGN_KEY_PASSPHRASE`, or
+      else asks for it at the terminal
 - `mfer fetch https://example.com/stuff/`
     - fetches `/stuff/index.mf` and downloads all files listed in manifest into
       the current directory, or the one given with `--dest`, and assures

docs/FORMAT.md

@@ -6,7 +6,7 @@ Version 1.0
 
 An `.mf` file is a binary manifest that describes a directory tree of files,
 including their paths, sizes, and cryptographic checksums. It supports optional
-GPG signatures for integrity verification and optional timestamps and file
+OpenPGP signatures for integrity verification and optional timestamps and file
 permissions for metadata preservation.
 
 Nothing goes in the 1.0 manifest that 1.0 does not read or write: no field is
@@ -36,9 +36,9 @@ The outer message contains:
 | `sha256`          | 104    | bytes            | SHA-256 hash of the **compressed** `innerMessage` (corruption detection) |
 | `uuid`            | 105    | bytes            | Random v4 UUID; must match the inner message UUID                        |
 | `innerMessage`    | 199    | bytes            | Zstd-compressed serialized `MFFile` message                              |
-| `signature`       | 201    | bytes (optional) | GPG signature (ASCII-armored or binary)                                  |
+| `signature`       | 201    | bytes (optional) | OpenPGP detached signature (ASCII-armored or binary)                     |
 | `signer`          | 202    | bytes (optional) | Fingerprint of the signing key                                           |
-| `signingPubKey`   | 203    | bytes (optional) | Full GPG signing public key                                              |
+| `signingPubKey`   | 203    | bytes (optional) | Full OpenPGP public key of the signing key (ASCII-armored or binary)     |
 
 ### SHA-256 Hash
 
@@ -50,11 +50,14 @@ allows verifying data integrity before decompression.
 The `innerMessage` field is compressed with
 [Zstandard (zstd)](https://facebook.github.io/zstd/). Implementations must
 enforce a decompression size limit to prevent decompression bombs. The reference
-implementation limits decompressed size to 256 MB. It writes zstd frames with a
+implementation limits decompressed size to 256 MiB. It writes zstd frames with a
 window of at most 8 MiB, the largest window the zstd format recommends decoders
 support, and refuses frames that ask for a larger one. It also refuses an inner
 message whose file entries, hashes, timestamps and MIME types, counted at 176,
-112, 64 and 16 bytes each, add up to more than 8 times its size.
+112, 64 and 16 bytes each, add up to more than 8 times its size. It refuses a
+manifest file larger than 258 MiB without reading the rest of it: zstd's worst
+case grows a 256 MiB inner message by 1/256 to 257 MiB, and the last MiB is room
+for the signature, the signing key and the other outer fields.
 
 ## Inner Message (`MFFile`)
 
@@ -106,9 +109,12 @@ All `path` values must satisfy these invariants:
 - **No parent traversal**: no `..` path segments
 - **No empty segments**: no `//` sequences
 - **No trailing slash**: paths refer to files, not directories
+- **Listed once**: each path appears at most once in a manifest, compared byte
+  for byte, so `A.txt` and `a.txt` are two paths
 
 Implementations must validate these invariants when reading and writing
-manifests. Paths that violate these rules must be rejected.
+manifests. Paths that violate these rules must be rejected, and a reader must
+reject a manifest that lists a path more than once.
 
 ## Hash Format (`MFFileChecksum`)
 
@@ -136,17 +142,20 @@ Where:
 - `<SHA256>` is the hex-encoded SHA-256 hash from the outer message (covering
   compressed data)
 
-Components are separated by hyphens. The signature is produced by GPG over this
-canonical string and stored in the `signature` field of the outer message. The
-signing key's public key goes in `signingPubKey` and its fingerprint, in hex, in
-`signer`.
+Components are separated by hyphens. The signature is an OpenPGP detached
+signature over this canonical string, stored in the `signature` field of the
+outer message. The signing key's public key goes in `signingPubKey` and its
+fingerprint, in hex, in `signer`.
 
 A verifier accepts a signed manifest only if `signingPubKey` holds exactly one
 primary key, `signature` is one good signature over the canonical string made by
 that key (or one of its subkeys), and `signer` is that key's fingerprint. The
 reference implementation refuses to load a manifest that fails these checks;
 `check` and `fetch` given `--require-signature` then compare the required
-fingerprint with `signer`.
+fingerprint with `signer`. It also refuses a manifest whose `signingPubKey`
+holds a DSA key or subkey, or any secret key or subkey, since checking the
+self-signatures of a DSA key or the numbers of a secret key can take hours when
+those numbers are very large.
 
 ## Deterministic Serialization
 

go.mod

@@ -3,6 +3,8 @@ module sneak.berlin/go/mfer
 go 1.27.1
 
 require (
+	github.com/ProtonMail/go-crypto v1.5.2
+	github.com/creack/pty v1.1.25-0.20260601142114-9246436fffe8
 	github.com/davecgh/go-spew v1.1.1
 	github.com/dustin/go-humanize v1.1.0
 	github.com/klauspost/compress v1.20.1
@@ -15,6 +17,7 @@ require (
 )
 
 require (
+	github.com/cloudflare/circl v1.6.3 // indirect
 	github.com/klauspost/cpuid/v2 v2.4.0 // indirect
 	github.com/minio/sha256-simd v1.0.1 // indirect
 	github.com/mr-tron/base58 v1.3.0 // indirect

go.sum

@@ -1,3 +1,9 @@
+github.com/ProtonMail/go-crypto v1.5.2 h1:cucYnvqcY7UOXVD//mSyjeaPY0SSN3v5cDkYPxumINk=
+github.com/ProtonMail/go-crypto v1.5.2/go.mod h1:/RaSu30DaKO4RY+XdV/ACcCcZkGr7AhUIduq5sjzzCo=
+github.com/cloudflare/circl v1.6.3 h1:9GPOhQGF9MCYUeXyMYlqTR6a5gTrgR/fBLXvUgtVcg8=
+github.com/cloudflare/circl v1.6.3/go.mod h1:2eXP6Qfat4O/Yhh8BznvKnJ+uzEoTQ6jVKJRn81BiS4=
+github.com/creack/pty v1.1.25-0.20260601142114-9246436fffe8 h1:CY3gjC7naqYGLMiywvj3suPfa1i0p/QEr7o8ujxL/2M=
+github.com/creack/pty v1.1.25-0.20260601142114-9246436fffe8/go.mod h1:08sCNb52WyoAwi2QDyzUCTgcvVFhUzewun7wtTfvcwE=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/dustin/go-humanize v1.1.0 h1:dbKTrvD0klcbBV/h4AWJdMuZogJACoMlvWIWZ5b2xWg=

internal/cli/check.go

@@ -21,8 +21,8 @@ import (
 	"sneak.berlin/go/mfer/mfer"
 )
 
-// fingerprintHexLen is the length of a full GPG key fingerprint in hex
-// characters.
+// fingerprintHexLen is the length in hex characters of the fingerprint of
+// an OpenPGP version 4 key, the only version mfer signs with.
 const fingerprintHexLen = 40
 
 var (
@@ -108,18 +108,25 @@ func (mfa *CLIApp) fetchManifestToTemp(
 	if tmpErr != nil {
 		_ = rc.Close()
 
-		return "", fmt.Errorf("failed to create temp file: %w", tmpErr)
+		return "", tmpErr
 	}
 
 	tmpPath := tmpFile.Name()
-	_, cpErr := io.Copy(tmpFile, rc)
+
+	// Copying stops one byte past mfa.maxManifestSize, which is enough to
+	// tell that the manifest is too large.
+	written, cpErr := io.Copy(tmpFile, io.LimitReader(rc, mfa.maxManifestSize+1))
 	_ = rc.Close()
 	_ = tmpFile.Close()
 
+	if cpErr == nil && written > mfa.maxManifestSize {
+		cpErr = fmt.Errorf("%w of %d bytes", errManifestTooLarge, mfa.maxManifestSize)
+	}
+
 	if cpErr != nil {
 		_ = mfa.Fs.Remove(tmpPath)
 
-		return "", fmt.Errorf("failed to download manifest: %w", cpErr)
+		return "", fmt.Errorf("download manifest: %w", cpErr)
 	}
 
 	return tmpPath, nil
@@ -135,7 +142,7 @@ func verifyRequiredSigner(chk *mfer.Checker, requiredSigner string) error {
 
 	_, err := hex.DecodeString(requiredSigner)
 	if err != nil {
-		return fmt.Errorf("invalid fingerprint: must be valid hex: %w", err)
+		return fmt.Errorf("invalid fingerprint: %w", err)
 	}
 
 	if !chk.IsSigned() {
@@ -225,7 +232,7 @@ func findExtraFiles(
 
 	err := chk.FindExtraFiles(ctx, extraResults)
 	if err != nil {
-		return fmt.Errorf("failed to check for extra files: %w", err)
+		return fmt.Errorf("find extra files: %w", err)
 	}
 
 	<-extraDone
@@ -268,7 +275,7 @@ func runCheck(
 	progressWg.Wait()
 
 	if err != nil {
-		return 0, fmt.Errorf("check failed: %w", err)
+		return 0, fmt.Errorf("check files: %w", err)
 	}
 
 	// Wait for results processing to complete
@@ -289,14 +296,18 @@ func (mfa *CLIApp) checkManifestOperation(
 
 	manifestPath, err := mfa.resolveManifestArg(cmd)
 	if err != nil {
-		return fmt.Errorf("check: %w", err)
+		return err
 	}
 
+	// Done before a URL is swapped for the temp file it is downloaded to,
+	// whose directory is not the base.
+	basePath := resolveBasePath(cmd, manifestPath)
+
 	// URL manifests need to be downloaded to a temp file for the checker
 	if isHTTPURL(manifestPath) {
 		tmpPath, tmpErr := mfa.fetchManifestToTemp(ctx, manifestPath)
 		if tmpErr != nil {
-			return fmt.Errorf("check: %w", tmpErr)
+			return tmpErr
 		}
 
 		defer func() { _ = mfa.Fs.Remove(tmpPath) }()
@@ -304,20 +315,18 @@ func (mfa *CLIApp) checkManifestOperation(
 		manifestPath = tmpPath
 	}
 
-	basePath := cmd.String("base")
 	showProgress := cmd.Bool("progress")
 
 	log.Infof("checking manifest %s with base %s", manifestPath, basePath)
 
 	// Create checker
-	//nolint:contextcheck // mfer loads a manifest without a context
 	chk, err := mfer.NewChecker(&mfer.CheckerOptions{
 		ManifestPath: manifestPath,
 		BasePath:     basePath,
 		Fs:           mfa.Fs,
 	})
 	if err != nil {
-		return fmt.Errorf("failed to load manifest: %w", err)
+		return fmt.Errorf("load manifest: %w", err)
 	}
 
 	// Check signature requirement

internal/cli/entry.go

@@ -5,6 +5,7 @@ import (
 	"os"
 
 	"github.com/spf13/afero"
+	"sneak.berlin/go/mfer/mfer"
 )
 
 // NoColor disables colored output when set. Automatically true if the
@@ -60,6 +61,7 @@ func RunWithOptions(opts *RunOptions) int {
 		version:         opts.Version,
 		gitrev:          opts.Gitrev,
 		exitCode:        0,
+		maxManifestSize: mfer.MaxManifestSize,
 		Stdin:           opts.Stdin,
 		Stdout:          opts.Stdout,
 		Stderr:          opts.Stderr,

internal/cli/entry_test.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"io"
 	"math/rand"
+	"net/http/httptest"
 	"os"
 	"path/filepath"
 	"slices"
@@ -354,6 +355,44 @@ func TestGenerateCommand(t *testing.T) {
 	assert.True(t, exists)
 }
 
+// TestGenerateRefusesTwoFilesAtOnePath runs gen on arguments whose files
+// would share a path in the manifest: two directories that each hold a.txt,
+// and one directory given twice. gen must fail while it lists the files,
+// before it hashes any, naming the path, and write no manifest.
+func TestGenerateRefusesTwoFilesAtOnePath(t *testing.T) {
+	t.Parallel()
+
+	for _, tc := range []struct {
+		name          string
+		first, second string
+	}{
+		{"two directories", testDir, "/other"},
+		{"one directory twice", testDir, testDir},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			fs := afero.NewMemMapFs()
+			require.NoError(t, fs.MkdirAll(testDir, 0o755))
+			require.NoError(t, fs.MkdirAll("/other", 0o755))
+			writeTestFile(t, fs, "/testdir/a.txt", "first")
+			writeTestFile(t, fs, "/other/a.txt", "second")
+
+			opts := testOpts([]string{
+				testApp, cmdGenerate, "-q", "-o", testOutput, tc.first, tc.second,
+			}, fs)
+			assert.Equal(t, 1, runCLI(opts))
+			assert.Contains(t, testStderr(t, opts),
+				`enumerate files: duplicate path "a.txt": `+
+					tc.first+"/a.txt and "+tc.second+"/a.txt")
+
+			exists, err := afero.Exists(fs, testOutput)
+			require.NoError(t, err)
+			assert.False(t, exists)
+		})
+	}
+}
+
 // TestGenerateSeededManifestBytes pins the exact bytes `gen --seed` writes
 // for a fixed tree, so that a Go or dependency update that changes what
 // mfer writes fails here. testdata/seeded.mf was written by an mfer built
@@ -615,11 +654,10 @@ func runCheckAfterRewrite(t *testing.T, rewritten, msg string) {
 
 // TestCheckRequireSignatureRefusesOtherSigningKey runs check
 // --require-signature on a manifest signed by another key whose embedded
-// public key block also holds the required key. check must refuse it. It
-// needs gpg and is skipped without it, as the other signing tests are.
-//
-//nolint:paralleltest // signedManifest calls t.Setenv, which bars t.Parallel
+// public key block also holds the required key. check must refuse it.
 func TestCheckRequireSignatureRefusesOtherSigningKey(t *testing.T) {
+	t.Parallel()
+
 	content := []byte("signed file")
 	manifest, required := manifestSignedByAnotherKey(t,
 		map[string][]byte{testFileTxt: content})
@@ -636,7 +674,7 @@ func TestCheckRequireSignatureRefusesOtherSigningKey(t *testing.T) {
 	}, fs)
 	assert.Equal(t, 1, runCLI(opts))
 	assert.Contains(t, testStderr(t, opts),
-		"failed to load manifest: signature verification failed: "+
+		"load manifest: "+
 			"embedded public key block must hold exactly one key, found 2")
 }
 
@@ -941,6 +979,90 @@ func TestCheckNeverReportsManifest(t *testing.T) {
 	}
 }
 
+// The directory setupManifestInSubdir makes and the manifest it writes there,
+// relative to the working directory it sets.
+const (
+	testSubdir         = "sub"
+	testSubdirManifest = testSubdir + "/" + defaultManifestName
+)
+
+// setupManifestInSubdir makes a temp dir holding file.txt and sub/b.txt,
+// where sub/index.mf is the manifest gen writes for sub, and makes it the
+// working directory, so a test calling it cannot run in parallel. It returns
+// the temp dir.
+func setupManifestInSubdir(t *testing.T) string {
+	t.Helper()
+
+	root := t.TempDir()
+	sub := filepath.Join(root, testSubdir)
+
+	fs := afero.NewOsFs()
+	require.NoError(t, fs.MkdirAll(sub, 0o750))
+	writeTestFile(t, fs, filepath.Join(root, testFileTxt), "not in the manifest")
+	writeTestFile(t, fs, filepath.Join(sub, "b.txt"), "in the manifest")
+
+	opts := testOpts([]string{
+		testApp, cmdGenerate, "-q", "-o", filepath.Join(sub, defaultManifestName), sub,
+	}, fs)
+	require.Equal(t, 0, runCLI(opts), "stderr: %s", testStderr(t, opts))
+
+	t.Chdir(root)
+
+	return root
+}
+
+// TestCheckResolvesEntriesAgainstManifestDirectory runs check from the
+// directory above sub, on the manifest in sub. Without --base, the
+// manifest's entries are looked for in sub, whether check is given the
+// manifest or sub, and the files above sub are not reported. --base names
+// the directory to look in instead, the current one included.
+//
+//nolint:paralleltest // changes the process-global working directory
+func TestCheckResolvesEntriesAgainstManifestDirectory(t *testing.T) {
+	root := setupManifestInSubdir(t)
+
+	for _, tc := range []struct {
+		args     []string
+		exitCode int
+		failure  string // a line check must print, if any
+	}{
+		{[]string{testSubdir}, 0, ""},
+		{[]string{testSubdirManifest}, 0, ""},
+		{[]string{filepath.Join(root, testSubdir)}, 0, ""},
+		{[]string{testFlagBase, testSubdir, testSubdirManifest}, 0, ""},
+		{[]string{testFlagBase, ".", testSubdirManifest}, 1, "MISSING: b.txt"},
+	} {
+		t.Run(strings.Join(tc.args, " "), func(t *testing.T) {
+			opts := testOpts(slices.Concat(
+				[]string{testApp, cmdCheck, testFlagNoExtra}, tc.args,
+			), afero.NewOsFs())
+			assert.Equal(t, tc.exitCode, runCLI(opts), "stderr: %s", testStderr(t, opts))
+			assert.Contains(t, testStderr(t, opts), tc.failure)
+		})
+	}
+}
+
+// TestCheckURLManifestResolvesEntriesAgainstCurrentDirectory runs check on
+// a manifest given by URL, from a directory holding the file it lists: the
+// file is looked for there.
+//
+//nolint:paralleltest // changes the process-global working directory
+func TestCheckURLManifestResolvesEntriesAgainstCurrentDirectory(t *testing.T) {
+	files := map[string][]byte{testFileTxt: []byte("hello")}
+
+	server := httptest.NewServer(fetchTestHandler(manifestOf(t, files), files))
+	defer server.Close()
+
+	cwd := chdirTemp(t)
+	require.NoError(t,
+		os.WriteFile(filepath.Join(cwd, testFileTxt), files[testFileTxt], 0o600))
+
+	opts := testOpts([]string{
+		testApp, cmdCheck, testFlagNoExtra, server.URL + "/" + defaultManifestName,
+	}, afero.NewOsFs())
+	assert.Equal(t, 0, runCLI(opts), "stderr: %s", testStderr(t, opts))
+}
+
 // unlistableDirFs is a filesystem on which one directory cannot be listed.
 type unlistableDirFs struct {
 	afero.Fs
@@ -1166,6 +1288,225 @@ func TestGenerateLeavesLeftoverTempFileOutOfListing(t *testing.T) {
 	assert.Equal(t, []string{testFileTxt}, manifestPaths(t, fs, output))
 }
 
+// writeTestTree writes file.txt and sub/nested.txt under dir.
+func writeTestTree(t *testing.T, fs afero.Fs, dir string) {
+	t.Helper()
+
+	require.NoError(t, fs.MkdirAll(filepath.Join(dir, testSubdir), 0o750))
+	writeTestFile(t, fs, filepath.Join(dir, testFileTxt), "hello")
+	writeTestFile(t, fs, filepath.Join(dir, testSubdir, "nested.txt"), "in sub")
+}
+
+// TestGenerateDefaultOutput runs gen without --output on one directory or
+// one file: it writes index.mf in that directory, or beside that file,
+// listing each file by its path under the directory index.mf is in, and
+// check given that directory passes.
+func TestGenerateDefaultOutput(t *testing.T) {
+	t.Parallel()
+
+	// Paths are relative to a temp dir holding file.txt and sub/nested.txt.
+	for name, tc := range map[string]struct {
+		input, output string
+		listed        []string
+	}{
+		"directory": {
+			".", defaultManifestName, []string{testFileTxt, "sub/nested.txt"},
+		},
+		"subdirectory": {testSubdir, testSubdirManifest, []string{"nested.txt"}},
+		"file":         {testFileTxt, defaultManifestName, []string{testFileTxt}},
+	} {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			root := t.TempDir()
+			fs := afero.NewOsFs()
+			writeTestTree(t, fs, root)
+
+			opts := testOpts([]string{
+				testApp, cmdGenerate, "-q", filepath.Join(root, tc.input),
+			}, fs)
+			require.Equal(t, 0, runCLI(opts), "stderr: %s", testStderr(t, opts))
+
+			output := filepath.Join(root, tc.output)
+			assert.ElementsMatch(t, tc.listed, manifestPaths(t, fs, output))
+
+			opts = testOpts([]string{
+				testApp, cmdCheck, "-q", filepath.Dir(output),
+			}, fs)
+			assert.Equal(t, 0, runCLI(opts), "stderr: %s", testStderr(t, opts))
+		})
+	}
+}
+
+// TestGenerateSeveralPathsDefaultOutput runs gen without --output on two
+// directories: it writes index.mf in the current directory, listing both
+// directories' files, and writes no index.mf in either directory.
+//
+//nolint:paralleltest // changes the process-global working directory
+func TestGenerateSeveralPathsDefaultOutput(t *testing.T) {
+	root := t.TempDir()
+	fs := afero.NewOsFs()
+	dirs := []string{"first", "second"}
+
+	for _, dir := range dirs {
+		require.NoError(t, fs.MkdirAll(filepath.Join(root, dir), 0o750))
+		writeTestFile(t, fs, filepath.Join(root, dir, dir+".txt"), dir)
+	}
+
+	t.Chdir(root)
+
+	opts := testOpts(append([]string{testApp, cmdGenerate, "-q"}, dirs...), fs)
+	require.Equal(t, 0, runCLI(opts), "stderr: %s", testStderr(t, opts))
+
+	assert.ElementsMatch(t, []string{"first.txt", "second.txt"},
+		manifestPaths(t, fs, filepath.Join(root, defaultManifestName)))
+
+	for _, dir := range dirs {
+		exists, err := afero.Exists(fs, filepath.Join(root, dir, defaultManifestName))
+		require.NoError(t, err)
+		assert.False(t, exists, "index.mf written in %s", dir)
+	}
+}
+
+// testLink is the name of the symlink to a tree that the
+// DirectoryNamedThroughSymlink tests make in a temp dir.
+const testLink = "link"
+
+// TestGenerateDirectoryNamedThroughSymlink runs gen on a directory named
+// through a symlink, given as the argument or as the working directory: the
+// manifest lists the files in the directory the symlink points to, and
+// leaves out a symlink inside it, as gen does without --follow-symlinks.
+//
+//nolint:paralleltest // changes the process-global working directory
+func TestGenerateDirectoryNamedThroughSymlink(t *testing.T) {
+	// Paths are relative to a temp dir holding data and link, a symlink to
+	// data.
+	for name, tc := range map[string]struct {
+		workDir string
+		args    []string
+	}{
+		"argument":          {".", []string{testLink}},
+		"working directory": {testLink, nil},
+	} {
+		t.Run(name, func(t *testing.T) {
+			root := t.TempDir()
+			data := filepath.Join(root, "data")
+
+			fs := afero.NewOsFs()
+			writeTestTree(t, fs, data)
+			require.NoError(t,
+				os.Symlink(testFileTxt, filepath.Join(data, "alias.txt")))
+			require.NoError(t, os.Symlink(data, filepath.Join(root, testLink)))
+
+			// t.Chdir sets PWD to the path it is given, as a shell does, and
+			// os.Getwd returns PWD when it names the working directory.
+			t.Chdir(filepath.Join(root, tc.workDir))
+
+			opts := testOpts(slices.Concat(
+				[]string{testApp, cmdGenerate, "-q"}, tc.args,
+			), fs)
+			require.Equal(t, 0, runCLI(opts), "stderr: %s", testStderr(t, opts))
+
+			assert.ElementsMatch(t, []string{testFileTxt, "sub/nested.txt"},
+				manifestPaths(t, fs, filepath.Join(data, defaultManifestName)))
+		})
+	}
+}
+
+// TestGenerateBytesDoNotDependOnOutput runs gen --seed on one tree, each
+// time writing to another file, over a file already there and beside an
+// earlier run's temp file: neither is listed, and what is listed depends
+// only on the tree, so every manifest has the same bytes.
+func TestGenerateBytesDoNotDependOnOutput(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	tree := filepath.Join(root, "tree")
+	defaultOutput := filepath.Join(tree, defaultManifestName)
+
+	fs := afero.NewOsFs()
+	writeTestTree(t, fs, tree)
+
+	var first []byte
+
+	for _, output := range []string{
+		defaultOutput,
+		filepath.Join(tree, "listing.mf"),
+		filepath.Join(tree, testSubdir, "listing.mf"),
+		filepath.Join(root, "outside.mf"),
+	} {
+		writeTestFile(t, fs, output, "previous manifest")
+		writeTestFile(t, fs, manifestTempPath(output), "part of a manifest")
+
+		args := []string{testApp, cmdGenerate, "-q", "-f", "--seed", "mfer"}
+		if output != defaultOutput {
+			args = append(args, "-o", output)
+		}
+
+		args = append(args, tree)
+
+		opts := testOpts(args, fs)
+		require.Equal(t, 0, runCLI(opts),
+			"output %s, stderr: %s", output, testStderr(t, opts))
+
+		got, err := afero.ReadFile(fs, output)
+		require.NoError(t, err)
+		require.NoError(t, fs.Remove(output))
+
+		if first == nil {
+			first = got
+		}
+
+		assert.Equal(t, first, got, "output %s", output)
+	}
+}
+
+// TestGenerateRefusesExistingDefaultOutput runs gen without --output or
+// --force on a directory already holding index.mf: gen fails, naming that
+// file, and leaves it as it was.
+func TestGenerateRefusesExistingDefaultOutput(t *testing.T) {
+	t.Parallel()
+
+	output := filepath.Join(testDir, defaultManifestName)
+
+	fs := afero.NewMemMapFs()
+	require.NoError(t, fs.MkdirAll(testDir, 0o755))
+	writeTestFile(t, fs, testFile1, "hello")
+	writeTestFile(t, fs, output, "previous manifest")
+
+	opts := testOpts([]string{testApp, cmdGenerate, "-q", testDir}, fs)
+	assert.Equal(t, 1, runCLI(opts))
+	assert.Contains(t, testStderr(t, opts),
+		"output file "+output+" already exists (use --force to overwrite)")
+
+	content, err := afero.ReadFile(fs, output)
+	require.NoError(t, err)
+	assert.Equal(t, "previous manifest", string(content))
+}
+
+// TestGenerateRefusesEmptyOutput runs gen with --force and an --output
+// given an empty value, as an unset shell variable gives it, on a
+// directory already holding index.mf: gen fails and leaves that file as it
+// was.
+func TestGenerateRefusesEmptyOutput(t *testing.T) {
+	t.Parallel()
+
+	output := filepath.Join(testDir, defaultManifestName)
+
+	fs := afero.NewMemMapFs()
+	require.NoError(t, fs.MkdirAll(testDir, 0o755))
+	writeTestFile(t, fs, testFile1, "hello")
+	writeTestFile(t, fs, output, "previous manifest")
+
+	opts := testOpts([]string{testApp, cmdGenerate, "-q", "-f", "-o", "", testDir}, fs)
+	assert.Equal(t, 1, runCLI(opts))
+	assert.Contains(t, testStderr(t, opts), errEmptyOutput.Error())
+
+	content, err := afero.ReadFile(fs, output)
+	require.NoError(t, err)
+	assert.Equal(t, "previous manifest", string(content))
+}
+
 func TestGenerateAtomicWriteUsesTemp(t *testing.T) {
 	t.Parallel()
 

internal/cli/errmsg_test.go

@@ -4,14 +4,18 @@ package cli
 import (
 	"bytes"
 	"context"
+	"encoding/hex"
+	"io"
 	"net/http"
 	"net/http/httptest"
 	"os"
-	"os/exec"
 	"path/filepath"
-	"slices"
+	"strings"
 	"testing"
 
+	"github.com/ProtonMail/go-crypto/openpgp"
+	"github.com/ProtonMail/go-crypto/openpgp/armor"
+	"github.com/ProtonMail/go-crypto/openpgp/packet"
 	"github.com/spf13/afero"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -108,11 +112,10 @@ func TestVerifyRequiredSignerMessages(t *testing.T) {
 // manifest. The signing key's fingerprint is whatever the generated key
 // produced, so it is read back from the checker and substituted into the
 // expected string; the required signer is a fixed value that cannot match
-// it. Requires gpg and is skipped where it is absent, as the other signing
-// tests are.
-//
-//nolint:paralleltest // signedManifest calls t.Setenv, which bars t.Parallel
+// it.
 func TestSignerMismatchMessage(t *testing.T) {
+	t.Parallel()
+
 	chk := signedChecker(t,
 		signedManifest(t, map[string][]byte{"f.txt": []byte("signed file")}))
 
@@ -123,43 +126,48 @@ func TestSignerMismatchMessage(t *testing.T) {
 			" does not match required "+msgFpB)
 }
 
-// signedManifest returns a manifest of files signed by a throwaway GPG key
-// generated in a temporary GNUPGHOME, which it leaves set for the rest of
-// the test.
+// testSecretKey returns a new OpenPGP key with its secret key, armored, as
+// gpg --export-secret-keys --armor writes it, and the key's fingerprint.
+// The key is protected by passphrase unless that is nil. config sets how
+// the key is made; without one it is an Ed25519 key, which is quick to
+// make.
+func testSecretKey(
+	t *testing.T, passphrase []byte, config *packet.Config,
+) ([]byte, string) {
+	t.Helper()
+
+	if config == nil {
+		config = &packet.Config{Algorithm: packet.PubKeyAlgoEdDSA}
+	}
+
+	key, err := openpgp.NewEntity("MFER Test Key", "", "test@mfer.test", config)
+	require.NoError(t, err)
+
+	if passphrase != nil {
+		require.NoError(t, key.EncryptPrivateKeys(passphrase, nil))
+	}
+
+	var buf bytes.Buffer
+
+	w, err := armor.Encode(&buf, openpgp.PrivateKeyType, nil)
+	require.NoError(t, err)
+	require.NoError(t, key.SerializePrivateWithoutSigning(w, nil))
+	require.NoError(t, w.Close())
+
+	return buf.Bytes(), strings.ToUpper(hex.EncodeToString(key.PrimaryKey.Fingerprint))
+}
+
+// signedManifest returns a manifest of files signed by a new OpenPGP key.
 func signedManifest(t *testing.T, files map[string][]byte) []byte {
 	t.Helper()
 
-	_, err := exec.LookPath("gpg")
-	if err != nil {
-		t.Skip("gpg not installed, skipping signing test")
-	}
-
-	gpgHome := t.TempDir()
-	params := "%no-protection\n" +
-		"Key-Type: RSA\nKey-Length: 2048\n" +
-		"Name-Real: MFER Test Key\nName-Email: test@mfer.test\n" +
-		"Expire-Date: 0\n%commit\n"
-	paramsFile := filepath.Join(gpgHome, "key-params")
-	require.NoError(t, os.WriteFile(paramsFile, []byte(params), 0o600))
-
-	//nolint:gosec // paramsFile is a test-controlled path inside t.TempDir()
-	cmd := exec.CommandContext(context.Background(), "gpg",
-		"--batch", "--gen-key", paramsFile)
-
-	cmd.Env = append(os.Environ(), "GNUPGHOME="+gpgHome)
-
-	out, err := cmd.CombinedOutput()
-	if err != nil {
-		t.Skipf("failed to generate test GPG key: %v: %s", err, out)
-	}
-
-	t.Setenv("GNUPGHOME", gpgHome)
+	secretKey, _ := testSecretKey(t, nil, nil)
 
 	b := mfer.NewBuilder()
-	b.SetSigningOptions(&mfer.SigningOptions{KeyID: mfer.GPGKeyID("test@mfer.test")})
+	b.SetSigningOptions(&mfer.SigningOptions{SecretKey: secretKey})
 
 	for path, content := range files {
-		_, err = b.AddFile(mfer.RelFilePath(path), mfer.FileSize(len(content)),
+		_, err := b.AddFile(mfer.RelFilePath(path), mfer.FileSize(len(content)),
 			mfer.ModTime{}, 0, bytes.NewReader(content), nil)
 		require.NoError(t, err)
 	}
@@ -190,8 +198,8 @@ func signedChecker(t *testing.T, manifest []byte) *mfer.Checker {
 }
 
 // manifestSignedByAnotherKey returns a manifest of files and the
-// fingerprint of a throwaway key, the required key, that did not sign it.
-// The manifest is signed by a second throwaway key; its embedded public key
+// fingerprint of a new key, the required key, that did not sign it.
+// The manifest is signed by a second new key; its embedded public key
 // block holds the required key followed by the second key, and its signer
 // field names the required key.
 func manifestSignedByAnotherKey(
@@ -207,8 +215,26 @@ func manifestSignedByAnotherKey(
 	require.NoError(t, proto.Unmarshal(
 		signedManifest(t, files)[len(mfer.MAGIC):], outer))
 
-	outer.SigningPubKey = slices.Concat(
-		required.GetSigningPubKey(), outer.GetSigningPubKey())
+	// One armored block holding both keys, as gpg --export --armor writes
+	// two keys.
+	var block bytes.Buffer
+
+	w, err := armor.Encode(&block, openpgp.PublicKeyType, nil)
+	require.NoError(t, err)
+
+	for _, key := range [][]byte{
+		required.GetSigningPubKey(), outer.GetSigningPubKey(),
+	} {
+		decoded, err := armor.Decode(bytes.NewReader(key))
+		require.NoError(t, err)
+
+		_, err = io.Copy(w, decoded.Body)
+		require.NoError(t, err)
+	}
+
+	require.NoError(t, w.Close())
+
+	outer.SigningPubKey = block.Bytes()
 	outer.Signer = required.GetSigner()
 
 	data, err := proto.Marshal(outer)
@@ -303,7 +329,7 @@ func TestManifestLoaderHTTPStatusMessage(t *testing.T) {
 	_, err := mfa.openManifestReader(context.Background(), server.URL+"/foo.mf")
 	require.ErrorIs(t, err, errHTTPStatus)
 	assert.EqualError(t, err,
-		"failed to fetch "+server.URL+"/foo.mf: HTTP 404")
+		"download manifest "+server.URL+"/foo.mf: unexpected HTTP status 404")
 }
 
 func TestFetchManifestHTTPStatusMessage(t *testing.T) {
@@ -325,7 +351,7 @@ func TestFetchManifestHTTPStatusMessage(t *testing.T) {
 		return cmd.Run(context.Background(), []string{cmdFetch, server.URL})
 	})
 	require.ErrorIs(t, err, errHTTPStatus)
-	assert.EqualError(t, err, "failed to fetch manifest: HTTP 404")
+	assert.EqualError(t, err, "download manifest: unexpected HTTP status 404")
 }
 
 func TestFetchFileHTTPStatusMessage(t *testing.T) {
@@ -343,7 +369,108 @@ func TestFetchFileHTTPStatusMessage(t *testing.T) {
 			&mfer.MFFilePath{}, nil)
 	})
 	require.ErrorIs(t, err, errHTTPStatus)
-	assert.EqualError(t, err, "HTTP 500")
+	assert.EqualError(t, err, "unexpected HTTP status 500")
+}
+
+// TestCheckCorruptManifestMessage runs check on a file that is not a
+// manifest.
+func TestCheckCorruptManifestMessage(t *testing.T) {
+	t.Parallel()
+
+	fs := afero.NewMemMapFs()
+	require.NoError(t, afero.WriteFile(fs, "/bad.mf", []byte("not a manifest"), 0o644))
+
+	mfa := &CLIApp{Fs: fs}
+	cmd := mfa.checkCommand()
+	cmd.Action = mfa.checkManifestOperation
+
+	// checkManifestOperation logs to the process-global logger.
+	err := runLocked(func() error {
+		return cmd.Run(context.Background(), []string{cmdCheck, "/bad.mf"})
+	})
+	assert.EqualError(t, err, "load manifest: invalid file format")
+}
+
+// TestListMissingManifestMessage runs list on a manifest file that does not
+// exist.
+func TestListMissingManifestMessage(t *testing.T) {
+	t.Parallel()
+
+	mfa := &CLIApp{Fs: afero.NewMemMapFs()}
+	cmd := mfa.listCommand()
+
+	// listManifestOperation sets the process-global log level.
+	err := runLocked(func() error {
+		return cmd.Run(context.Background(), []string{cmdList, "/nope.mf"})
+	})
+	require.ErrorIs(t, err, os.ErrNotExist)
+	assert.EqualError(t, err, "open /nope.mf: file does not exist")
+}
+
+// TestFetchHashMismatchMessage runs fetch against a server that sends a
+// listed file with other content of the same size.
+func TestFetchHashMismatchMessage(t *testing.T) {
+	t.Parallel()
+
+	manifest := builtManifest(t, map[string][]byte{testFileTxt: []byte("listed")})
+
+	server := httptest.NewServer(fetchTestHandler(manifest,
+		map[string][]byte{testFileTxt: []byte("served")}))
+	defer server.Close()
+
+	mfa := &CLIApp{Fs: afero.NewMemMapFs(), maxManifestSize: mfer.MaxManifestSize}
+	cmd := mfa.fetchCommand()
+	cmd.Action = mfa.fetchManifestOperation
+
+	// fetchManifestOperation logs to the process-global logger.
+	err := runLocked(func() error {
+		return cmd.Run(context.Background(),
+			[]string{cmdFetch, "--" + flagDest, t.TempDir(), server.URL})
+	})
+	require.ErrorIs(t, err, errHashMismatch)
+	assert.EqualError(t, err, "download "+testFileTxt+": hash mismatch")
+}
+
+// TestFreshenBackslashPathMessage runs freshen on a tree that has gained a
+// file whose name holds a backslash, which a manifest path may not contain.
+func TestFreshenBackslashPathMessage(t *testing.T) {
+	t.Parallel()
+
+	fs := afero.NewOsFs()
+	root, manifestPath := setupFreshenDir(t, fs,
+		map[string]string{testFileTxt: "content"})
+	writeTestFile(t, fs, filepath.Join(root, `a\b.txt`), "new")
+
+	mfa := &CLIApp{Fs: fs}
+	cmd := mfa.freshenCommand()
+	cmd.Action = mfa.freshenManifestOperation
+
+	// freshenManifestOperation logs to the process-global logger.
+	err := runLocked(func() error {
+		return cmd.Run(context.Background(),
+			[]string{cmdFreshen, testFlagBase, root, manifestPath})
+	})
+	assert.EqualError(t, err,
+		`path "a\\b.txt" contains backslash; use forward slashes only`)
+}
+
+// TestFreshenReadErrorMessage has freshen hash a directory as though it
+// were a file, so reading it fails.
+func TestFreshenReadErrorMessage(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	require.NoError(t, os.Mkdir(filepath.Join(root, "sub"), 0o750))
+
+	hasher := &freshenHasher{
+		fs:      afero.NewOsFs(),
+		absBase: root,
+		builder: mfer.NewBuilder(),
+	}
+
+	err := hasher.processEntry(&freshenEntry{path: "sub", needsHash: true})
+	assert.EqualError(t, err,
+		"read "+filepath.Join(root, "sub")+": is a directory")
 }
 
 func TestURLRequiredMessage(t *testing.T) {

internal/cli/export.go

@@ -26,20 +26,19 @@ func (mfa *CLIApp) exportManifestOperation(
 ) error {
 	pathOrURL, err := mfa.resolveManifestArg(cmd)
 	if err != nil {
-		return fmt.Errorf("export: %w", err)
+		return err
 	}
 
 	rc, err := mfa.openManifestReader(ctx, pathOrURL)
 	if err != nil {
-		return fmt.Errorf("export: %w", err)
+		return err
 	}
 
 	defer func() { _ = rc.Close() }()
 
-	//nolint:contextcheck // mfer loads a manifest without a context
 	manifest, err := mfer.NewManifestFromReader(rc)
 	if err != nil {
-		return fmt.Errorf("export: failed to parse manifest: %w", err)
+		return fmt.Errorf("parse manifest: %w", err)
 	}
 
 	files := manifest.Files()
@@ -76,7 +75,7 @@ func (mfa *CLIApp) exportManifestOperation(
 
 	err = enc.Encode(entries)
 	if err != nil {
-		return fmt.Errorf("export: failed to encode JSON: %w", err)
+		return fmt.Errorf("encode JSON: %w", err)
 	}
 
 	return nil

internal/cli/fetch.go

@@ -239,7 +239,7 @@ func reportDownloadProgress(progress <-chan DownloadProgress, done chan<- struct
 func manifestBaseURL(manifestURL string) (*url.URL, error) {
 	parsed, err := url.Parse(manifestURL)
 	if err != nil {
-		return nil, fmt.Errorf("fetch: invalid manifest URL: %w", err)
+		return nil, fmt.Errorf("invalid manifest URL: %w", err)
 	}
 
 	// JoinPath cleans the path it builds, so ".." drops the manifest's
@@ -268,7 +268,7 @@ func downloadManifestFiles(
 		// Sanitize the path to prevent path traversal attacks
 		localPath, err := sanitizePath(f.GetPath())
 		if err != nil {
-			return 0, 0, fmt.Errorf("invalid path in manifest: %w", err)
+			return 0, 0, fmt.Errorf("invalid file entry: %w", err)
 		}
 
 		if alreadyPresent(dest, localPath, f) {
@@ -284,7 +284,7 @@ func downloadManifestFiles(
 
 		err = downloadFile(ctx, client, fileURL, dest, localPath, f, progress)
 		if err != nil {
-			return 0, 0, fmt.Errorf("failed to download %s: %w", f.GetPath(), err)
+			return 0, 0, fmt.Errorf("download %s: %w", f.GetPath(), err)
 		}
 
 		downloaded++
@@ -361,7 +361,7 @@ func (mfa *CLIApp) fetchManifestOperation(
 		firstDelay: firstRetryDelay,
 	}
 
-	manifestData, files, err := fetchManifest(ctx, cmd, client, manifestURL)
+	manifestData, files, err := mfa.fetchManifest(ctx, cmd, client, manifestURL)
 	if err != nil {
 		return err
 	}
@@ -376,7 +376,7 @@ func (mfa *CLIApp) fetchManifestOperation(
 
 	err = os.MkdirAll(dest, dirPerms)
 	if err != nil {
-		return fmt.Errorf("failed to create destination directory %s: %w", dest, err)
+		return err
 	}
 
 	// Create progress channel and start progress reporter goroutine
@@ -403,7 +403,7 @@ func (mfa *CLIApp) fetchManifestOperation(
 	// "mfer check" can verify the tree later.
 	err = saveManifest(dest, manifestData)
 	if err != nil {
-		return fmt.Errorf("failed to save manifest: %w", err)
+		return fmt.Errorf("save manifest: %w", err)
 	}
 
 	// Print summary
@@ -426,37 +426,42 @@ func (mfa *CLIApp) fetchManifestOperation(
 // that lists a file where fetch writes another or a mode outside 0777. It
 // returns the manifest as downloaded, to be saved once the files are in
 // place, and the files it lists.
-func fetchManifest(
+func (mfa *CLIApp) fetchManifest(
 	ctx context.Context, cmd *cli.Command, client retryingClient, manifestURL string,
 ) ([]byte, []*mfer.MFFilePath, error) {
 	log.Infof("fetching manifest from %s", manifestURL)
 
 	// Read the whole manifest before parsing it, so that a connection
 	// lost partway through is retried rather than reported as a bad
-	// manifest.
+	// manifest. Reading stops one byte past mfa.maxManifestSize, which is
+	// enough to tell that the manifest is too large.
 	var manifestData []byte
 
 	err := client.get(ctx, manifestURL, func(resp *http.Response) error {
 		var readErr error
 
-		manifestData, readErr = io.ReadAll(resp.Body)
+		manifestData, readErr = io.ReadAll(
+			io.LimitReader(resp.Body, mfa.maxManifestSize+1))
 
 		return readErr
 	})
 	if err != nil {
-		return nil, nil, fmt.Errorf("failed to fetch manifest: %w", err)
+		return nil, nil, fmt.Errorf("download manifest: %w", err)
+	}
+
+	if int64(len(manifestData)) > mfa.maxManifestSize {
+		return nil, nil, fmt.Errorf("download manifest: %w of %d bytes",
+			errManifestTooLarge, mfa.maxManifestSize)
 	}
 
 	// Parse manifest
-	//nolint:contextcheck // mfer loads a manifest without a context
 	manifest, err := mfer.NewManifestFromReader(bytes.NewReader(manifestData))
 	if err != nil {
-		return nil, nil, fmt.Errorf("failed to parse manifest: %w", err)
+		return nil, nil, fmt.Errorf("parse manifest: %w", err)
 	}
 
 	requiredSigner := cmd.String(flagRequireSignature)
 	if requiredSigner != "" {
-		//nolint:contextcheck // mfer loads a manifest without a context
 		err = verifyFetchedSigner(manifestData, requiredSigner)
 		if err != nil {
 			return nil, nil, err
@@ -554,7 +559,7 @@ func verifyFetchedSigner(manifestData []byte, requiredSigner string) error {
 		Fs:           memFs,
 	})
 	if err != nil {
-		return fmt.Errorf("failed to load manifest: %w", err)
+		return fmt.Errorf("load manifest: %w", err)
 	}
 
 	return verifyRequiredSigner(chk, requiredSigner)
@@ -646,7 +651,7 @@ func checkNoSymlinks(dest, p string) error {
 		}
 
 		if err != nil {
-			return fmt.Errorf("failed to check %s for a symlink: %w", current, err)
+			return err
 		}
 
 		if info.Mode()&os.ModeSymlink != 0 {
@@ -763,7 +768,7 @@ func tempPathFor(localPath string) string {
 func verifyDownloadedHash(digest []byte, entry *mfer.MFFilePath) error {
 	computed, err := multihash.Encode(digest, multihash.SHA2_256)
 	if err != nil {
-		return fmt.Errorf("failed to encode hash: %w", err)
+		return fmt.Errorf("encode hash: %w", err)
 	}
 
 	for _, hash := range entry.GetHashes() {
@@ -790,7 +795,7 @@ func downloadFile(
 	// so every entry point to downloadFile gets the same treatment.
 	localPath, err := sanitizePath(localPath)
 	if err != nil {
-		return fmt.Errorf("invalid path: %w", err)
+		return fmt.Errorf("invalid file entry: %w", err)
 	}
 
 	// Create parent directories if needed
@@ -805,7 +810,7 @@ func downloadFile(
 
 		err = os.MkdirAll(dir, dirPerms)
 		if err != nil {
-			return fmt.Errorf("failed to create directory %s: %w", dir, err)
+			return err
 		}
 	}
 
@@ -839,7 +844,7 @@ func createTempFile(dest, tmpPath string) (*os.File, error) {
 	out, err := os.OpenFile( //nolint:gosec // G304: see comment above
 		path, os.O_RDWR|os.O_CREATE|os.O_EXCL, filePerms)
 	if err != nil {
-		return nil, fmt.Errorf("failed to create temp file: %w", err)
+		return nil, err
 	}
 
 	return out, nil
@@ -852,12 +857,7 @@ func moveIntoPlace(dest, tmpPath, localPath string) error {
 		return err
 	}
 
-	err = os.Rename(filepath.Join(dest, tmpPath), filepath.Join(dest, localPath))
-	if err != nil {
-		return fmt.Errorf("failed to rename temp file: %w", err)
-	}
-
-	return nil
+	return os.Rename(filepath.Join(dest, tmpPath), filepath.Join(dest, localPath))
 }
 
 // saveResponse writes resp's body to tmpPath, verifies it against entry,
@@ -892,7 +892,7 @@ func saveResponse(
 			_ = out.Close()
 			_ = os.Remove(filepath.Join(dest, tmpPath))
 
-			return fmt.Errorf("failed to set mode: %w", err)
+			return err
 		}
 	}
 
@@ -908,8 +908,10 @@ func saveResponse(
 		progress:  progress,
 	}
 
-	// Copy content while hashing and reporting progress
-	written, copyErr := io.Copy(pw, resp.Body)
+	// Copy content while hashing and reporting progress. One byte past
+	// the listed size is enough for finishDownload to report a size
+	// mismatch.
+	written, copyErr := io.Copy(pw, io.LimitReader(resp.Body, expectedSize+1))
 
 	// Close file before checking errors (to flush writes)
 	closeErr := out.Close()

internal/cli/fetch_test.go

@@ -26,6 +26,7 @@ import (
 	"github.com/spf13/afero"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	urfcli "github.com/urfave/cli/v3"
 	"google.golang.org/protobuf/proto"
 	"sneak.berlin/go/mfer/mfer"
 )
@@ -441,6 +442,76 @@ func TestFetchSizeMismatch(t *testing.T) {
 		"temp file should be cleaned up on size mismatch")
 }
 
+// zeros is an io.Reader of zero bytes without end.
+type zeros struct{}
+
+func (zeros) Read(p []byte) (int, error) {
+	clear(p)
+
+	return len(p), nil
+}
+
+// TestFetchStopsReadingFilePastListedSize serves a body that never ends
+// for a file listed at 16 bytes. fetch must stop reading one byte past
+// the listed size, report the size mismatch and remove its temp file.
+//
+//nolint:paralleltest // changes the process-global working directory
+func TestFetchStopsReadingFilePastListedSize(t *testing.T) {
+	server := httptest.NewServer(
+		http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+			_, _ = io.Copy(w, zeros{})
+		}))
+	defer server.Close()
+
+	chdirTemp(t)
+
+	err := downloadFile(context.Background(), testClient(),
+		server.URL+"/"+testFileTxt, ".", testFileTxt,
+		&mfer.MFFilePath{Path: testFileTxt, Size: 16}, nil)
+	require.ErrorIs(t, err, errSizeMismatch)
+	require.EqualError(t, err, "size mismatch: expected 16 bytes, got 17")
+	assert.NoFileExists(t, tempPathFor(testFileTxt))
+}
+
+// TestManifestDownloadStopsAtLimit serves a manifest that never ends to
+// fetch and to check, with the most they download of a manifest lowered
+// to 64 KiB. Each must stop reading at that limit, fail with an error
+// naming it and leave no temp file.
+func TestManifestDownloadStopsAtLimit(t *testing.T) {
+	server := httptest.NewServer(
+		http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+			_, _ = io.Copy(w, zeros{})
+		}))
+	defer server.Close()
+
+	tmpDir := t.TempDir()
+	t.Setenv("TMPDIR", tmpDir)
+
+	mfa := &CLIApp{Fs: afero.NewOsFs(), maxManifestSize: 64 << 10}
+
+	fetch := mfa.fetchCommand()
+	fetch.Action = mfa.fetchManifestOperation
+
+	check := mfa.checkCommand()
+	check.Action = mfa.checkManifestOperation
+
+	for _, cmd := range []*urfcli.Command{fetch, check} {
+		// Both operations log to the process-global logger.
+		err := runLocked(func() error {
+			return cmd.Run(context.Background(),
+				[]string{cmd.Name, server.URL + "/index.mf"})
+		})
+		require.ErrorIs(t, err, errManifestTooLarge, cmd.Name)
+		require.EqualError(t, err,
+			"download manifest: file exceeds maximum allowed size of 65536 bytes",
+			cmd.Name)
+	}
+
+	leftover, err := os.ReadDir(tmpDir)
+	require.NoError(t, err)
+	assert.Empty(t, leftover)
+}
+
 //nolint:paralleltest // changes the process-global working directory
 func TestFetchProgress(t *testing.T) {
 	// Create source filesystem with a larger test file
@@ -535,27 +606,27 @@ func TestFetchRefusesSymlinks(t *testing.T) {
 	}{
 		{
 			"parent directory", "sub/deeper/file.txt", "sub", ".",
-			"failed to download sub/deeper/file.txt",
+			"download sub/deeper/file.txt",
 		},
 		{
 			"directory inside a plain directory", "docs/data/passwd", "docs/data", ".",
-			"failed to download docs/data/passwd",
+			"download docs/data/passwd",
 		},
 		{
 			"temp file", testFileTxt, ".file.txt.tmp", newFile,
-			"failed to download " + testFileTxt,
+			"download " + testFileTxt,
 		},
 		{
 			"file", testFileTxt, testFileTxt, newFile,
-			"failed to download " + testFileTxt,
+			"download " + testFileTxt,
 		},
 		{
 			"manifest temp file", testFileTxt, tempPathFor(defaultManifestName), newFile,
-			"failed to save manifest",
+			"save manifest",
 		},
 		{
 			"manifest", testFileTxt, defaultManifestName, newFile,
-			"failed to save manifest",
+			"save manifest",
 		},
 	}
 
@@ -622,7 +693,7 @@ func TestFetchDoesNotSkipThroughSymlink(t *testing.T) {
 	}, afero.NewOsFs())
 	assert.Equal(t, 1, runCLI(opts))
 	assert.Contains(t, testStderr(t, opts),
-		"failed to download sub/"+testFileTxt+": symlink in path not allowed: "+link)
+		"download sub/"+testFileTxt+": symlink in path not allowed: "+link)
 	assert.Equal(t, map[string][]byte{testFileTxt: content}, filesUnder(t, outside))
 }
 
@@ -692,7 +763,8 @@ func TestGetRetriesTransientStatusesOnly(t *testing.T) {
 
 			err := getNothing(testClient(), server.URL, 10*time.Second)
 			require.ErrorIs(t, err, errHTTPStatus)
-			require.EqualError(t, err, fmt.Sprintf("HTTP %d", tt.status))
+			require.EqualError(t, err,
+				fmt.Sprintf("unexpected HTTP status %d", tt.status))
 			assert.Equal(t, tt.requests, requests.Load())
 		})
 	}
@@ -1119,20 +1191,23 @@ func TestFetchIntoDest(t *testing.T) {
 // with check's message before it downloads or writes anything; the
 // required key lets it through. A manifest signed by another key whose
 // embedded public key block also holds the required key must stop fetch
-// too. The signed cases need gpg and are skipped without it, as the other
-// signing tests are.
-//
-//nolint:paralleltest // signedManifest calls t.Setenv, which bars t.Parallel
+// too.
 func TestFetchRequireSignature(t *testing.T) {
+	t.Parallel()
+
 	files := map[string][]byte{testFileTxt: []byte("signed file")}
 
 	t.Run("unsigned", func(t *testing.T) {
+		t.Parallel()
+
 		assertFetchRefused(t, manifestOf(t, files), files,
 			"manifest is not signed, but signature from "+msgFpA+" is required",
 			"--"+flagRequireSignature, msgFpA)
 	})
 
 	t.Run("signed", func(t *testing.T) {
+		t.Parallel()
+
 		manifest := signedManifest(t, files)
 
 		signer := string(signedChecker(t, manifest).Signer())
@@ -1155,10 +1230,12 @@ func TestFetchRequireSignature(t *testing.T) {
 	})
 
 	t.Run("signed by another key embedded after the required one", func(t *testing.T) {
+		t.Parallel()
+
 		manifest, required := manifestSignedByAnotherKey(t, files)
 
 		assertFetchRefused(t, manifest, files,
-			"failed to parse manifest: signature verification failed: "+
+			"parse manifest: "+
 				"embedded public key block must hold exactly one key, found 2",
 			"--"+flagRequireSignature, required)
 	})

internal/cli/freshen.go

@@ -165,8 +165,7 @@ func (s *freshenScanner) walk(path string, info fs.FileInfo, walkErr error) erro
 	// Get relative path
 	relPath, err := filepath.Rel(s.absBase, path)
 	if err != nil {
-		return fmt.Errorf(
-			"freshen: failed to compute relative path for %s: %w", path, err)
+		return err
 	}
 
 	// Handle dotfiles
@@ -280,13 +279,8 @@ func (h *freshenHasher) reportProgress(n int64) {
 // processEntry hashes the entry if needed and adds it to the builder.
 func (h *freshenHasher) processEntry(e *freshenEntry) error {
 	if !e.needsHash {
-		// Use existing entry
-		err := addExistingToBuilder(h.builder, e.existing)
-		if err != nil {
-			return fmt.Errorf("failed to add %s: %w", e.path, err)
-		}
-
-		return nil
+		// Use existing entry; the error names the entry
+		return addExistingToBuilder(h.builder, e.existing)
 	}
 
 	// Need to read and hash the file
@@ -294,26 +288,21 @@ func (h *freshenHasher) processEntry(e *freshenEntry) error {
 
 	f, err := h.fs.Open(absPath)
 	if err != nil {
-		return fmt.Errorf("failed to open %s: %w", e.path, err)
+		return err
 	}
 
 	hash, bytesRead, err := hashFile(f, h.reportProgress)
 	_ = f.Close()
 
 	if err != nil {
-		return fmt.Errorf("failed to hash %s: %w", e.path, err)
+		return err
 	}
 
 	h.hashedBytes += bytesRead
 	h.hashedFiles++
 
-	// Add to builder with computed hash
-	err = addFileToBuilder(h.builder, e.path, e.size, e.mtime, e.mode, hash)
-	if err != nil {
-		return fmt.Errorf("failed to add %s: %w", e.path, err)
-	}
-
-	return nil
+	// Add to builder with computed hash; a refused path is named in the error
+	return addFileToBuilder(h.builder, e.path, e.size, e.mtime, e.mode, hash)
 }
 
 // writeFreshenedManifest writes the manifest atomically (write to a
@@ -325,7 +314,7 @@ func writeFreshenedManifest(
 
 	outFile, err := afs.Create(tmpPath)
 	if err != nil {
-		return fmt.Errorf("failed to create temp file: %w", err)
+		return err
 	}
 
 	err = builder.Build(ctx, outFile)
@@ -334,7 +323,7 @@ func writeFreshenedManifest(
 	if err != nil {
 		_ = afs.Remove(tmpPath)
 
-		return fmt.Errorf("failed to write manifest: %w", err)
+		return fmt.Errorf("build manifest: %w", err)
 	}
 
 	// Rename temp to final
@@ -342,7 +331,7 @@ func writeFreshenedManifest(
 	if err != nil {
 		_ = afs.Remove(tmpPath)
 
-		return fmt.Errorf("failed to rename manifest: %w", err)
+		return err
 	}
 
 	return nil
@@ -350,7 +339,7 @@ func writeFreshenedManifest(
 
 // newFreshenBuilder constructs the manifest builder configured from CLI
 // flags.
-func newFreshenBuilder(cmd *cli.Command) *mfer.Builder {
+func (mfa *CLIApp) newFreshenBuilder(cmd *cli.Command) (*mfer.Builder, error) {
 	builder := mfer.NewBuilder()
 	if cmd.Bool("include-timestamps") {
 		builder.SetIncludeTimestamps(true)
@@ -358,13 +347,15 @@ func newFreshenBuilder(cmd *cli.Command) *mfer.Builder {
 
 	// Set up signing options if sign-key is provided
 	if signKey := cmd.String("sign-key"); signKey != "" {
-		builder.SetSigningOptions(&mfer.SigningOptions{
-			KeyID: mfer.GPGKeyID(signKey),
-		})
-		log.Infof("signing manifest with GPG key: %s", signKey)
+		signing, err := mfa.signingOptions(signKey)
+		if err != nil {
+			return nil, err
 		}
 
-	return builder
+		builder.SetSigningOptions(signing)
+	}
+
+	return builder, nil
 }
 
 // freshenScan runs the scan phase against the loaded manifest entries
@@ -390,6 +381,14 @@ func (mfa *CLIApp) freshenScan(
 		}
 	}
 
+	// The walk does not follow a symlink at its top, so a base directory
+	// named through one is resolved first. If that fails, the base is
+	// walked as named and the walk reports the problem.
+	resolved, err := filepath.EvalSymlinks(absBase)
+	if err == nil {
+		absBase = resolved
+	}
+
 	scanner := &freshenScanner{
 		fs:                 mfa.Fs,
 		absBase:            absBase,
@@ -401,14 +400,14 @@ func (mfa *CLIApp) freshenScan(
 		existingByPath:     existingByPath,
 	}
 
-	err := afero.Walk(mfa.Fs, absBase, scanner.walk)
+	err = afero.Walk(mfa.Fs, absBase, scanner.walk)
 
 	if showProgress {
 		log.ProgressDone()
 	}
 
 	if err != nil {
-		return nil, 0, fmt.Errorf("failed to scan filesystem: %w", err)
+		return nil, 0, fmt.Errorf("scan filesystem: %w", err)
 	}
 
 	// Remaining entries in existingByPath are removed files
@@ -444,7 +443,7 @@ func hashTotals(entries []*freshenEntry) (int64, int64) {
 }
 
 // runFreshenHash processes every entry through the hasher, aborting if
-// the context is canceled.
+// the context is canceled, and ends the hasher's progress line.
 func runFreshenHash(
 	ctx context.Context, hasher *freshenHasher, entries []*freshenEntry,
 ) error {
@@ -461,6 +460,10 @@ func runFreshenHash(
 		}
 	}
 
+	if hasher.showProgress && hasher.filesToHash > 0 {
+		log.ProgressDone()
+	}
+
 	return nil
 }
 
@@ -477,7 +480,7 @@ func (mfa *CLIApp) loadExistingEntries(
 		Fs:   mfa.Fs,
 	})
 	if err != nil {
-		return nil, fmt.Errorf("failed to load manifest: %w", err)
+		return nil, fmt.Errorf("load manifest: %w", err)
 	}
 
 	existingFiles := manifest.Files()
@@ -497,24 +500,27 @@ func (mfa *CLIApp) freshenManifestOperation(
 ) error {
 	log.Debug("freshenManifestOperation()")
 
-	basePath := cmd.String("base")
 	showProgress := cmd.Bool("progress")
 
 	// Find manifest file
 	manifestPath, err := mfa.resolveFreshenManifestPath(cmd)
 	if err != nil {
-		return fmt.Errorf("freshen: %w", err)
+		return err
+	}
+
+	builder, err := mfa.newFreshenBuilder(cmd)
+	if err != nil {
+		return err
 	}
 
-	//nolint:contextcheck // mfer loads a manifest without a context
 	existingByPath, err := mfa.loadExistingEntries(manifestPath)
 	if err != nil {
 		return err
 	}
 
-	absBase, err := filepath.Abs(basePath)
+	absBase, err := filepath.Abs(resolveBasePath(cmd, manifestPath))
 	if err != nil {
-		return fmt.Errorf("freshen: invalid base path: %w", err)
+		return fmt.Errorf("invalid base path: %w", err)
 	}
 
 	// Phase 1: Scan filesystem
@@ -540,7 +546,7 @@ func (mfa *CLIApp) freshenManifestOperation(
 		totalHashBytes: totalHashBytes,
 		filesToHash:    filesToHash,
 		startHash:      time.Now(),
-		builder:        newFreshenBuilder(cmd),
+		builder:        builder,
 	}
 
 	err = runFreshenHash(ctx, hasher, scanner.entries)
@@ -548,10 +554,6 @@ func (mfa *CLIApp) freshenManifestOperation(
 		return err
 	}
 
-	if showProgress && filesToHash > 0 {
-		log.ProgressDone()
-	}
-
 	// Print summary
 	log.Infof("freshen complete: %d unchanged, %d changed, %d added, %d removed",
 		scanner.unchanged, scanner.changed, scanner.added, removed)
@@ -606,9 +608,7 @@ func hashFile(r io.Reader, progress func(int64)) ([]byte, int64, error) {
 			break
 		}
 
-		// Returned unwrapped: the caller renders this as
-		// "failed to hash <path>: <err>" and adding a second layer here
-		// would change that message.
+		// Returned unwrapped: a read error already names the file.
 		if err != nil {
 			return nil, total, err
 		}

internal/cli/freshen_test.go

@@ -7,6 +7,7 @@ import (
 	"os"
 	"path/filepath"
 	"slices"
+	"strings"
 	"testing"
 	"time"
 
@@ -300,6 +301,84 @@ func TestFreshenLeavesLeftoverTempFileOutOfListing(t *testing.T) {
 		manifestPaths(t, fs, manifestPath))
 }
 
+// TestFreshenResolvesEntriesAgainstManifestDirectory adds sub/c.txt, then
+// runs freshen from the directory above sub, on the manifest in sub.
+// Without --base, the manifest then lists the files in sub, whether freshen
+// is given the manifest or sub. --base names the directory to list instead,
+// the current one included.
+//
+//nolint:paralleltest // changes the process-global working directory
+func TestFreshenResolvesEntriesAgainstManifestDirectory(t *testing.T) {
+	for _, tc := range []struct {
+		args []string
+		want []string // the paths the manifest lists afterwards
+	}{
+		{[]string{testSubdir}, []string{"b.txt", "c.txt"}},
+		{[]string{testSubdirManifest}, []string{"b.txt", "c.txt"}},
+		{
+			[]string{testFlagBase, ".", testSubdirManifest},
+			[]string{testFileTxt, "sub/b.txt", "sub/c.txt"},
+		},
+	} {
+		t.Run(strings.Join(tc.args, " "), func(t *testing.T) {
+			fs := afero.NewOsFs()
+			root := setupManifestInSubdir(t)
+			writeTestFile(t, fs, filepath.Join(root, testSubdir, "c.txt"), "added")
+
+			opts := testOpts(slices.Concat(
+				[]string{testApp, cmdFreshen, "-q"}, tc.args,
+			), fs)
+			require.Equal(t, 0, runCLI(opts), "stderr: %s", testStderr(t, opts))
+
+			assert.ElementsMatch(t, tc.want, manifestPaths(t, fs,
+				filepath.Join(root, testSubdirManifest)))
+		})
+	}
+}
+
+// TestFreshenDirectoryNamedThroughSymlink adds a file to a tree after gen
+// made its manifest, then freshens it with the tree named through a symlink,
+// given as the argument or as the working directory: the manifest lists the
+// files in the tree, and leaves out a symlink inside it, as freshen does
+// without --follow-symlinks.
+//
+//nolint:paralleltest // changes the process-global working directory
+func TestFreshenDirectoryNamedThroughSymlink(t *testing.T) {
+	// Paths are relative to a temp dir holding link, a symlink to the tree.
+	for name, tc := range map[string]struct {
+		workDir string
+		args    []string
+	}{
+		"argument":          {".", []string{testLink}},
+		"working directory": {testLink, nil},
+	} {
+		t.Run(name, func(t *testing.T) {
+			fs := afero.NewOsFs()
+			tree, manifestPath := setupFreshenDir(t, fs,
+				map[string]string{testFileTxt: "in the tree"})
+			writeTestFile(t, fs, filepath.Join(tree, "later.txt"), "added later")
+			require.NoError(t,
+				os.Symlink(testFileTxt, filepath.Join(tree, "alias.txt")))
+
+			root := t.TempDir()
+			require.NoError(t, os.Symlink(tree, filepath.Join(root, testLink)))
+
+			// t.Chdir sets PWD to the path it is given, as a shell does, and
+			// os.Getwd returns PWD when it names the working directory.
+			t.Chdir(filepath.Join(root, tc.workDir))
+
+			opts := testOpts(slices.Concat(
+				[]string{testApp, cmdFreshen, "-q"}, tc.args,
+			), fs)
+			require.Equal(t, 0, runCLI(opts), "stderr: %s", testStderr(t, opts))
+
+			assertManifestLists(t, fs, tree, manifestPath, map[string]string{
+				testFileTxt: "in the tree", "later.txt": "added later",
+			})
+		})
+	}
+}
+
 // TestFreshenRecordEntryMtimePresence pins the behavior of recordEntry
 // with respect to MFFilePath.Mtime, which is a message pointer with
 // proto3 field presence and may legitimately be absent.

internal/cli/gen.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"io"
 	"os"
 	"os/signal"
 	"path/filepath"
@@ -26,6 +27,8 @@ var (
 	// rendered message stays exactly as mfer has always printed it.
 	errOutputExists = errors.New(
 		"already exists (use --force to overwrite)")
+	// errEmptyOutput indicates --output given with an empty value.
+	errEmptyOutput = errors.New("--output must not be empty")
 )
 
 // reportEnumProgress renders enumeration progress until the channel
@@ -74,7 +77,7 @@ func (mfa *CLIApp) collectInputPaths(args cli.Args) ([]string, error) {
 
 		ap, err := filepath.Abs(inputPath)
 		if err != nil {
-			return nil, fmt.Errorf("generate: invalid path %q: %w", inputPath, err)
+			return nil, fmt.Errorf("invalid path %q: %w", inputPath, err)
 		}
 		// Validate path exists before adding to list
 		if exists, _ := afero.Exists(mfa.Fs, ap); !exists {
@@ -88,9 +91,40 @@ func (mfa *CLIApp) collectInputPaths(args cli.Args) ([]string, error) {
 	return paths, nil
 }
 
-// buildScannerOptions constructs scanner options from the CLI flags.
-func (mfa *CLIApp) buildScannerOptions(cmd *cli.Command) *mfer.ScannerOptions {
+// outputPath returns the file gen writes the manifest to: the one --output
+// names, or else index.mf in the directory the only argument names, or
+// beside the file it names, or else in the current directory. An --output
+// given with an empty value is refused.
+func (mfa *CLIApp) outputPath(cmd *cli.Command) (string, error) {
+	if cmd.IsSet("output") {
 		output := cmd.String("output")
+		if output == "" {
+			return "", errEmptyOutput
+		}
+
+		return output, nil
+	}
+
+	if cmd.Args().Len() != 1 {
+		return defaultManifestName, nil
+	}
+
+	arg := cmd.Args().First()
+
+	// A path that does not exist is refused when it is enumerated.
+	info, err := mfa.Fs.Stat(arg)
+	if err == nil && !info.IsDir() {
+		return filepath.Join(filepath.Dir(arg), defaultManifestName), nil
+	}
+
+	return filepath.Join(arg, defaultManifestName), nil
+}
+
+// buildScannerOptions constructs scanner options from the CLI flags and
+// the path the manifest is written to.
+func (mfa *CLIApp) buildScannerOptions(
+	cmd *cli.Command, output string,
+) (*mfer.ScannerOptions, error) {
 	opts := &mfer.ScannerOptions{
 		IncludeDotfiles:    cmd.Bool("include-dotfiles"),
 		FollowSymLinks:     cmd.Bool("follow-symlinks"),
@@ -111,13 +145,15 @@ func (mfa *CLIApp) buildScannerOptions(cmd *cli.Command) *mfer.ScannerOptions {
 
 	// Set up signing options if sign-key is provided
 	if signKey := cmd.String("sign-key"); signKey != "" {
-		opts.SigningOptions = &mfer.SigningOptions{
-			KeyID: mfer.GPGKeyID(signKey),
-		}
-		log.Infof("signing manifest with GPG key: %s", signKey)
+		signing, err := mfa.signingOptions(signKey)
+		if err != nil {
+			return nil, err
 		}
 
-	return opts
+		opts.SigningOptions = signing
+	}
+
+	return opts, nil
 }
 
 // enumerateInputs runs the enumeration phase over the argument paths,
@@ -129,8 +165,7 @@ func (mfa *CLIApp) enumerateInputs(
 		// Default to current directory
 		err := s.EnumeratePath(".", enumProgress)
 		if err != nil {
-			return fmt.Errorf(
-				"generate: failed to enumerate current directory: %w", err)
+			return fmt.Errorf("enumerate current directory: %w", err)
 		}
 
 		return nil
@@ -144,7 +179,7 @@ func (mfa *CLIApp) enumerateInputs(
 
 	err = s.EnumeratePaths(enumProgress, paths...)
 	if err != nil {
-		return fmt.Errorf("generate: failed to enumerate paths: %w", err)
+		return fmt.Errorf("enumerate files: %w", err)
 	}
 
 	return nil
@@ -203,23 +238,59 @@ func (mfa *CLIApp) runEnumeratePhase(cmd *cli.Command, s *mfer.Scanner) error {
 	return nil
 }
 
+// runScanPhase reads the enumerated files and writes the manifest to out,
+// with optional progress reporting.
+func (mfa *CLIApp) runScanPhase(
+	ctx context.Context, cmd *cli.Command, s *mfer.Scanner, out io.Writer,
+) error {
+	var (
+		scanProgress chan mfer.ScanStatus
+		scanWg       sync.WaitGroup
+	)
+
+	if cmd.Bool("progress") {
+		scanProgress = make(chan mfer.ScanStatus, 1)
+
+		scanWg.Add(1)
+
+		go reportScanProgress(scanProgress, &scanWg)
+	}
+
+	err := s.ToManifest(ctx, out, scanProgress)
+
+	scanWg.Wait()
+
+	if err != nil {
+		return fmt.Errorf("generate manifest: %w", err)
+	}
+
+	return nil
+}
+
 func (mfa *CLIApp) generateManifestOperation(
 	ctx context.Context, cmd *cli.Command,
 ) error {
 	log.Debug("generateManifestOperation()")
 
-	s := mfer.NewScannerWithOptions(mfa.buildScannerOptions(cmd))
-
-	// Phase 1: Enumeration - collect paths and stat files
-	err := mfa.runEnumeratePhase(cmd, s)
+	outputPath, err := mfa.outputPath(cmd)
 	if err != nil {
 		return err
 	}
 
-	showProgress := cmd.Bool("progress")
+	opts, err := mfa.buildScannerOptions(cmd, outputPath)
+	if err != nil {
+		return err
+	}
+
+	s := mfer.NewScannerWithOptions(opts)
+
+	// Phase 1: Enumeration - collect paths and stat files
+	err = mfa.runEnumeratePhase(cmd, s)
+	if err != nil {
+		return err
+	}
 
 	// Check if output file exists
-	outputPath := cmd.String("output")
 	if exists, _ := afero.Exists(mfa.Fs, outputPath); exists && !cmd.Bool("force") {
 		return fmt.Errorf("output file %s %w", outputPath, errOutputExists)
 	}
@@ -229,7 +300,7 @@ func (mfa *CLIApp) generateManifestOperation(
 
 	outFile, err := mfa.Fs.Create(tmpPath)
 	if err != nil {
-		return fmt.Errorf("failed to create temp file: %w", err)
+		return err
 	}
 
 	// Set up signal handler to clean up temp file on Ctrl-C
@@ -250,37 +321,21 @@ func (mfa *CLIApp) generateManifestOperation(
 	}()
 
 	// Phase 2: Scan - read file contents and generate manifest
-	var (
-		scanProgress chan mfer.ScanStatus
-		scanWg       sync.WaitGroup
-	)
-
-	if showProgress {
-		scanProgress = make(chan mfer.ScanStatus, 1)
-
-		scanWg.Add(1)
-
-		go reportScanProgress(scanProgress, &scanWg)
-	}
-
-	err = s.ToManifest(ctx, outFile, scanProgress)
-
-	scanWg.Wait()
-
+	err = mfa.runScanPhase(ctx, cmd, s, outFile)
 	if err != nil {
-		return fmt.Errorf("failed to generate manifest: %w", err)
+		return err
 	}
 
 	// Close file before rename to ensure all data is flushed
 	err = outFile.Close()
 	if err != nil {
-		return fmt.Errorf("failed to close temp file: %w", err)
+		return err
 	}
 
 	// Atomic rename
 	err = mfa.Fs.Rename(tmpPath, outputPath)
 	if err != nil {
-		return fmt.Errorf("failed to rename temp file: %w", err)
+		return err
 	}
 
 	success = true

internal/cli/list.go

@@ -19,20 +19,19 @@ func (mfa *CLIApp) listManifestOperation(ctx context.Context, cmd *cli.Command)
 
 	pathOrURL, err := mfa.resolveManifestArg(cmd)
 	if err != nil {
-		return fmt.Errorf("list: %w", err)
+		return err
 	}
 
 	rc, err := mfa.openManifestReader(ctx, pathOrURL)
 	if err != nil {
-		return fmt.Errorf("list: %w", err)
+		return err
 	}
 
 	defer func() { _ = rc.Close() }()
 
-	//nolint:contextcheck // mfer loads a manifest without a context
 	manifest, err := mfer.NewManifestFromReader(rc)
 	if err != nil {
-		return fmt.Errorf("list: failed to parse manifest: %w", err)
+		return fmt.Errorf("parse manifest: %w", err)
 	}
 
 	files := manifest.Files()

internal/cli/manifest_loader.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"path/filepath"
 	"strings"
 	"time"
 
@@ -15,13 +16,13 @@ import (
 // manifestFetchTimeout bounds HTTP requests made to fetch a manifest.
 const manifestFetchTimeout = 30 * time.Second
 
-// errHTTPStatus indicates an HTTP response with a non-OK status code.
-//
-// Its text is the literal "HTTP" prefix of the rendered "HTTP <code>"
-// message that mfer has always printed, so that wrapping it does not
-// change any user-visible output. Match it with errors.Is; do not read
-// its message.
-var errHTTPStatus = errors.New("HTTP")
+// errHTTPStatus indicates an HTTP response with a non-OK status code. It is
+// followed by the code, as in "unexpected HTTP status 404".
+var errHTTPStatus = errors.New("unexpected HTTP status")
+
+// errManifestTooLarge indicates a manifest download that passed
+// CLIApp.maxManifestSize.
+var errManifestTooLarge = errors.New("file exceeds maximum allowed size")
 
 // isHTTPURL returns true if the string starts with http:// or https://.
 func isHTTPURL(s string) bool {
@@ -36,20 +37,22 @@ func (mfa *CLIApp) openManifestReader(
 	if isHTTPURL(pathOrURL) {
 		client := &http.Client{Timeout: manifestFetchTimeout}
 
+		// The *url.Error that NewRequestWithContext and Do return names
+		// the URL.
 		req, err := http.NewRequestWithContext(ctx, http.MethodGet, pathOrURL, nil)
 		if err != nil {
-			return nil, fmt.Errorf("failed to fetch %s: %w", pathOrURL, err)
+			return nil, fmt.Errorf("download manifest: %w", err)
 		}
 
 		resp, err := client.Do(req)
 		if err != nil {
-			return nil, fmt.Errorf("failed to fetch %s: %w", pathOrURL, err)
+			return nil, fmt.Errorf("download manifest: %w", err)
 		}
 
 		if resp.StatusCode != http.StatusOK {
 			_ = resp.Body.Close()
 
-			return nil, fmt.Errorf("failed to fetch %s: %w %d",
+			return nil, fmt.Errorf("download manifest %s: %w %d",
 				pathOrURL, errHTTPStatus, resp.StatusCode)
 		}
 
@@ -84,3 +87,17 @@ func (mfa *CLIApp) resolveManifestArg(cmd *cli.Command) (string, error) {
 
 	return findManifest(mfa.Fs, ".")
 }
+
+// resolveBasePath returns the directory a manifest's paths are resolved
+// against: the one --base names, or else the directory holding the manifest,
+// or the current directory for a manifest URL.
+func resolveBasePath(cmd *cli.Command, manifestPath string) string {
+	switch {
+	case cmd.IsSet(flagBase):
+		return cmd.String(flagBase)
+	case isHTTPURL(manifestPath):
+		return "."
+	default:
+		return filepath.Dir(manifestPath)
+	}
+}

internal/cli/mfer.go

@@ -24,6 +24,7 @@ const (
 	cmdList     = "list"
 	cmdVersion  = "version"
 
+	flagBase               = "base"
 	flagProgress           = "progress"
 	flagTimeout            = "timeout"
 	flagDest               = "dest"
@@ -59,6 +60,10 @@ type CLIApp struct {
 	exitCode    int
 	app         *cli.Command
 
+	// maxManifestSize is the most of a manifest that fetch, and check
+	// given a URL, download: mfer.MaxManifestSize, which tests lower.
+	maxManifestSize int64
+
 	Stdin  io.Reader // Standard input stream
 	Stdout io.Writer // Standard output stream for normal output
 	Stderr io.Writer // Standard error stream for diagnostics
@@ -164,7 +169,7 @@ func requireSignatureFlag() *cli.StringFlag {
 	return &cli.StringFlag{
 		Name:    flagRequireSignature,
 		Aliases: []string{"S"},
-		Usage:   "Require manifest to be signed by the specified GPG key ID",
+		Usage:   "Require manifest to be signed by the OpenPGP key with this fingerprint",
 		Sources: cli.EnvVars("MFER_REQUIRE_SIGNATURE"),
 	}
 }
@@ -206,9 +211,10 @@ func (mfa *CLIApp) generateCommand() *cli.Command {
 			},
 			&cli.StringFlag{
 				Name:    "output",
-				Value:   defaultManifestName,
 				Aliases: []string{"o"},
-				Usage:   "Specify output filename",
+				Usage: "File to write the manifest to (default: index.mf in " +
+					"the directory given, or beside the file given; with no " +
+					"path or several, index.mf in the current directory)",
 			},
 			&cli.BoolFlag{
 				Name:    "force",
@@ -223,7 +229,7 @@ func (mfa *CLIApp) generateCommand() *cli.Command {
 			&cli.StringFlag{
 				Name:    "sign-key",
 				Aliases: []string{"s"},
-				Usage:   "GPG key ID to sign the manifest with",
+				Usage:   "OpenPGP secret key file to sign the manifest with",
 				Sources: cli.EnvVars("MFER_SIGN_KEY"),
 			},
 			&cli.StringFlag{
@@ -255,10 +261,11 @@ func (mfa *CLIApp) checkCommand() *cli.Command {
 		},
 		Flags: append(commonFlags(),
 			&cli.StringFlag{
-				Name:    "base",
+				Name:    flagBase,
 				Aliases: []string{"b"},
-				Value:   ".",
-				Usage:   "Base directory for resolving relative paths from manifest",
+				Usage: "Base directory for resolving relative paths from manifest " +
+					"(by default the directory holding the manifest, or the " +
+					"current directory for a manifest URL)",
 			},
 			&cli.BoolFlag{
 				Name:    flagProgress,
@@ -288,10 +295,10 @@ func (mfa *CLIApp) freshenCommand() *cli.Command {
 		},
 		Flags: append(commonFlags(),
 			&cli.StringFlag{
-				Name:    "base",
+				Name:    flagBase,
 				Aliases: []string{"b"},
-				Value:   ".",
-				Usage:   "Base directory for resolving relative paths",
+				Usage: "Base directory for resolving relative paths " +
+					"(by default the directory holding the manifest)",
 			},
 			&cli.BoolFlag{
 				Name:    "follow-symlinks",
@@ -312,7 +319,7 @@ func (mfa *CLIApp) freshenCommand() *cli.Command {
 			&cli.StringFlag{
 				Name:    "sign-key",
 				Aliases: []string{"s"},
-				Usage:   "GPG key ID to sign the manifest with",
+				Usage:   "OpenPGP secret key file to sign the manifest with",
 				Sources: cli.EnvVars("MFER_SIGN_KEY"),
 			},
 			&cli.BoolFlag{

internal/cli/signing.go

@@ -0,0 +1,86 @@
+package cli
+
+import (
+	"errors"
+	"fmt"
+	"os"
+
+	"github.com/spf13/afero"
+	"golang.org/x/term"
+	"sneak.berlin/go/mfer/internal/log"
+	"sneak.berlin/go/mfer/mfer"
+)
+
+// envSignKeyPassphrase names the environment variable holding the
+// passphrase of a protected signing key.
+//
+//nolint:gosec // G101: the name of a variable, not a credential
+const envSignKeyPassphrase = "MFER_SIGN_KEY_PASSPHRASE"
+
+// errNoPassphrase indicates a protected signing key whose passphrase is
+// neither in the environment nor can be asked for on a terminal.
+var errNoPassphrase = errors.New(
+	"signing key is protected: set " + envSignKeyPassphrase + " to its passphrase")
+
+// signingOptions returns the signing options for the OpenPGP secret key in
+// the file path, which must be able to sign. The passphrase of a protected
+// key comes from MFER_SIGN_KEY_PASSPHRASE, or else from the terminal on
+// stdin, and must unlock the key.
+func (mfa *CLIApp) signingOptions(path string) (*mfer.SigningOptions, error) {
+	secretKey, err := afero.ReadFile(mfa.Fs, path)
+	if err != nil {
+		return nil, fmt.Errorf("read signing key: %w", err)
+	}
+
+	protected, err := mfer.SecretKeyIsProtected(secretKey)
+	if err != nil {
+		return nil, fmt.Errorf("%s: %w", path, err)
+	}
+
+	log.Infof("signing manifest with the OpenPGP key in %s", path)
+
+	opts := &mfer.SigningOptions{SecretKey: secretKey}
+	if protected {
+		opts.Passphrase, err = mfa.readPassphrase(path)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	// gen and freshen read the signing options before any file, so a key
+	// that cannot sign, or a wrong passphrase, stops them before they hash
+	// anything.
+	err = mfer.CheckSigningKey(opts)
+	if err != nil {
+		return nil, fmt.Errorf("%s: %w", path, err)
+	}
+
+	return opts, nil
+}
+
+// readPassphrase returns MFER_SIGN_KEY_PASSPHRASE when it is set, or else
+// asks for the passphrase of the key in the file path on the terminal on
+// stdin.
+func (mfa *CLIApp) readPassphrase(path string) ([]byte, error) {
+	passphrase := os.Getenv(envSignKeyPassphrase)
+	if passphrase != "" {
+		return []byte(passphrase), nil
+	}
+
+	stdin, ok := mfa.Stdin.(*os.File)
+	if !ok || !term.IsTerminal(int(stdin.Fd())) {
+		return nil, errNoPassphrase
+	}
+
+	_, _ = fmt.Fprintf(mfa.Stderr, "Passphrase for %s: ", path)
+
+	typed, err := term.ReadPassword(int(stdin.Fd()))
+
+	_, _ = fmt.Fprintln(mfa.Stderr)
+
+	if err != nil {
+		return nil, fmt.Errorf("read passphrase: %w", err)
+	}
+
+	return typed, nil
+}

internal/cli/signing_test.go

@@ -0,0 +1,209 @@
+//nolint:testpackage // white-box tests exercise unexported internals
+package cli
+
+import (
+	"bufio"
+	"io"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/ProtonMail/go-crypto/openpgp/packet"
+	"github.com/creack/pty"
+	"github.com/spf13/afero"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+const (
+	testFlagSignKey = "--sign-key"
+	testKeyFile     = "/key.asc"
+)
+
+// TestGenAndFreshenSignWithKeyFile runs gen, then freshen after a file is
+// added, with --sign-key naming a key file: one key with no passphrase and
+// one protected by the passphrase in MFER_SIGN_KEY_PASSPHRASE. check
+// --require-signature must accept each manifest as signed by that key.
+// freshen leaves its manifest out of the listing only on the real
+// filesystem, so the test uses that.
+func TestGenAndFreshenSignWithKeyFile(t *testing.T) {
+	for name, passphrase := range map[string][]byte{
+		"unprotected": nil,
+		"protected":   []byte("passphrase"),
+	} {
+		t.Run(name, func(t *testing.T) {
+			t.Setenv(envSignKeyPassphrase, string(passphrase))
+
+			secretKey, fingerprint := testSecretKey(t, passphrase, nil)
+
+			fs := afero.NewOsFs()
+			keyFile := filepath.Join(t.TempDir(), "key.asc")
+			root := t.TempDir()
+			manifestPath := filepath.Join(root, defaultManifestName)
+
+			require.NoError(t, afero.WriteFile(fs, keyFile, secretKey, 0o600))
+			writeTestFile(t, fs, filepath.Join(root, testFileTxt), "hello")
+
+			opts := testOpts([]string{
+				testApp, cmdGenerate, "-q", testFlagSignKey, keyFile,
+				"-o", manifestPath, root,
+			}, fs)
+			require.Equal(t, 0, runCLI(opts), testStderr(t, opts))
+
+			check := []string{
+				testApp, cmdCheck, "-q",
+				"--" + flagRequireSignature, fingerprint, manifestPath,
+			}
+
+			opts = testOpts(check, fs)
+			require.Equal(t, 0, runCLI(opts), testStderr(t, opts))
+
+			writeTestFile(t, fs, filepath.Join(root, "added.txt"), "added")
+
+			opts = testOpts([]string{
+				testApp, cmdFreshen, "-q", testFlagSignKey, keyFile, manifestPath,
+			}, fs)
+			require.Equal(t, 0, runCLI(opts), testStderr(t, opts))
+
+			opts = testOpts(check, fs)
+			require.Equal(t, 0, runCLI(opts), testStderr(t, opts))
+			assert.Len(t, manifestFiles(t, fs, manifestPath), 2)
+		})
+	}
+}
+
+// TestSignWithProtectedKeyNeedsPassphrase runs gen with a protected key,
+// with MFER_SIGN_KEY_PASSPHRASE empty and no terminal to ask on. gen must
+// fail, naming the variable, and write no manifest.
+func TestSignWithProtectedKeyNeedsPassphrase(t *testing.T) {
+	t.Setenv(envSignKeyPassphrase, "")
+
+	secretKey, _ := testSecretKey(t, []byte("secret"), nil)
+
+	fs := afero.NewMemMapFs()
+	require.NoError(t, afero.WriteFile(fs, testKeyFile, secretKey, 0o600))
+	require.NoError(t, fs.MkdirAll(testDir, 0o755))
+	writeTestFile(t, fs, testFile1, "hello")
+
+	opts := testOpts([]string{
+		testApp, cmdGenerate, "-q", testFlagSignKey, testKeyFile,
+		"-o", testMF, testDir,
+	}, fs)
+	assert.Equal(t, 1, runCLI(opts))
+	assert.Contains(t, testStderr(t, opts),
+		"signing key is protected: set MFER_SIGN_KEY_PASSPHRASE to its passphrase")
+
+	exists, err := afero.Exists(fs, testMF)
+	require.NoError(t, err)
+	assert.False(t, exists)
+}
+
+// TestSignWithKeyThatCannotSignFailsFirst runs gen on a directory and
+// freshen on a manifest, neither of which exists, with keys that cannot
+// sign: a protected key with a wrong MFER_SIGN_KEY_PASSPHRASE, a key that
+// expired in 2020, and a version 6 key. Each run must fail on the key: it
+// checks the key before it reads any file, so a missing file goes
+// unnoticed.
+func TestSignWithKeyThatCannotSignFailsFirst(t *testing.T) {
+	t.Setenv(envSignKeyPassphrase, "wrong")
+
+	wrongPassphrase, _ := testSecretKey(t, []byte("right"), nil)
+
+	made := time.Date(2020, 1, 1, 0, 0, 0, 0, time.UTC)
+	expired, _ := testSecretKey(t, nil, &packet.Config{
+		Algorithm:       packet.PubKeyAlgoEdDSA,
+		Time:            func() time.Time { return made },
+		KeyLifetimeSecs: uint32((24 * time.Hour).Seconds()),
+	})
+
+	version6, _ := testSecretKey(t, nil, &packet.Config{
+		Algorithm: packet.PubKeyAlgoEd25519,
+		V6Keys:    true,
+	})
+
+	for want, secretKey := range map[string][]byte{
+		"unlock signing key":                           wrongPassphrase,
+		"signing key cannot sign":                      expired,
+		"signing key must be an OpenPGP version 4 key": version6,
+	} {
+		fs := afero.NewMemMapFs()
+		require.NoError(t, afero.WriteFile(fs, testKeyFile, secretKey, 0o600))
+
+		for _, args := range [][]string{
+			{
+				testApp, cmdGenerate, "-q", testFlagSignKey, testKeyFile,
+				"-o", testMF, "/missing",
+			},
+			{testApp, cmdFreshen, "-q", testFlagSignKey, testKeyFile, "/missing.mf"},
+		} {
+			opts := testOpts(args, fs)
+			assert.Equal(t, 1, runCLI(opts), args[1], want)
+			assert.Contains(t, testStderr(t, opts), testKeyFile+": "+want, args[1])
+		}
+	}
+}
+
+// TestGenAsksForPassphraseOnTerminal runs gen with a protected key, no
+// MFER_SIGN_KEY_PASSPHRASE, and a terminal as stdin and stderr. gen must
+// ask for the passphrase on stderr, and sign with what is typed after the
+// prompt.
+func TestGenAsksForPassphraseOnTerminal(t *testing.T) {
+	t.Setenv(envSignKeyPassphrase, "")
+
+	secretKey, fingerprint := testSecretKey(t, []byte("passphrase"), nil)
+
+	fs := afero.NewOsFs()
+	keyFile := filepath.Join(t.TempDir(), "key.asc")
+	root := t.TempDir()
+	manifestPath := filepath.Join(root, defaultManifestName)
+
+	require.NoError(t, afero.WriteFile(fs, keyFile, secretKey, 0o600))
+	writeTestFile(t, fs, filepath.Join(root, testFileTxt), "hello")
+
+	terminal, tty, err := pty.Open()
+	require.NoError(t, err)
+
+	t.Cleanup(func() { _ = terminal.Close() })
+
+	opts := testOpts([]string{
+		testApp, cmdGenerate, "-q", testFlagSignKey, keyFile,
+		"-o", manifestPath, root,
+	}, fs)
+	opts.Stdin = tty
+	opts.Stderr = tty
+
+	exitCode := make(chan int, 1)
+
+	go func() {
+		exitCode <- runCLI(opts)
+
+		// Once gen has ended, reading the terminal fails instead of
+		// waiting for a prompt that will not come.
+		_ = tty.Close()
+	}()
+
+	prompt := "Passphrase for " + keyFile + ": "
+	output := bufio.NewReader(terminal)
+	written := ""
+
+	for !strings.HasSuffix(written, prompt) {
+		b, err := output.ReadByte()
+		require.NoError(t, err, "gen wrote %q and no prompt", written)
+
+		written += string(b)
+	}
+
+	_, err = terminal.WriteString("passphrase\n")
+	require.NoError(t, err)
+
+	code := <-exitCode
+	rest, _ := io.ReadAll(output)
+	require.Equal(t, 0, code, "gen wrote %q", rest)
+
+	check := testOpts([]string{
+		testApp, cmdCheck, "-q",
+		"--" + flagRequireSignature, fingerprint, manifestPath,
+	}, fs)
+	require.Equal(t, 0, runCLI(check), testStderr(t, check))
+}

mfer/builder.go

@@ -38,6 +38,7 @@ var (
 	errNegativeSize     = errors.New("size cannot be negative")
 	errHashNotMultihash = errors.New("hash is not a valid multihash")
 	errHashTooShort     = errors.New("hash digest is too short")
+	errDuplicatePath    = errors.New("duplicate path")
 )
 
 // ValidatePath checks that a file path conforms to manifest path invariants:
@@ -115,6 +116,7 @@ type FileHashProgress struct {
 type Builder struct {
 	mu                sync.Mutex
 	files             []*MFFilePath
+	paths             map[string]bool // the path of each entry in files
 	createdAt         time.Time
 	includeTimestamps bool
 	signingOptions    *SigningOptions
@@ -125,6 +127,7 @@ type Builder struct {
 func NewBuilder() *Builder {
 	return &Builder{
 		files:     make([]*MFFilePath, 0),
+		paths:     make(map[string]bool),
 		createdAt: time.Now(),
 	}
 }
@@ -138,6 +141,7 @@ func (b *Builder) SetSeed(seed string) {
 }
 
 // AddFile reads file content from reader, computes hashes, and adds to manifest.
+// A path already added is refused once the file is read.
 // Only mode's permission bits (mode.Perm()) are recorded; 0 records none.
 // Progress updates are sent to the progress channel (if non-nil) without blocking.
 // Returns the number of bytes read.
@@ -204,11 +208,7 @@ func (b *Builder) AddFile(
 		Mode:  uint32(mode.Perm()),
 	}
 
-	b.mu.Lock()
-	b.files = append(b.files, entry)
-	b.mu.Unlock()
-
-	return totalRead, nil
+	return totalRead, b.addEntry(entry)
 }
 
 // sendFileHashProgress sends a progress update without blocking.
@@ -234,8 +234,9 @@ func (b *Builder) FileCount() int {
 // AddFileWithHash adds a file entry with a pre-computed hash.
 // This is useful when the hash is already known (e.g., from an existing manifest).
 // Only mode's permission bits (mode.Perm()) are recorded; 0 records none.
-// Returns an error if path is invalid, size is negative, or hash is not a
-// multihash with a digest of at least 32 bytes, as long as SHA-256's.
+// Returns an error if path is invalid or already added, size is negative,
+// or hash is not a multihash with a digest of at least 32 bytes, as long
+// as SHA-256's.
 func (b *Builder) AddFileWithHash(
 	path RelFilePath,
 	size FileSize,
@@ -245,7 +246,7 @@ func (b *Builder) AddFileWithHash(
 ) error {
 	err := ValidatePath(string(path))
 	if err != nil {
-		return fmt.Errorf("add file: %w", err)
+		return err
 	}
 
 	if size < 0 {
@@ -277,11 +278,7 @@ func (b *Builder) AddFileWithHash(
 		Mode:  uint32(mode.Perm()),
 	}
 
-	b.mu.Lock()
-	b.files = append(b.files, entry)
-	b.mu.Unlock()
-
-	return nil
+	return b.addEntry(entry)
 }
 
 // SetIncludeTimestamps controls whether the manifest includes a createdAt timestamp.
@@ -293,7 +290,7 @@ func (b *Builder) SetIncludeTimestamps(include bool) {
 	b.includeTimestamps = include
 }
 
-// SetSigningOptions sets the GPG signing options for the manifest.
+// SetSigningOptions sets the key the manifest is signed with.
 // If opts is non-nil, the manifest will be signed when Build() is called.
 func (b *Builder) SetSigningOptions(opts *SigningOptions) {
 	b.mu.Lock()
@@ -302,8 +299,8 @@ func (b *Builder) SetSigningOptions(opts *SigningOptions) {
 	b.signingOptions = opts
 }
 
-// Build finalizes the manifest and writes it to the writer. ctx bounds the
-// gpg runs that sign the manifest when signing options are set.
+// Build finalizes the manifest and writes it to the writer. When signing
+// options are set, it does not sign once ctx has ended.
 func (b *Builder) Build(ctx context.Context, w io.Writer) error {
 	b.mu.Lock()
 	defer b.mu.Unlock()
@@ -332,20 +329,33 @@ func (b *Builder) Build(ctx context.Context, w io.Writer) error {
 	// Generate outer wrapper
 	err := m.generateOuter(ctx)
 	if err != nil {
-		return fmt.Errorf("build: generate outer: %w", err)
+		return err
 	}
 
 	// Generate final output
 	err = m.generate(ctx)
 	if err != nil {
-		return fmt.Errorf("build: generate: %w", err)
+		return err
 	}
 
 	// Write to output
 	_, err = w.Write(m.output.Bytes())
-	if err != nil {
-		return fmt.Errorf("build: write output: %w", err)
+
+	return err
+}
+
+// addEntry adds entry to the manifest unless an entry with its path is
+// already there.
+func (b *Builder) addEntry(entry *MFFilePath) error {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
+	if b.paths[entry.GetPath()] {
+		return fmt.Errorf("%w %q", errDuplicatePath, entry.GetPath())
 	}
 
+	b.paths[entry.GetPath()] = true
+	b.files = append(b.files, entry)
+
 	return nil
 }

mfer/builder_test.go

@@ -125,6 +125,32 @@ func TestBuilderAddFileWithHashRejectsBadHashes(t *testing.T) {
 	}
 }
 
+// TestBuilderRefusesPathAlreadyAdded adds a path, then adds it again with
+// AddFile and with AddFileWithHash. Each must refuse it, naming it, and
+// keep the one entry already added.
+func TestBuilderRefusesPathAlreadyAdded(t *testing.T) {
+	t.Parallel()
+
+	hash, err := multihash.Encode(make([]byte, sha256.Size), multihash.SHA2_256)
+	require.NoError(t, err)
+
+	b := NewBuilder()
+	require.NoError(t, b.AddFileWithHash("dir/a.txt", 4, ModTime{}, 0, hash))
+
+	content := []byte("data")
+	_, err = b.AddFile(
+		"dir/a.txt", FileSize(len(content)), ModTime{}, 0, bytes.NewReader(content), nil,
+	)
+	require.ErrorIs(t, err, errDuplicatePath)
+	require.EqualError(t, err, `duplicate path "dir/a.txt"`)
+
+	err = b.AddFileWithHash("dir/a.txt", 4, ModTime{}, 0, hash)
+	require.ErrorIs(t, err, errDuplicatePath)
+	require.EqualError(t, err, `duplicate path "dir/a.txt"`)
+
+	assert.Equal(t, 1, b.FileCount())
+}
+
 func TestBuilderBuild(t *testing.T) {
 	t.Parallel()
 

mfer/constants.go

@@ -8,10 +8,17 @@ const (
 	ReleaseDate = "2025-12-17"
 
 	// MaxDecompressedSize is the maximum allowed size of decompressed manifest
-	// data (256 MB). This prevents decompression bombs from consuming excessive
+	// data (256 MiB). This prevents decompression bombs from consuming excessive
 	// memory.
 	MaxDecompressedSize int64 = 256 * 1024 * 1024
 
+	// MaxManifestSize is the largest manifest file mfer reads (258 MiB).
+	// zstd's worst case grows data it cannot compress by 1/256, so an inner
+	// message of MaxDecompressedSize compresses to at most 257 MiB; the
+	// last MiB is room for the signature, the signing key and the other
+	// outer fields.
+	MaxManifestSize = MaxDecompressedSize + MaxDecompressedSize/256 + 1<<20
+
 	// zstdWindowSize is the zstd window zstd.SpeedBestCompression gives mfer's writer.
 	zstdWindowSize = 8 << 20
 

mfer/deserialize.go

@@ -2,7 +2,6 @@ package mfer
 
 import (
 	"bytes"
-	"context"
 	"crypto/sha256"
 	"errors"
 	"fmt"
@@ -24,11 +23,11 @@ var (
 	errCompressedHashWrong  = errors.New("compressed data hash mismatch")
 	errSignatureNoPubKey    = errors.New("signature present but no public key")
 	errDecompressedTooLarge = errors.New("decompressed data exceeds maximum allowed size")
+	errManifestTooLarge     = errors.New("file exceeds maximum allowed size")
 	errUUIDMismatch         = errors.New("outer and inner UUID mismatch")
 	errInvalidFileFormat    = errors.New("invalid file format")
-	errInvalidManifestPath  = errors.New("manifest contains invalid path")
-	errDecodedTooLarge      = errors.New(
-		"manifest would take too much memory to decode")
+	errInvalidManifestPath  = errors.New("invalid file entry")
+	errDecodedTooLarge      = errors.New("too much memory needed")
 	errSignerNotSigningKey  = errors.New(
 		"signer is not the fingerprint of the key that made the signature")
 )
@@ -57,7 +56,7 @@ func (m *manifest) validateOuterHeader() error {
 	// Validate outer UUID before any decompression
 	err := validateUUID(m.pbOuter.GetUuid())
 	if err != nil {
-		return fmt.Errorf("outer UUID invalid: %w", err)
+		return fmt.Errorf("outer message: %w", err)
 	}
 
 	return nil
@@ -72,7 +71,7 @@ func (m *manifest) verifyOuterIntegrity() error {
 
 	_, err := h.Write(m.pbOuter.GetInnerMessage())
 	if err != nil {
-		return fmt.Errorf("deserialize: hash write: %w", err)
+		return fmt.Errorf("hash inner message: %w", err)
 	}
 
 	sha256Hash := h.Sum(nil)
@@ -90,20 +89,16 @@ func (m *manifest) verifyOuterIntegrity() error {
 
 	sigString, err := m.signatureString()
 	if err != nil {
-		return fmt.Errorf(
-			"failed to generate signature string for verification: %w", err,
-		)
+		return fmt.Errorf("build signature string: %w", err)
 	}
 
-	// Loading a manifest takes no context; gpgTimeout still bounds gpg.
-	signingKey, err := gpgVerify(
-		context.Background(),
+	signingKey, err := verifySignature(
 		[]byte(sigString),
 		m.pbOuter.GetSignature(),
 		m.pbOuter.GetSigningPubKey(),
 	)
 	if err != nil {
-		return fmt.Errorf("signature verification failed: %w", err)
+		return err
 	}
 
 	if !strings.EqualFold(string(m.pbOuter.GetSigner()), signingKey) {
@@ -134,7 +129,7 @@ func (m *manifest) decompressInner() ([]byte, error) {
 		zstd.WithDecodeBuffersBelow(0),
 		zstd.WithDecoderMaxWindow(zstdWindowSize))
 	if err != nil {
-		return nil, fmt.Errorf("deserialize: zstd reader: %w", err)
+		return nil, fmt.Errorf("create decompressor: %w", err)
 	}
 	defer zr.Close()
 
@@ -149,7 +144,7 @@ func (m *manifest) decompressInner() ([]byte, error) {
 
 	dat, err := io.ReadAll(limitedReader)
 	if err != nil {
-		return nil, fmt.Errorf("deserialize: decompress: %w", err)
+		return nil, fmt.Errorf("decompress inner message: %w", err)
 	}
 
 	if int64(len(dat)) >= MaxDecompressedSize {
@@ -265,7 +260,7 @@ func (m *manifest) deserializeInner() error {
 
 	err = checkDecodedSize(dat)
 	if err != nil {
-		return fmt.Errorf("deserialize: unmarshal inner: %w", err)
+		return fmt.Errorf("unmarshal inner message: %w", err)
 	}
 
 	// Deserialize inner message
@@ -274,13 +269,17 @@ func (m *manifest) deserializeInner() error {
 	// Unknown fields would cost memory; mfer never writes a loaded manifest out.
 	err = proto.UnmarshalOptions{DiscardUnknown: true}.Unmarshal(dat, m.pbInner)
 	if err != nil {
-		return fmt.Errorf("deserialize: unmarshal inner: %w", err)
+		return fmt.Errorf("unmarshal inner message: %w", err)
+	}
+
+	if m.pbInner.GetVersion() != MFFile_VERSION_ONE {
+		return errUnknownVersion
 	}
 
 	// Validate inner UUID
 	err = validateUUID(m.pbInner.GetUuid())
 	if err != nil {
-		return fmt.Errorf("inner UUID invalid: %w", err)
+		return fmt.Errorf("inner message: %w", err)
 	}
 
 	// Verify UUIDs match
@@ -293,12 +292,21 @@ func (m *manifest) deserializeInner() error {
 	// extract path tomorrow — acts on a traversal or absolute path from an
 	// untrusted .mf. Reject loudly on the first offender rather than
 	// dropping entries, which would let a hostile manifest hide files from a
-	// check.
+	// check. A path listed twice is refused too: check would check the one
+	// file against both entries.
+	seen := make(map[string]bool, len(m.pbInner.GetFiles()))
+
 	for _, f := range m.pbInner.GetFiles() {
 		err = ValidatePath(f.GetPath())
 		if err != nil {
 			return fmt.Errorf("%w: %w", errInvalidManifestPath, err)
 		}
+
+		if seen[f.GetPath()] {
+			return fmt.Errorf("%w %q", errDuplicatePath, f.GetPath())
+		}
+
+		seen[f.GetPath()] = true
 	}
 
 	log.Infof("loaded manifest with %d files", len(m.pbInner.GetFiles()))
@@ -318,13 +326,14 @@ func validateMagic(dat []byte) bool {
 	return bytes.Equal(got, expected)
 }
 
-// NewManifestFromReader reads a manifest from an io.Reader.
+// NewManifestFromReader reads a manifest from an io.Reader. It refuses a
+// manifest larger than MaxManifestSize, reading at most one byte past it.
 //
 //nolint:revive // unexported-return: exporting manifest is owner question 13
 func NewManifestFromReader(input io.Reader) (*manifest, error) {
 	m := &manifest{}
 
-	dat, err := io.ReadAll(input)
+	dat, err := readAtMost(input, MaxManifestSize)
 	if err != nil {
 		return nil, err
 	}
@@ -356,6 +365,21 @@ func NewManifestFromReader(input io.Reader) (*manifest, error) {
 	return m, nil
 }
 
+// readAtMost reads all of input, or refuses it with errManifestTooLarge
+// once it passes maxSize bytes, after reading one byte past maxSize.
+func readAtMost(input io.Reader, maxSize int64) ([]byte, error) {
+	dat, err := io.ReadAll(io.LimitReader(input, maxSize+1))
+	if err != nil {
+		return nil, err
+	}
+
+	if int64(len(dat)) > maxSize {
+		return nil, fmt.Errorf("%w of %d bytes", errManifestTooLarge, maxSize)
+	}
+
+	return dat, nil
+}
+
 // ManifestFromFileOptions configures NewManifestFromFile.
 type ManifestFromFileOptions struct {
 	// Path is the manifest file to read (required).

mfer/deserialize_fuzz_test.go

@@ -19,14 +19,6 @@ import (
 // input and of the decompressed data it may read, plus room for the
 // decoder's window buffers. A panic or a hang fails the test on its own.
 func FuzzNewManifestFromReader(f *testing.F) {
-	// A signed manifest makes the parser write the key and signature to a
-	// temporary directory and run gpg on them. With gpg off the PATH and
-	// temporary files kept in the test's own directory, no process is
-	// started and nothing is written elsewhere; such input ends in an
-	// error instead.
-	f.Setenv("PATH", "")
-	f.Setenv("TMPDIR", f.TempDir())
-
 	f.Fuzz(func(t *testing.T, data []byte) {
 		var before, after runtime.MemStats
 

mfer/deserialize_path_test.go

@@ -7,7 +7,6 @@ import (
 	"crypto/sha256"
 	"fmt"
 	"strconv"
-	"strings"
 	"testing"
 	"time"
 	"uuid"
@@ -118,12 +117,61 @@ func TestDeserializeRejectsInvalidEntryPaths(t *testing.T) {
 	}
 }
 
+// A manifest that lists a path twice is refused as it is loaded, naming the
+// path. Paths are compared byte for byte: two that differ only in letter case
+// load, and fetch refuses those itself. Each entry has a hash, as entries mfer
+// writes do; entries of a path alone would take too much memory to decode.
+func TestDeserializeRefusesPathListedTwice(t *testing.T) {
+	t.Parallel()
+
+	hash, err := multihash.Encode(make([]byte, sha256.Size), multihash.SHA2_256)
+	require.NoError(t, err)
+
+	tests := []struct {
+		name    string
+		paths   []string
+		refused bool
+	}{
+		{"same path twice", []string{"dir/a.txt", "other.txt", "dir/a.txt"}, true},
+		{"paths differing in letter case", []string{"dir/b.txt", "dir/B.txt"}, false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			id := uuid.NewV4()
+			inner := &MFFile{Version: MFFile_VERSION_ONE, Uuid: id[:]}
+
+			for _, p := range tt.paths {
+				inner.Files = append(inner.Files, &MFFilePath{
+					Path:   p,
+					Hashes: []*MFFileChecksum{{MultiHash: hash}},
+				})
+			}
+
+			innerData, err := proto.Marshal(inner)
+			require.NoError(t, err)
+
+			m, err := NewManifestFromReader(bytes.NewReader(wrapInner(t, id, innerData)))
+			if tt.refused {
+				require.ErrorIs(t, err, errDuplicatePath)
+				require.EqualError(t, err, `duplicate path "dir/a.txt"`)
+			} else {
+				require.NoError(t, err)
+				assert.Len(t, m.Files(), len(tt.paths))
+			}
+		})
+	}
+}
+
 // Entries of a path, an empty hash, an empty MIME type and empty modification
 // and change times are counted at 432 bytes each (176 + 112 + 16 + 64 + 64)
 // and take 16 bytes plus the path to encode. A 37-character path makes that
 // 53 bytes, about 8.2 times: refused, and leaving any one of the five
 // uncounted, even the MIME type, brings it under 8. A 39-character path makes
-// it 55 bytes, about 7.9 times: loaded.
+// it 55 bytes, about 7.9 times: loaded. Each entry's path is its number,
+// padded with zeros to that length, since a manifest lists a path only once.
 func TestDeserializeRefusesEntriesThatDecodeTooLarge(t *testing.T) {
 	t.Parallel()
 
@@ -139,8 +187,15 @@ func TestDeserializeRefusesEntriesThatDecodeTooLarge(t *testing.T) {
 		t.Run(strconv.Itoa(tt.pathLen), func(t *testing.T) {
 			t.Parallel()
 
+			id := uuid.NewV4()
+			inner := protowire.AppendTag(nil, 100, protowire.VarintType) // MFFile.version
+			inner = protowire.AppendVarint(inner, uint64(MFFile_VERSION_ONE))
+			inner = protowire.AppendTag(inner, 102, protowire.BytesType) // MFFile.uuid
+			inner = protowire.AppendBytes(inner, id[:])
+
+			for i := range 1000 {
 				entry := protowire.AppendTag(nil, 1, protowire.BytesType) // MFFilePath.path
-			entry = protowire.AppendString(entry, strings.Repeat("a", tt.pathLen))
+				entry = protowire.AppendString(entry, fmt.Sprintf("%0*d", tt.pathLen, i))
 				entry = protowire.AppendTag(entry, 3, protowire.BytesType) // MFFilePath.hashes
 				entry = protowire.AppendBytes(entry, nil)
 				entry = protowire.AppendTag(entry, 301, protowire.BytesType) // MFFilePath.mimeType
@@ -150,11 +205,6 @@ func TestDeserializeRefusesEntriesThatDecodeTooLarge(t *testing.T) {
 				entry = protowire.AppendTag(entry, 303, protowire.BytesType) // MFFilePath.ctime
 				entry = protowire.AppendBytes(entry, nil)
 
-			id := uuid.NewV4()
-			inner := protowire.AppendTag(nil, 102, protowire.BytesType) // MFFile.uuid
-			inner = protowire.AppendBytes(inner, id[:])
-
-			for range 1000 {
 				inner = protowire.AppendTag(inner, 101, protowire.BytesType) // MFFile.files
 				inner = protowire.AppendBytes(inner, entry)
 			}
@@ -182,7 +232,9 @@ func TestDeserializeDropsUnknownFields(t *testing.T) {
 	entry = append(entry, unknown...)
 
 	id := uuid.NewV4()
-	inner := protowire.AppendTag(nil, 101, protowire.BytesType) // MFFile.files
+	inner := protowire.AppendTag(nil, 100, protowire.VarintType) // MFFile.version
+	inner = protowire.AppendVarint(inner, uint64(MFFile_VERSION_ONE))
+	inner = protowire.AppendTag(inner, 101, protowire.BytesType) // MFFile.files
 	inner = protowire.AppendBytes(inner, entry)
 	inner = protowire.AppendTag(inner, 102, protowire.BytesType) // MFFile.uuid
 	inner = protowire.AppendBytes(inner, id[:])

mfer/deserialize_test.go

@@ -0,0 +1,54 @@
+//nolint:testpackage // white-box tests exercise unexported internals
+package mfer
+
+import (
+	"bytes"
+	"testing"
+	"uuid"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"google.golang.org/protobuf/proto"
+)
+
+// An inner message whose version is not VERSION_ONE, whether version 0 or a
+// later one, is refused with the same error as an outer message's.
+func TestDeserializeRefusesUnknownInnerVersion(t *testing.T) {
+	t.Parallel()
+
+	for _, version := range []MFFile_Version{MFFile_VERSION_NONE, MFFile_VERSION_ONE + 1} {
+		t.Run(version.String(), func(t *testing.T) {
+			t.Parallel()
+
+			id := uuid.NewV4()
+			inner, err := proto.Marshal(&MFFile{Version: version, Uuid: id[:]})
+			require.NoError(t, err)
+
+			_, err = NewManifestFromReader(bytes.NewReader(wrapInner(t, id, inner)))
+			require.ErrorIs(t, err, errUnknownVersion)
+		})
+	}
+}
+
+// TestReadAtMost gives readAtMost exactly its maximum, which it must
+// return whole, and twice its maximum, which it must refuse after reading
+// one byte past the maximum, and no more. NewManifestFromReader reads
+// through it with MaxManifestSize; the test uses 64 KiB, since reading
+// MaxManifestSize under the race detector takes gigabytes of memory.
+func TestReadAtMost(t *testing.T) {
+	t.Parallel()
+
+	const maxSize = 64 << 10
+
+	dat, err := readAtMost(bytes.NewReader(make([]byte, maxSize)), maxSize)
+	require.NoError(t, err)
+	assert.Len(t, dat, maxSize)
+
+	input := bytes.NewReader(make([]byte, 2*maxSize))
+
+	_, err = readAtMost(input, maxSize)
+	require.ErrorIs(t, err, errManifestTooLarge)
+	require.EqualError(t, err,
+		"file exceeds maximum allowed size of 65536 bytes")
+	assert.Equal(t, maxSize-1, input.Len(), "bytes left unread")
+}

mfer/errmsg_test.go

@@ -74,14 +74,18 @@ func TestValidatePathMessagesVerbatim(t *testing.T) {
 	}
 }
 
-// TestSerializeInternalErrorMessagesVerbatim pins the two distinct
-// "internal error" messages, which differ between generate and
-// generateOuter and have always done so.
-func TestSerializeInternalErrorMessagesVerbatim(t *testing.T) {
+// TestSerializeInnerNotSetMessagesVerbatim pins the messages generate and
+// generateOuter return when the inner message is missing.
+func TestSerializeInnerNotSetMessagesVerbatim(t *testing.T) {
 	t.Parallel()
 
 	m := &manifest{}
-	require.EqualError(t, m.generate(context.Background()),
-		"internal error: pbInner not set")
-	require.EqualError(t, m.generateOuter(context.Background()), "internal error")
+
+	err := m.generate(context.Background())
+	require.ErrorIs(t, err, errInnerNotSet)
+	require.EqualError(t, err, "inner message not set")
+
+	err = m.generateOuter(context.Background())
+	require.ErrorIs(t, err, errInternal)
+	require.EqualError(t, err, "inner message not set")
 }

mfer/gpg.go

@@ -1,336 +0,0 @@
-package mfer
-
-import (
-	"bytes"
-	"context"
-	"errors"
-	"fmt"
-	"io"
-	"os"
-	"os/exec"
-	"path/filepath"
-	"strings"
-	"time"
-)
-
-const (
-	// gpgTimeout bounds every gpg run, which can otherwise wait forever on
-	// a passphrase prompt or a stalled gpg-agent. A minute leaves a person
-	// time to type a passphrase or touch a smartcard.
-	gpgTimeout = time.Minute
-
-	// gpgWaitDelay is how long a gpg run keeps waiting for gpg's stdout
-	// and stderr to close once gpg has been killed or has exited. Reading
-	// what gpg itself wrote takes far less; only a process gpg left behind
-	// holds them open longer.
-	gpgWaitDelay = time.Second
-
-	// privateDirPerms is the permission mode for temporary GPG home
-	// directories.
-	privateDirPerms os.FileMode = 0o700
-
-	// privateFilePerms is the permission mode for temporary key,
-	// signature, and data files.
-	privateFilePerms os.FileMode = 0o600
-
-	// gpgFingerprintField is the record type tag for fingerprint lines
-	// in gpg --with-colons output.
-	gpgFingerprintField = "fpr"
-
-	// gpgFingerprintMinFields is the minimum number of colon-separated
-	// fields in a gpg fingerprint record (the fingerprint is field 10).
-	gpgFingerprintMinFields = 10
-
-	// gpgPrimaryKeyRecord starts the record of each primary public key in
-	// gpg --with-colons output.
-	gpgPrimaryKeyRecord = "pub:"
-
-	// gpgValidSigStatus starts the status line gpg --verify writes for a
-	// good signature. Its last field is the fingerprint of the primary key
-	// that made the signature.
-	gpgValidSigStatus = "[GNUPG:] VALIDSIG "
-
-	// gpg option names used from more than one call site.
-	gpgOptArmor   = "--armor"
-	gpgOptHomedir = "--homedir"
-	gpgOptVerify  = "--verify"
-)
-
-var (
-	errGPGKeyNotFound      = errors.New("gpg key not found")
-	errFingerprintNotFound = errors.New("fingerprint not found for key")
-	errSigningKeyCount     = errors.New(
-		"embedded public key block must hold exactly one key")
-	errNotOneGoodSignature = errors.New(
-		"gpg did not report exactly one good signature")
-)
-
-// GPGKeyID represents a GPG key identifier (fingerprint or key ID).
-type GPGKeyID string
-
-// SigningOptions contains options for GPG signing.
-type SigningOptions struct {
-	KeyID GPGKeyID
-}
-
-// gpgArgs builds a gpg argument list from opts followed by positional
-// arguments, separated by an explicit "--" end-of-options marker.
-//
-// This matters because key IDs reach gpg as bare positional arguments
-// (from --sign-key / MFER_SIGN_KEY) and gpg would otherwise parse a value
-// beginning with "-" as one of its own options. Callers must route every
-// non-option argument through here.
-func gpgArgs(opts []string, positional ...string) []string {
-	args := make([]string, 0, len(opts)+1+len(positional))
-	args = append(args, opts...)
-	args = append(args, "--")
-	args = append(args, positional...)
-
-	return args
-}
-
-// runGPG runs the gpg binary in batch mode with the given arguments and
-// optional stdin, returning captured stdout and stderr. gpg is killed when
-// ctx ends or gpgTimeout passes, whichever comes first.
-func runGPG(
-	ctx context.Context, stdin io.Reader, args ...string,
-) (*bytes.Buffer, *bytes.Buffer, error) {
-	// exec.CommandContext kills only gpg itself. A gpg-agent that gpg
-	// starts runs detached and holds none of gpg's output, but another
-	// process gpg leaves behind (a wrapper script that runs the real gpg
-	// without exec, for example) can keep gpg's stdout or stderr open, and
-	// Run would wait for it to exit. WaitDelay stops that wait
-	// gpgWaitDelay after the kill; that process is left running.
-	ctx, cancel := context.WithTimeout(ctx, gpgTimeout)
-	defer cancel()
-
-	fullArgs := append([]string{"--batch", "--no-tty"}, args...)
-
-	// G204: the executable name is a compile-time constant. The arguments
-	// are not, so the guarantee that matters is placement: every
-	// caller-supplied value is passed either as the value of a named
-	// option or after the "--" end-of-options marker inserted by gpgArgs,
-	// and therefore cannot be reinterpreted by gpg as an option.
-	cmd := exec.CommandContext( //nolint:gosec // G204: see comment above
-		ctx, "gpg", fullArgs...)
-	cmd.WaitDelay = gpgWaitDelay
-	cmd.Stdin = stdin
-
-	var stdout, stderr bytes.Buffer
-
-	cmd.Stdout = &stdout
-	cmd.Stderr = &stderr
-
-	err := cmd.Run()
-	if err != nil && ctx.Err() != nil {
-		// gpg was killed because ctx ended, which Run reports only as
-		// "signal: killed"; return the reason instead.
-		err = ctx.Err()
-		if errors.Is(err, context.DeadlineExceeded) {
-			err = fmt.Errorf("gpg timed out: %w", err)
-		}
-	}
-
-	return &stdout, &stderr, err
-}
-
-// parseFingerprint extracts the first fingerprint from gpg --with-colons
-// output, or returns ok=false if none is present.
-func parseFingerprint(colonOutput string) (string, bool) {
-	for line := range strings.SplitSeq(colonOutput, "\n") {
-		fields := strings.Split(line, ":")
-		if len(fields) >= gpgFingerprintMinFields &&
-			fields[0] == gpgFingerprintField {
-			return fields[9], true
-		}
-	}
-
-	return "", false
-}
-
-// countPrimaryKeys returns the number of primary keys in gpg --with-colons
-// key listing output.
-func countPrimaryKeys(colonOutput string) int {
-	count := 0
-
-	for line := range strings.SplitSeq(colonOutput, "\n") {
-		if strings.HasPrefix(line, gpgPrimaryKeyRecord) {
-			count++
-		}
-	}
-
-	return count
-}
-
-// parseSigningKey returns the fingerprint of the primary key that made a
-// signature, the last field of the VALIDSIG line in gpg --verify status
-// output, or ok=false unless there is exactly one such line.
-func parseSigningKey(statusOutput string) (string, bool) {
-	var fingerprints []string
-
-	for line := range strings.SplitSeq(statusOutput, "\n") {
-		if strings.HasPrefix(line, gpgValidSigStatus) {
-			fields := strings.Fields(line)
-			fingerprints = append(fingerprints, fields[len(fields)-1])
-		}
-	}
-
-	if len(fingerprints) != 1 {
-		return "", false
-	}
-
-	return fingerprints[0], true
-}
-
-// gpgSign creates a detached signature of the data using the specified key.
-// Returns the armored detached signature.
-func gpgSign(ctx context.Context, data []byte, keyID GPGKeyID) ([]byte, error) {
-	stdout, stderr, err := runGPG(ctx, bytes.NewReader(data),
-		"--detach-sign",
-		gpgOptArmor,
-		"--local-user", string(keyID),
-	)
-	if err != nil {
-		return nil, fmt.Errorf("gpg sign failed: %w: %s", err, stderr.String())
-	}
-
-	return stdout.Bytes(), nil
-}
-
-// gpgExportPublicKey exports the public key for the specified key ID.
-// Returns the armored public key.
-func gpgExportPublicKey(ctx context.Context, keyID GPGKeyID) ([]byte, error) {
-	stdout, stderr, err := runGPG(ctx, nil,
-		gpgArgs([]string{"--export", gpgOptArmor}, string(keyID))...,
-	)
-	if err != nil {
-		return nil, fmt.Errorf("gpg export failed: %w: %s", err, stderr.String())
-	}
-
-	if stdout.Len() == 0 {
-		return nil, fmt.Errorf("%w: %s", errGPGKeyNotFound, keyID)
-	}
-
-	return stdout.Bytes(), nil
-}
-
-// gpgGetKeyFingerprint gets the full fingerprint for a key ID.
-func gpgGetKeyFingerprint(ctx context.Context, keyID GPGKeyID) ([]byte, error) {
-	stdout, stderr, err := runGPG(ctx, nil,
-		gpgArgs([]string{"--with-colons", "--fingerprint"}, string(keyID))...,
-	)
-	if err != nil {
-		return nil, fmt.Errorf(
-			"gpg fingerprint lookup failed: %w: %s", err, stderr.String(),
-		)
-	}
-
-	fpr, ok := parseFingerprint(stdout.String())
-	if !ok {
-		return nil, fmt.Errorf("%w: %s", errFingerprintNotFound, keyID)
-	}
-
-	return []byte(fpr), nil
-}
-
-// gpgImportOneKey imports the public key in pubKeyFile into the keyring in
-// gpgHome, which must then hold exactly one primary key.
-func gpgImportOneKey(ctx context.Context, gpgHome, pubKeyFile string) error {
-	// Import the public key into the keyring
-	_, importStderr, err := runGPG(ctx, nil,
-		gpgArgs([]string{gpgOptHomedir, gpgHome, "--import"}, pubKeyFile)...,
-	)
-	if err != nil {
-		return fmt.Errorf(
-			"failed to import public key: %w: %s", err, importStderr.String(),
-		)
-	}
-
-	// List keys to count them
-	listStdout, listStderr, err := runGPG(ctx, nil,
-		gpgOptHomedir, gpgHome,
-		"--with-colons",
-		"--list-keys",
-	)
-	if err != nil {
-		return fmt.Errorf(
-			"failed to list keys: %w: %s", err, listStderr.String(),
-		)
-	}
-
-	keys := countPrimaryKeys(listStdout.String())
-	if keys != 1 {
-		return fmt.Errorf("%w, found %d", errSigningKeyCount, keys)
-	}
-
-	return nil
-}
-
-// gpgVerify verifies a detached signature against data using the provided
-// public key, imported into a temporary keyring, and returns the
-// fingerprint of the primary key that made the signature. The public key
-// must hold exactly one primary key, so that a good signature can come
-// from no other key.
-func gpgVerify(ctx context.Context, data, signature, pubKey []byte) (string, error) {
-	// Create temporary directory for GPG operations
-	tmpDir, err := os.MkdirTemp("", "mfer-gpg-verify-*")
-	if err != nil {
-		return "", fmt.Errorf("failed to create temp dir: %w", err)
-	}
-
-	defer func() { _ = os.RemoveAll(tmpDir) }()
-
-	// Set restrictive permissions
-	err = os.Chmod(tmpDir, privateDirPerms)
-	if err != nil {
-		return "", fmt.Errorf("failed to set temp dir permissions: %w", err)
-	}
-
-	// Write public key to temp file
-	pubKeyFile := filepath.Join(tmpDir, "pubkey.asc")
-
-	err = os.WriteFile(pubKeyFile, pubKey, privateFilePerms)
-	if err != nil {
-		return "", fmt.Errorf("failed to write public key: %w", err)
-	}
-
-	// Write signature to temp file
-	sigFile := filepath.Join(tmpDir, "signature.asc")
-
-	err = os.WriteFile(sigFile, signature, privateFilePerms)
-	if err != nil {
-		return "", fmt.Errorf("failed to write signature: %w", err)
-	}
-
-	// Write data to temp file
-	dataFile := filepath.Join(tmpDir, "data")
-
-	err = os.WriteFile(dataFile, data, privateFilePerms)
-	if err != nil {
-		return "", fmt.Errorf("failed to write data: %w", err)
-	}
-
-	err = gpgImportOneKey(ctx, tmpDir, pubKeyFile)
-	if err != nil {
-		return "", err
-	}
-
-	// --status-fd 1 sends gpg's status lines to stdout, which verifying a
-	// detached signature otherwise leaves empty; its messages go to stderr.
-	verifyStdout, verifyStderr, err := runGPG(ctx, nil,
-		gpgArgs([]string{gpgOptHomedir, tmpDir, "--status-fd", "1", gpgOptVerify},
-			sigFile, dataFile)...,
-	)
-	if err != nil {
-		return "", fmt.Errorf(
-			"signature verification failed: %w: %s", err, verifyStderr.String(),
-		)
-	}
-
-	fingerprint, ok := parseSigningKey(verifyStdout.String())
-	if !ok {
-		return "", errNotOneGoodSignature
-	}
-
-	return fingerprint, nil
-}

mfer/gpg_test.go

@@ -1,588 +0,0 @@
-//nolint:testpackage // white-box tests exercise unexported internals
-package mfer
-
-import (
-	"bytes"
-	"context"
-	"io"
-	"os"
-	"os/exec"
-	"path/filepath"
-	"slices"
-	"strconv"
-	"strings"
-	"syscall"
-	"testing"
-	"time"
-
-	"github.com/spf13/afero"
-	"github.com/stretchr/testify/assert"
-	"github.com/stretchr/testify/require"
-	"google.golang.org/protobuf/proto"
-)
-
-// testGPGEnv sets up a temporary GPG home directory with a test key.
-// Returns the key ID and the GPG home directory; callers must point
-// GNUPGHOME at the returned directory (via t.Setenv) before using the
-// gpg helpers under test.
-func testGPGEnv(t *testing.T) (GPGKeyID, string) {
-	t.Helper()
-
-	// Check if gpg is installed
-	_, err := exec.LookPath("gpg")
-	if err != nil {
-		t.Skip("gpg not installed, skipping signing test")
-	}
-
-	// Create temporary GPG home directory (0700 by default)
-	gpgHome := t.TempDir()
-
-	genTestKey(t, gpgHome)
-
-	ctx, cancel := context.WithTimeout(context.Background(), gpgTimeout)
-	defer cancel()
-
-	// Get the key fingerprint
-	cmd := exec.CommandContext(ctx, "gpg",
-		"--list-keys", "--with-colons", "test@mfer.test")
-
-	cmd.Env = append(os.Environ(), "GNUPGHOME="+gpgHome)
-
-	output, err := cmd.Output()
-	if err != nil {
-		t.Fatalf("failed to list test key: %v", err)
-	}
-
-	// Parse fingerprint from output
-	var keyID string
-
-	for line := range strings.SplitSeq(string(output), "\n") {
-		fields := strings.Split(line, ":")
-		if len(fields) >= gpgFingerprintMinFields &&
-			fields[0] == gpgFingerprintField {
-			keyID = fields[9]
-
-			break
-		}
-	}
-
-	if keyID == "" {
-		t.Fatal("failed to find test key fingerprint")
-	}
-
-	return GPGKeyID(keyID), gpgHome
-}
-
-// genTestKey generates a key with no passphrase for
-// "MFER Test Key <test@mfer.test>" in gpgHome, which may already hold one.
-func genTestKey(t *testing.T, gpgHome string) {
-	t.Helper()
-
-	keyParams := `%no-protection
-Key-Type: RSA
-Key-Length: 2048
-Name-Real: MFER Test Key
-Name-Email: test@mfer.test
-Expire-Date: 0
-%commit
-`
-	paramsFile := filepath.Join(gpgHome, "key-params")
-	require.NoError(t, os.WriteFile(paramsFile, []byte(keyParams), 0o600))
-
-	ctx, cancel := context.WithTimeout(context.Background(), gpgTimeout)
-	defer cancel()
-
-	//nolint:gosec // paramsFile is a test-controlled path inside t.TempDir()
-	cmd := exec.CommandContext(ctx, "gpg",
-		"--batch", "--gen-key", paramsFile)
-
-	cmd.Env = append(os.Environ(), "GNUPGHOME="+gpgHome)
-
-	output, err := cmd.CombinedOutput()
-	if err != nil {
-		t.Skipf("failed to generate test GPG key: %v: %s", err, output)
-	}
-}
-
-// signedTestManifest returns a manifest of one file signed with keyID.
-func signedTestManifest(t *testing.T, keyID GPGKeyID) []byte {
-	t.Helper()
-
-	b := NewBuilder()
-	b.SetSigningOptions(&SigningOptions{KeyID: keyID})
-
-	content := []byte("signed file content")
-	_, err := b.AddFile("test.txt", FileSize(len(content)), ModTime{}, 0,
-		bytes.NewReader(content), nil)
-	require.NoError(t, err)
-
-	var buf bytes.Buffer
-
-	require.NoError(t, b.Build(context.Background(), &buf))
-
-	return buf.Bytes()
-}
-
-// rewriteOuter returns manifest with its outer message changed by edit.
-// A signature stays good as long as edit leaves the UUID and hash alone.
-func rewriteOuter(t *testing.T, manifest []byte, edit func(*MFFileOuter)) []byte {
-	t.Helper()
-
-	outer := new(MFFileOuter)
-	require.NoError(t, proto.Unmarshal(manifest[len(MAGIC):], outer))
-
-	edit(outer)
-
-	data, err := proto.Marshal(outer)
-	require.NoError(t, err)
-
-	return append([]byte(MAGIC), data...)
-}
-
-func TestGPGSign(t *testing.T) {
-	keyID, gpgHome := testGPGEnv(t)
-	t.Setenv("GNUPGHOME", gpgHome)
-
-	data := []byte("test data to sign")
-	sig, err := gpgSign(context.Background(), data, keyID)
-	require.NoError(t, err)
-	assert.NotEmpty(t, sig)
-	assert.Contains(t, string(sig), "-----BEGIN PGP SIGNATURE-----")
-	assert.Contains(t, string(sig), "-----END PGP SIGNATURE-----")
-}
-
-func TestGPGExportPublicKey(t *testing.T) {
-	keyID, gpgHome := testGPGEnv(t)
-	t.Setenv("GNUPGHOME", gpgHome)
-
-	pubKey, err := gpgExportPublicKey(context.Background(), keyID)
-	require.NoError(t, err)
-	assert.NotEmpty(t, pubKey)
-	assert.Contains(t, string(pubKey), "-----BEGIN PGP PUBLIC KEY BLOCK-----")
-	assert.Contains(t, string(pubKey), "-----END PGP PUBLIC KEY BLOCK-----")
-}
-
-func TestGPGGetKeyFingerprint(t *testing.T) {
-	keyID, gpgHome := testGPGEnv(t)
-	t.Setenv("GNUPGHOME", gpgHome)
-
-	fingerprint, err := gpgGetKeyFingerprint(context.Background(), keyID)
-	require.NoError(t, err)
-	assert.NotEmpty(t, fingerprint)
-	// The fingerprint should be 40 hex chars
-	assert.Len(t, fingerprint, 40, "fingerprint should be 40 hex chars")
-}
-
-// TestGPGArgsSeparatesPositionals pins that caller-supplied values are
-// placed after an end-of-options marker. Key IDs arrive from --sign-key
-// and MFER_SIGN_KEY as bare positional arguments, so without the marker
-// a value beginning with "-" would be parsed by gpg as one of its own
-// options.
-func TestGPGArgsSeparatesPositionals(t *testing.T) {
-	t.Parallel()
-
-	assert.Equal(t,
-		[]string{"--opt-a", "--opt-b", "--", "--version"},
-		gpgArgs([]string{"--opt-a", "--opt-b"}, "--version"))
-
-	assert.Equal(t,
-		[]string{"--opt-c", "--", "sig", "data"},
-		gpgArgs([]string{"--opt-c"}, "sig", "data"))
-
-	assert.Equal(t, []string{"--opt-d", "--"},
-		gpgArgs([]string{"--opt-d"}))
-}
-
-// TestGPGOptionLikeKeyIDIsNotAnOption drives real gpg with a key ID that
-// looks like an option and asserts it is treated as a (nonexistent) key
-// rather than executed as gpg's own --version.
-func TestGPGOptionLikeKeyIDIsNotAnOption(t *testing.T) {
-	_, gpgHome := testGPGEnv(t)
-	t.Setenv("GNUPGHOME", gpgHome)
-
-	pubKey, err := gpgExportPublicKey(context.Background(), GPGKeyID("--version"))
-	require.Error(t, err)
-	require.ErrorIs(t, err, errGPGKeyNotFound)
-	assert.NotContains(t, string(pubKey), "gpg (GnuPG)")
-
-	fpr, err := gpgGetKeyFingerprint(context.Background(), GPGKeyID("--version"))
-	require.Error(t, err)
-	assert.NotContains(t, string(fpr), "gpg (GnuPG)")
-}
-
-func TestGPGSignInvalidKey(t *testing.T) {
-	// Set up test environment (we need GNUPGHOME set)
-	_, gpgHome := testGPGEnv(t)
-	t.Setenv("GNUPGHOME", gpgHome)
-
-	data := []byte("test data")
-	_, err := gpgSign(context.Background(), data,
-		GPGKeyID("NONEXISTENT_KEY_ID_12345"))
-	assert.Error(t, err)
-}
-
-func TestBuilderWithSigning(t *testing.T) {
-	keyID, gpgHome := testGPGEnv(t)
-	t.Setenv("GNUPGHOME", gpgHome)
-
-	// Create a builder with signing options
-	b := NewBuilder()
-	b.SetSigningOptions(&SigningOptions{
-		KeyID: keyID,
-	})
-
-	// Add a test file
-	content := []byte("test file content")
-	reader := bytes.NewReader(content)
-	_, err := b.AddFile("test.txt", FileSize(len(content)), ModTime{}, 0, reader, nil)
-	require.NoError(t, err)
-
-	// Build the manifest
-	var buf bytes.Buffer
-
-	err = b.Build(context.Background(), &buf)
-	require.NoError(t, err)
-
-	// Parse the manifest and verify signature fields are populated
-	manifest, err := NewManifestFromReader(&buf)
-	require.NoError(t, err)
-	require.NotNil(t, manifest.pbOuter)
-
-	assert.NotEmpty(t, manifest.pbOuter.GetSignature(),
-		"signature should be populated")
-	assert.NotEmpty(t, manifest.pbOuter.GetSigner(), "signer should be populated")
-	assert.NotEmpty(t, manifest.pbOuter.GetSigningPubKey(),
-		"signing public key should be populated")
-
-	// Verify signature is a valid PGP signature
-	assert.Contains(t, string(manifest.pbOuter.GetSignature()),
-		"-----BEGIN PGP SIGNATURE-----")
-
-	// Verify public key is a valid PGP public key block
-	assert.Contains(t, string(manifest.pbOuter.GetSigningPubKey()),
-		"-----BEGIN PGP PUBLIC KEY BLOCK-----")
-}
-
-func TestScannerWithSigning(t *testing.T) {
-	keyID, gpgHome := testGPGEnv(t)
-	t.Setenv("GNUPGHOME", gpgHome)
-
-	// Create in-memory filesystem with test files
-	fs := afero.NewMemMapFs()
-	require.NoError(t, fs.MkdirAll("/testdir", 0o755))
-	require.NoError(t,
-		afero.WriteFile(fs, "/testdir/file1.txt", []byte("content1"), 0o644))
-	require.NoError(t,
-		afero.WriteFile(fs, "/testdir/file2.txt", []byte("content2"), 0o644))
-
-	// Create scanner with signing options
-	opts := &ScannerOptions{
-		Fs: fs,
-		SigningOptions: &SigningOptions{
-			KeyID: keyID,
-		},
-	}
-	s := NewScannerWithOptions(opts)
-
-	// Enumerate files
-	require.NoError(t, s.EnumeratePath("/testdir", nil))
-	assert.Equal(t, FileCount(2), s.FileCount())
-
-	// Generate signed manifest
-	var buf bytes.Buffer
-	require.NoError(t, s.ToManifest(context.Background(), &buf, nil))
-
-	// Parse and verify
-	manifest, err := NewManifestFromReader(&buf)
-	require.NoError(t, err)
-
-	assert.NotEmpty(t, manifest.pbOuter.GetSignature())
-	assert.NotEmpty(t, manifest.pbOuter.GetSigner())
-	assert.NotEmpty(t, manifest.pbOuter.GetSigningPubKey())
-}
-
-func TestGPGVerify(t *testing.T) {
-	keyID, gpgHome := testGPGEnv(t)
-	t.Setenv("GNUPGHOME", gpgHome)
-
-	data := []byte("test data to sign and verify")
-	sig, err := gpgSign(context.Background(), data, keyID)
-	require.NoError(t, err)
-
-	pubKey, err := gpgExportPublicKey(context.Background(), keyID)
-	require.NoError(t, err)
-
-	// Verify the signature; it names the key that made it
-	signingKey, err := gpgVerify(context.Background(), data, sig, pubKey)
-	require.NoError(t, err)
-	assert.Equal(t, string(keyID), signingKey)
-}
-
-func TestGPGVerifyInvalidSignature(t *testing.T) {
-	keyID, gpgHome := testGPGEnv(t)
-	t.Setenv("GNUPGHOME", gpgHome)
-
-	data := []byte("test data to sign")
-	sig, err := gpgSign(context.Background(), data, keyID)
-	require.NoError(t, err)
-
-	pubKey, err := gpgExportPublicKey(context.Background(), keyID)
-	require.NoError(t, err)
-
-	// Try to verify with different data - should fail
-	wrongData := []byte("different data")
-	_, err = gpgVerify(context.Background(), wrongData, sig, pubKey)
-	assert.Error(t, err)
-}
-
-func TestGPGVerifyBadPublicKey(t *testing.T) {
-	keyID, gpgHome := testGPGEnv(t)
-	t.Setenv("GNUPGHOME", gpgHome)
-
-	data := []byte("test data")
-	sig, err := gpgSign(context.Background(), data, keyID)
-	require.NoError(t, err)
-
-	// Try to verify with invalid public key - should fail
-	badPubKey := []byte("not a valid public key")
-	_, err = gpgVerify(context.Background(), data, sig, badPubKey)
-	assert.Error(t, err)
-}
-
-func TestManifestSignatureVerification(t *testing.T) {
-	keyID, gpgHome := testGPGEnv(t)
-	t.Setenv("GNUPGHOME", gpgHome)
-
-	// Create a builder with signing options
-	b := NewBuilder()
-	b.SetSigningOptions(&SigningOptions{
-		KeyID: keyID,
-	})
-
-	// Add a test file
-	content := []byte("test file content for verification")
-	reader := bytes.NewReader(content)
-	_, err := b.AddFile("test.txt", FileSize(len(content)), ModTime{}, 0, reader, nil)
-	require.NoError(t, err)
-
-	// Build the manifest
-	var buf bytes.Buffer
-
-	err = b.Build(context.Background(), &buf)
-	require.NoError(t, err)
-
-	// Parse the manifest - signature should be verified during load
-	manifest, err := NewManifestFromReader(&buf)
-	require.NoError(t, err)
-	require.NotNil(t, manifest)
-
-	// Signature should be present and valid
-	assert.NotEmpty(t, manifest.pbOuter.GetSignature())
-}
-
-func TestManifestTamperedSignatureFails(t *testing.T) {
-	keyID, gpgHome := testGPGEnv(t)
-	t.Setenv("GNUPGHOME", gpgHome)
-
-	// Create a signed manifest
-	b := NewBuilder()
-	b.SetSigningOptions(&SigningOptions{
-		KeyID: keyID,
-	})
-
-	content := []byte("test file content")
-	reader := bytes.NewReader(content)
-	_, err := b.AddFile("test.txt", FileSize(len(content)), ModTime{}, 0, reader, nil)
-	require.NoError(t, err)
-
-	var buf bytes.Buffer
-
-	err = b.Build(context.Background(), &buf)
-	require.NoError(t, err)
-
-	// Tamper with the signature by replacing some bytes
-	data := buf.Bytes()
-	// Find and modify a byte in the signature portion
-	for i := range data {
-		if i > 100 && data[i] == 'A' {
-			data[i] = 'B'
-
-			break
-		}
-	}
-
-	// Try to load the tampered manifest - should fail
-	_, err = NewManifestFromReader(bytes.NewReader(data))
-	assert.Error(t, err)
-}
-
-// TestManifestRefusesSecondEmbeddedKey loads a manifest whose embedded
-// public key block holds another key before the key that signed it.
-// Loading must refuse it, although the signature is good and the signer
-// field names the key that made it.
-func TestManifestRefusesSecondEmbeddedKey(t *testing.T) {
-	otherKey, otherHome := testGPGEnv(t)
-	t.Setenv("GNUPGHOME", otherHome)
-
-	otherPubKey, err := gpgExportPublicKey(context.Background(), otherKey)
-	require.NoError(t, err)
-
-	keyID, gpgHome := testGPGEnv(t)
-	t.Setenv("GNUPGHOME", gpgHome)
-
-	manifest := rewriteOuter(t, signedTestManifest(t, keyID),
-		func(outer *MFFileOuter) {
-			outer.SigningPubKey = slices.Concat(otherPubKey, outer.GetSigningPubKey())
-		})
-
-	_, err = NewManifestFromReader(bytes.NewReader(manifest))
-	require.ErrorIs(t, err, errSigningKeyCount)
-}
-
-// TestManifestRefusesSignerOtherThanSigningKey loads a manifest whose
-// signer field names a key other than the one that made the signature.
-func TestManifestRefusesSignerOtherThanSigningKey(t *testing.T) {
-	keyID, gpgHome := testGPGEnv(t)
-	t.Setenv("GNUPGHOME", gpgHome)
-
-	manifest := rewriteOuter(t, signedTestManifest(t, keyID),
-		func(outer *MFFileOuter) {
-			outer.Signer = []byte(strings.Repeat("A", len(keyID)))
-		})
-
-	_, err := NewManifestFromReader(bytes.NewReader(manifest))
-	require.ErrorIs(t, err, errSignerNotSigningKey)
-}
-
-// TestBuilderSigningKeyIDMatchingTwoKeys signs with a key ID that two keys
-// in the keyring match. The manifest must embed and name only the key that
-// signed it, or loading refuses it.
-func TestBuilderSigningKeyIDMatchingTwoKeys(t *testing.T) {
-	_, gpgHome := testGPGEnv(t)
-	genTestKey(t, gpgHome)
-	t.Setenv("GNUPGHOME", gpgHome)
-
-	manifest := signedTestManifest(t, GPGKeyID("test@mfer.test"))
-
-	_, err := NewManifestFromReader(bytes.NewReader(manifest))
-	require.NoError(t, err)
-}
-
-func TestBuilderWithoutSigning(t *testing.T) {
-	t.Parallel()
-
-	// Create a builder without signing options
-	b := NewBuilder()
-
-	// Add a test file
-	content := []byte("test file content")
-	reader := bytes.NewReader(content)
-	_, err := b.AddFile("test.txt", FileSize(len(content)), ModTime{}, 0, reader, nil)
-	require.NoError(t, err)
-
-	// Build the manifest
-	var buf bytes.Buffer
-
-	err = b.Build(context.Background(), &buf)
-	require.NoError(t, err)
-
-	// Parse the manifest and verify signature fields are empty
-	manifest, err := NewManifestFromReader(&buf)
-	require.NoError(t, err)
-	require.NotNil(t, manifest.pbOuter)
-
-	assert.Empty(t, manifest.pbOuter.GetSignature(),
-		"signature should be empty when not signing")
-	assert.Empty(t, manifest.pbOuter.GetSigner(),
-		"signer should be empty when not signing")
-	assert.Empty(t, manifest.pbOuter.GetSigningPubKey(),
-		"signing public key should be empty when not signing")
-}
-
-// fakeGPGPath writes script as an executable named gpg into a temporary
-// directory and returns a PATH value with that directory first.
-func fakeGPGPath(t *testing.T, script string) string {
-	t.Helper()
-
-	binDir := t.TempDir()
-	//nolint:gosec // G306: the fake gpg has to be executable
-	require.NoError(t, os.WriteFile(filepath.Join(binDir, "gpg"),
-		[]byte(script), 0o700))
-
-	return binDir + string(os.PathListSeparator) + os.Getenv("PATH")
-}
-
-// TestGPGTimeoutKillsGPG puts a fake gpg that never finishes first on
-// PATH and checks that a run past its deadline is killed and reported as
-// a timeout of the named operation, instead of hanging.
-func TestGPGTimeoutKillsGPG(t *testing.T) {
-	t.Setenv("PATH", fakeGPGPath(t, "#!/bin/sh\nexec sleep 10\n"))
-
-	ctx, cancel := context.WithTimeout(context.Background(), 100*time.Millisecond)
-	defer cancel()
-
-	_, err := gpgSign(ctx, []byte("data"), GPGKeyID("any"))
-	require.ErrorIs(t, err, context.DeadlineExceeded)
-	assert.Contains(t, err.Error(), "gpg sign failed: gpg timed out")
-}
-
-// TestGPGCancelWhenChildHoldsOutput uses a fake gpg that runs sleep as a
-// child instead of exec-ing it, the way a wrapper script around the real
-// gpg might. Killing the fake gpg leaves sleep holding its stdout and
-// stderr open; the call must still return once ctx ends instead of waiting
-// for sleep to exit. The fake gpg writes the process ID of sleep to a named
-// pipe; the test ends ctx only after reading it, so sleep is running by
-// then, and kills sleep before returning.
-func TestGPGCancelWhenChildHoldsOutput(t *testing.T) {
-	pidPipe := filepath.Join(t.TempDir(), "sleep.pid")
-	require.NoError(t, syscall.Mkfifo(pidPipe, 0o600))
-	// sleep outlasts the 10 s wait below, so a call that waits for it fails.
-	t.Setenv("PATH", fakeGPGPath(t,
-		"#!/bin/sh\nsleep 60 &\necho $! >'"+pidPipe+"'\nwait\n"))
-
-	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
-
-	signErr := make(chan error, 1)
-
-	go func() {
-		_, err := gpgSign(ctx, []byte("data"), GPGKeyID("any"))
-		signErr <- err
-	}()
-
-	pid, err := os.ReadFile(pidPipe) //nolint:gosec // G304: path inside t.TempDir()
-	require.NoError(t, err)
-
-	n, err := strconv.Atoi(strings.TrimSpace(string(pid)))
-	require.NoError(t, err)
-
-	sleep, err := os.FindProcess(n)
-	require.NoError(t, err)
-	t.Cleanup(func() { require.NoError(t, sleep.Kill()) })
-
-	cancel()
-
-	// The call should return about gpgWaitDelay (one second) after the
-	// cancel. 10 s is far above that and well under the 30 s test timeout,
-	// which would abort the whole package before the cleanup kills sleep.
-	select {
-	case err := <-signErr:
-		require.ErrorIs(t, err, context.Canceled)
-	case <-time.After(10 * time.Second):
-		t.Fatal("the call waited for the child holding gpg's output to exit")
-	}
-}
-
-// TestBuildPassesContextToSigning checks that a caller can cancel the gpg
-// runs that sign a manifest through the context given to Build.
-func TestBuildPassesContextToSigning(t *testing.T) {
-	t.Parallel()
-
-	b := NewBuilder()
-	b.SetSigningOptions(&SigningOptions{KeyID: "any"})
-
-	ctx, cancel := context.WithCancel(context.Background())
-	cancel()
-
-	require.ErrorIs(t, b.Build(ctx, io.Discard), context.Canceled)
-}

mfer/manifest.go

@@ -10,7 +10,7 @@ import (
 )
 
 var (
-	errOuterNotSet  = errors.New("pbOuter not set")
+	errOuterNotSet  = errors.New("outer message not set")
 	errUUIDNotSet   = errors.New("UUID not set")
 	errSHA256NotSet = errors.New("SHA256 hash not set")
 )
@@ -65,7 +65,7 @@ func (m *manifest) signatureString() (string, error) {
 
 	mh, err := multihash.Encode(m.pbOuter.GetSha256(), multihash.SHA2_256)
 	if err != nil {
-		return "", fmt.Errorf("failed to encode multihash: %w", err)
+		return "", fmt.Errorf("encode multihash: %w", err)
 	}
 
 	uuidStr := hex.EncodeToString(m.pbOuter.GetUuid())

mfer/openpgp.go

@@ -0,0 +1,376 @@
+package mfer
+
+import (
+	"bytes"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"io"
+	"slices"
+	"strings"
+
+	"github.com/ProtonMail/go-crypto/openpgp"
+	"github.com/ProtonMail/go-crypto/openpgp/armor"
+	pgperrors "github.com/ProtonMail/go-crypto/openpgp/errors"
+	"github.com/ProtonMail/go-crypto/openpgp/packet"
+)
+
+const (
+	// The tags of OpenPGP signature, key and subkey packets (RFC 9580,
+	// section 5). Subkeys have tags of their own, so each secret or public
+	// key packet is one primary key.
+	signaturePacketTag    = 2
+	secretKeyPacketTag    = 5
+	publicKeyPacketTag    = 6
+	secretSubkeyPacketTag = 7
+	publicSubkeyPacketTag = 14
+
+	// In the body of a key or subkey packet the algorithm octet follows
+	// the version octet and the four-octet creation time, and from version
+	// 5 on a four-octet length as well (RFC 9580, section 5.5.2).
+	keyAlgorithmOffset            = 5
+	firstKeyVersionWithLength     = 5
+	keyAlgorithmOffsetAfterLength = 9
+
+	// signingKeyVersion is the only OpenPGP key version mfer signs with.
+	// Its fingerprints are 40 hex characters, the length
+	// --require-signature takes.
+	signingKeyVersion = 4
+
+	// armorBegin and armorEnd start the lines that begin and end an
+	// armored block.
+	armorBegin = "-----BEGIN "
+	armorEnd   = "-----END "
+)
+
+var (
+	errKeyCount    = errors.New("must hold exactly one key")
+	errDSAKey      = errors.New("must not hold a DSA key")
+	errSecretKey   = errors.New("must not hold a secret key")
+	errNoSecretKey = errors.New("signing key file holds no secret key")
+	errNotV4Key    = errors.New("signing key must be an OpenPGP version 4 key, " +
+		"the only kind whose fingerprint --require-signature takes")
+	errNoPassphrase = errors.New(
+		"signing key is protected and no passphrase was given")
+	errNotOneSignature = errors.New(
+		"signature must hold exactly one signature")
+	errNotOneArmoredBlock = errors.New(
+		"must be exactly one armored block and nothing else")
+	errMalformedArmor = errors.New("armor is malformed")
+)
+
+// SigningOptions holds the key a manifest is signed with.
+type SigningOptions struct {
+	// SecretKey is an OpenPGP secret key, armored or binary, as
+	// gpg --export-secret-keys writes it. It must hold one primary key.
+	SecretKey []byte
+	// Passphrase unlocks SecretKey when it is protected.
+	Passphrase []byte
+}
+
+// SecretKeyIsProtected reports whether the OpenPGP secret key secretKey,
+// armored or binary, needs a passphrase to sign. It fails unless
+// secretKey holds one version 4 primary key with its secret key.
+func SecretKeyIsProtected(secretKey []byte) (bool, error) {
+	key, err := readSecretKey(secretKey)
+	if err != nil {
+		return false, err
+	}
+
+	return isProtected(key), nil
+}
+
+// CheckSigningKey fails unless opts can sign now: opts.SecretKey must hold
+// one version 4 primary key with a secret key that may sign and has not
+// expired or been revoked, and opts.Passphrase must unlock it when it is
+// protected. It lets a caller find a key that cannot sign before it builds
+// a manifest.
+func CheckSigningKey(opts *SigningOptions) error {
+	key, err := readSigningKey(opts)
+	if err != nil {
+		return err
+	}
+
+	// Signing nothing fails wherever signing the manifest would.
+	err = openpgp.DetachSign(io.Discard, key, bytes.NewReader(nil), nil)
+	if err != nil {
+		return fmt.Errorf("signing key cannot sign: %w", err)
+	}
+
+	return nil
+}
+
+// readSigningKey returns the key in opts.SecretKey, unlocked with
+// opts.Passphrase if it is protected.
+func readSigningKey(opts *SigningOptions) (*openpgp.Entity, error) {
+	key, err := readSecretKey(opts.SecretKey)
+	if err != nil {
+		return nil, err
+	}
+
+	if !isProtected(key) {
+		return key, nil
+	}
+
+	if len(opts.Passphrase) == 0 {
+		return nil, errNoPassphrase
+	}
+
+	err = key.DecryptPrivateKeys(opts.Passphrase)
+	if err != nil {
+		return nil, fmt.Errorf("unlock signing key: %w", err)
+	}
+
+	return key, nil
+}
+
+// readSecretKey returns the one key in secretKey, armored or binary,
+// which must be a version 4 key and include its secret key.
+func readSecretKey(secretKey []byte) (*openpgp.Entity, error) {
+	key, err := readOneKey(secretKey, "signing key file", false)
+	if err != nil {
+		return nil, err
+	}
+
+	if key.PrimaryKey.Version != signingKeyVersion {
+		return nil, fmt.Errorf("%w; this key is version %d",
+			errNotV4Key, key.PrimaryKey.Version)
+	}
+
+	if key.PrivateKey == nil {
+		return nil, errNoSecretKey
+	}
+
+	return key, nil
+}
+
+// readOneKey returns the key in data, armored or binary, which must hold
+// exactly one primary key and no DSA key or subkey, and when publicOnly no
+// secret key or subkey either. what names data in errors.
+func readOneKey(data []byte, what string, publicOnly bool) (*openpgp.Entity, error) {
+	packets, err := dearmor(data)
+	if err != nil {
+		return nil, fmt.Errorf("read %s: %w", what, err)
+	}
+
+	keys, err := countPackets(packets, secretKeyPacketTag, publicKeyPacketTag)
+	if err != nil {
+		return nil, fmt.Errorf("read %s: %w", what, err)
+	}
+
+	if keys != 1 {
+		return nil, fmt.Errorf("%s %w, found %d", what, errKeyCount, keys)
+	}
+
+	// openpgp.ReadKeyRing checks the numbers of every secret key it reads,
+	// and an ElGamal secret subkey with a very large prime makes that take
+	// minutes.
+	secretKeys, err := countPackets(packets, secretKeyPacketTag, secretSubkeyPacketTag)
+	if err != nil {
+		return nil, fmt.Errorf("read %s: %w", what, err)
+	}
+
+	if publicOnly && secretKeys != 0 {
+		return nil, fmt.Errorf("%s %w", what, errSecretKey)
+	}
+
+	// openpgp.ReadKeyRing also checks every self-signature, and a DSA key
+	// with very large numbers makes each check take seconds to minutes.
+	dsa, err := holdsDSAKey(packets)
+	if err != nil {
+		return nil, fmt.Errorf("read %s: %w", what, err)
+	}
+
+	if dsa {
+		return nil, fmt.Errorf("%s %w", what, errDSAKey)
+	}
+
+	keyring, err := openpgp.ReadKeyRing(bytes.NewReader(packets))
+	if err != nil {
+		return nil, fmt.Errorf("read %s: %w", what, err)
+	}
+
+	// openpgp.ReadKeyRing also reads a subkey packet at the start as a
+	// primary key.
+	if len(keyring) != 1 {
+		return nil, fmt.Errorf("%s %w, found %d", what, errKeyCount, len(keyring))
+	}
+
+	return keyring[0], nil
+}
+
+// isProtected reports whether any secret key in key needs a passphrase.
+func isProtected(key *openpgp.Entity) bool {
+	if key.PrivateKey.Encrypted {
+		return true
+	}
+
+	for _, subkey := range key.Subkeys {
+		if subkey.PrivateKey != nil && subkey.PrivateKey.Encrypted {
+			return true
+		}
+	}
+
+	return false
+}
+
+// armoredPublicKey returns the public part of key, armored.
+func armoredPublicKey(key *openpgp.Entity) ([]byte, error) {
+	var buf bytes.Buffer
+
+	w, err := armor.Encode(&buf, openpgp.PublicKeyType, nil)
+	if err != nil {
+		return nil, err
+	}
+
+	err = key.Serialize(w)
+	if err != nil {
+		return nil, fmt.Errorf("write public key: %w", err)
+	}
+
+	err = w.Close()
+	if err != nil {
+		return nil, err
+	}
+
+	return buf.Bytes(), nil
+}
+
+// fingerprint returns the fingerprint of key's primary key in upper-case
+// hex, as gpg prints it.
+func fingerprint(key *openpgp.Entity) string {
+	return strings.ToUpper(hex.EncodeToString(key.PrimaryKey.Fingerprint))
+}
+
+// verifySignature checks that signature is one good OpenPGP signature
+// over data, made by the one primary key in pubKey or one of its subkeys,
+// and returns that primary key's fingerprint. signature and pubKey may each
+// be armored or binary.
+func verifySignature(data, signature, pubKey []byte) (string, error) {
+	key, err := readOneKey(pubKey, "embedded public key block", true)
+	if err != nil {
+		return "", err
+	}
+
+	sigData, err := dearmor(signature)
+	if err != nil {
+		return "", fmt.Errorf("read signature: %w", err)
+	}
+
+	sigs, err := countPackets(sigData, signaturePacketTag)
+	if err != nil {
+		return "", fmt.Errorf("read signature: %w", err)
+	}
+
+	if sigs != 1 {
+		return "", fmt.Errorf("%w, found %d", errNotOneSignature, sigs)
+	}
+
+	_, err = openpgp.CheckDetachedSignature(openpgp.EntityList{key},
+		bytes.NewReader(data), bytes.NewReader(sigData), nil)
+	// A manifest outlives its signing key, so a signature by a key that
+	// has expired since is still good.
+	if err != nil && !errors.Is(err, pgperrors.ErrKeyExpired) {
+		return "", fmt.Errorf("verify signature: %w", err)
+	}
+
+	return fingerprint(key), nil
+}
+
+// dearmor returns the binary OpenPGP data in data: data itself when it is
+// not armored, or else the body of its armored block. Armored data must be
+// one block and nothing else: its first line is the only BEGIN line and
+// its last line the only END line, white space around them aside.
+// armor.Decode skips any text before a BEGIN line and reads only the first
+// block, so without this a second key or signature would go unseen.
+func dearmor(data []byte) ([]byte, error) {
+	if !bytes.Contains(data, []byte(armorBegin)) {
+		return data, nil
+	}
+
+	text := bytes.TrimSpace(data)
+	lastLine := text[bytes.LastIndexByte(text, '\n')+1:]
+
+	if !bytes.HasPrefix(text, []byte(armorBegin)) ||
+		bytes.Count(text, []byte(armorBegin)) != 1 ||
+		!bytes.HasPrefix(lastLine, []byte(armorEnd)) ||
+		bytes.Count(text, []byte(armorEnd)) != 1 {
+		return nil, errNotOneArmoredBlock
+	}
+
+	// armor.Decode passes over a block it cannot read, such as one with a
+	// header line that has no colon, and returns io.EOF on finding no
+	// other.
+	block, err := armor.Decode(bytes.NewReader(text))
+	if err != nil {
+		return nil, errMalformedArmor
+	}
+
+	body, err := io.ReadAll(block.Body)
+	if err != nil {
+		return nil, fmt.Errorf("%w: %w", errMalformedArmor, err)
+	}
+
+	return body, nil
+}
+
+// countPackets returns how many packets in the binary OpenPGP data have
+// one of tags. It reads only each packet's header, so it also counts
+// packets that openpgp.ReadKeyRing skips, such as a key with no user ID
+// or of an algorithm it does not know.
+func countPackets(data []byte, tags ...uint8) (int, error) {
+	packets := packet.NewOpaqueReader(bytes.NewReader(data))
+	count := 0
+
+	for {
+		p, err := packets.Next()
+		if errors.Is(err, io.EOF) {
+			return count, nil
+		}
+
+		if err != nil {
+			return 0, err
+		}
+
+		if slices.Contains(tags, p.Tag) {
+			count++
+		}
+	}
+}
+
+// holdsDSAKey reports whether any key or subkey packet in the binary
+// OpenPGP data holds a DSA key. It reads each packet's algorithm octet
+// rather than parsing the packet, since parsing a secret key packet checks
+// its numbers, which for a DSA key with very large numbers is as slow as
+// checking a self-signature.
+func holdsDSAKey(data []byte) (bool, error) {
+	packets := packet.NewOpaqueReader(bytes.NewReader(data))
+
+	for {
+		p, err := packets.Next()
+		if errors.Is(err, io.EOF) {
+			return false, nil
+		}
+
+		if err != nil {
+			return false, err
+		}
+
+		if !slices.Contains([]uint8{
+			secretKeyPacketTag, publicKeyPacketTag,
+			secretSubkeyPacketTag, publicSubkeyPacketTag,
+		}, p.Tag) {
+			continue
+		}
+
+		offset := keyAlgorithmOffset
+		if len(p.Contents) > 0 && p.Contents[0] >= firstKeyVersionWithLength {
+			offset = keyAlgorithmOffsetAfterLength
+		}
+
+		if len(p.Contents) > offset &&
+			packet.PublicKeyAlgorithm(p.Contents[offset]) == packet.PubKeyAlgoDSA {
+			return true, nil
+		}
+	}
+}

mfer/openpgp_test.go

@@ -0,0 +1,803 @@
+//nolint:testpackage // white-box tests exercise unexported internals
+package mfer
+
+import (
+	"bytes"
+	"context"
+	"crypto/dsa" //nolint:staticcheck // SA1019: tests need a DSA key to refuse
+	"io"
+	"math/big"
+	"os"
+	"path/filepath"
+	"slices"
+	"strconv"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/ProtonMail/go-crypto/openpgp"
+	"github.com/ProtonMail/go-crypto/openpgp/armor"
+	"github.com/ProtonMail/go-crypto/openpgp/elgamal"
+	"github.com/ProtonMail/go-crypto/openpgp/packet"
+	"github.com/spf13/afero"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"google.golang.org/protobuf/proto"
+)
+
+// newTestKey returns a new Ed25519 key, which is quick to make, for
+// "MFER Test Key <test@mfer.test>". config may set when it is made and how
+// long it lasts.
+func newTestKey(t *testing.T, config *packet.Config) *openpgp.Entity {
+	t.Helper()
+
+	if config == nil {
+		config = &packet.Config{}
+	}
+
+	config.Algorithm = packet.PubKeyAlgoEdDSA
+
+	key, err := openpgp.NewEntity("MFER Test Key", "", "test@mfer.test", config)
+	require.NoError(t, err)
+
+	return key
+}
+
+// armoredSecretKeys returns keys with their secret keys in one armored
+// block, as gpg --export-secret-keys --armor writes them.
+func armoredSecretKeys(t *testing.T, keys ...*openpgp.Entity) []byte {
+	t.Helper()
+
+	var buf bytes.Buffer
+
+	w, err := armor.Encode(&buf, openpgp.PrivateKeyType, nil)
+	require.NoError(t, err)
+
+	for _, key := range keys {
+		require.NoError(t, key.SerializePrivateWithoutSigning(w, nil))
+	}
+
+	require.NoError(t, w.Close())
+
+	return buf.Bytes()
+}
+
+// armoredPublicKeys returns the public parts of keys in one armored block,
+// as gpg --export --armor writes them.
+func armoredPublicKeys(t *testing.T, keys ...*openpgp.Entity) []byte {
+	t.Helper()
+
+	var buf bytes.Buffer
+
+	w, err := armor.Encode(&buf, openpgp.PublicKeyType, nil)
+	require.NoError(t, err)
+
+	for _, key := range keys {
+		require.NoError(t, key.Serialize(w))
+	}
+
+	require.NoError(t, w.Close())
+
+	return buf.Bytes()
+}
+
+// testSigningOptions returns signing options for a new key with no
+// passphrase.
+func testSigningOptions(t *testing.T) *SigningOptions {
+	t.Helper()
+
+	return &SigningOptions{SecretKey: armoredSecretKeys(t, newTestKey(t, nil))}
+}
+
+// joinArmored returns the armored block first followed by the armored
+// block second on the next line. armor.Encode ends a block without a
+// newline, unlike gpg, and a block only starts at the start of a line.
+func joinArmored(first, second []byte) []byte {
+	return slices.Concat(first, []byte("\n"), second)
+}
+
+// signedTestManifest returns a manifest of one file signed with opts.
+func signedTestManifest(t *testing.T, opts *SigningOptions) []byte {
+	t.Helper()
+
+	b := NewBuilder()
+	b.SetSigningOptions(opts)
+
+	content := []byte("signed file content")
+	_, err := b.AddFile("test.txt", FileSize(len(content)), ModTime{}, 0,
+		bytes.NewReader(content), nil)
+	require.NoError(t, err)
+
+	var buf bytes.Buffer
+
+	require.NoError(t, b.Build(context.Background(), &buf))
+
+	return buf.Bytes()
+}
+
+// rewriteOuter returns manifest with its outer message changed by edit.
+// A signature stays good as long as edit leaves the UUID and hash alone.
+func rewriteOuter(t *testing.T, manifest []byte, edit func(*MFFileOuter)) []byte {
+	t.Helper()
+
+	outer := new(MFFileOuter)
+	require.NoError(t, proto.Unmarshal(manifest[len(MAGIC):], outer))
+
+	edit(outer)
+
+	data, err := proto.Marshal(outer)
+	require.NoError(t, err)
+
+	return append([]byte(MAGIC), data...)
+}
+
+func TestBuilderWithSigning(t *testing.T) {
+	t.Parallel()
+
+	key := newTestKey(t, nil)
+
+	// Create a builder with signing options
+	b := NewBuilder()
+	b.SetSigningOptions(&SigningOptions{SecretKey: armoredSecretKeys(t, key)})
+
+	// Add a test file
+	content := []byte("test file content")
+	reader := bytes.NewReader(content)
+	_, err := b.AddFile("test.txt", FileSize(len(content)), ModTime{}, 0, reader, nil)
+	require.NoError(t, err)
+
+	// Build the manifest
+	var buf bytes.Buffer
+
+	err = b.Build(context.Background(), &buf)
+	require.NoError(t, err)
+
+	// Parse the manifest and verify signature fields are populated
+	manifest, err := NewManifestFromReader(&buf)
+	require.NoError(t, err)
+	require.NotNil(t, manifest.pbOuter)
+
+	assert.NotEmpty(t, manifest.pbOuter.GetSignature(),
+		"signature should be populated")
+	assert.NotEmpty(t, manifest.pbOuter.GetSigningPubKey(),
+		"signing public key should be populated")
+
+	// The signer is the key's fingerprint in 40 upper-case hex characters.
+	assert.Equal(t, fingerprint(key), string(manifest.pbOuter.GetSigner()))
+	assert.Regexp(t, "^[0-9A-F]{40}$", string(manifest.pbOuter.GetSigner()))
+
+	// Verify signature is a valid PGP signature
+	assert.Contains(t, string(manifest.pbOuter.GetSignature()),
+		"-----BEGIN PGP SIGNATURE-----")
+
+	// Verify public key is a valid PGP public key block
+	assert.Contains(t, string(manifest.pbOuter.GetSigningPubKey()),
+		"-----BEGIN PGP PUBLIC KEY BLOCK-----")
+}
+
+func TestScannerWithSigning(t *testing.T) {
+	t.Parallel()
+
+	// Create in-memory filesystem with test files
+	fs := afero.NewMemMapFs()
+	require.NoError(t, fs.MkdirAll("/testdir", 0o755))
+	require.NoError(t,
+		afero.WriteFile(fs, "/testdir/file1.txt", []byte("content1"), 0o644))
+	require.NoError(t,
+		afero.WriteFile(fs, "/testdir/file2.txt", []byte("content2"), 0o644))
+
+	// Create scanner with signing options
+	opts := &ScannerOptions{
+		Fs:             fs,
+		SigningOptions: testSigningOptions(t),
+	}
+	s := NewScannerWithOptions(opts)
+
+	// Enumerate files
+	require.NoError(t, s.EnumeratePath("/testdir", nil))
+	assert.Equal(t, FileCount(2), s.FileCount())
+
+	// Generate signed manifest
+	var buf bytes.Buffer
+	require.NoError(t, s.ToManifest(context.Background(), &buf, nil))
+
+	// Parse and verify
+	manifest, err := NewManifestFromReader(&buf)
+	require.NoError(t, err)
+
+	assert.NotEmpty(t, manifest.pbOuter.GetSignature())
+	assert.NotEmpty(t, manifest.pbOuter.GetSigner())
+	assert.NotEmpty(t, manifest.pbOuter.GetSigningPubKey())
+}
+
+// TestSigningWithBinarySecretKey signs with a secret key that is not
+// armored, as gpg --export-secret-keys writes it without --armor.
+func TestSigningWithBinarySecretKey(t *testing.T) {
+	t.Parallel()
+
+	var secretKey bytes.Buffer
+
+	require.NoError(t, newTestKey(t, nil).SerializePrivateWithoutSigning(&secretKey, nil))
+
+	_, err := NewManifestFromReader(bytes.NewReader(
+		signedTestManifest(t, &SigningOptions{SecretKey: secretKey.Bytes()})))
+	require.NoError(t, err)
+}
+
+// TestSigningWithProtectedKey signs with a key protected by a passphrase:
+// with the passphrase, without one, and with a wrong one.
+func TestSigningWithProtectedKey(t *testing.T) {
+	t.Parallel()
+
+	key := newTestKey(t, nil)
+	require.NoError(t, key.EncryptPrivateKeys([]byte("right"), nil))
+	secretKey := armoredSecretKeys(t, key)
+
+	protected, err := SecretKeyIsProtected(secretKey)
+	require.NoError(t, err)
+	assert.True(t, protected)
+
+	_, err = NewManifestFromReader(bytes.NewReader(signedTestManifest(t,
+		&SigningOptions{SecretKey: secretKey, Passphrase: []byte("right")})))
+	require.NoError(t, err)
+
+	b := NewBuilder()
+	b.SetSigningOptions(&SigningOptions{SecretKey: secretKey})
+	require.ErrorIs(t, b.Build(context.Background(), io.Discard), errNoPassphrase)
+
+	b.SetSigningOptions(&SigningOptions{
+		SecretKey: secretKey, Passphrase: []byte("wrong"),
+	})
+	assert.ErrorContains(t, b.Build(context.Background(), io.Discard),
+		"unlock signing key")
+}
+
+func TestSecretKeyIsProtectedWithoutPassphrase(t *testing.T) {
+	t.Parallel()
+
+	protected, err := SecretKeyIsProtected(testSigningOptions(t).SecretKey)
+	require.NoError(t, err)
+	assert.False(t, protected)
+}
+
+// TestSigningKeyFileWithTwoKeys signs with a key file that holds two keys,
+// as gpg writes it for a user ID that two keys have. It names no one key
+// to sign with, so signing must fail.
+func TestSigningKeyFileWithTwoKeys(t *testing.T) {
+	t.Parallel()
+
+	b := NewBuilder()
+	b.SetSigningOptions(&SigningOptions{SecretKey: armoredSecretKeys(t,
+		newTestKey(t, nil), newTestKey(t, nil))})
+
+	err := b.Build(context.Background(), io.Discard)
+	require.ErrorIs(t, err, errKeyCount)
+	assert.EqualError(t, err, "signing key file must hold exactly one key, found 2")
+}
+
+// TestSigningKeyFileWithoutSecretKey signs with a file that holds only a
+// public key.
+func TestSigningKeyFileWithoutSecretKey(t *testing.T) {
+	t.Parallel()
+
+	b := NewBuilder()
+	b.SetSigningOptions(&SigningOptions{
+		SecretKey: armoredPublicKeys(t, newTestKey(t, nil)),
+	})
+	require.ErrorIs(t, b.Build(context.Background(), io.Discard), errNoSecretKey)
+}
+
+// TestCheckSigningKeyRefusesKeyThatCannotSign checks a key that can sign,
+// and keys that read and need no passphrase but cannot sign now: one that
+// expired in 2020 and one that has been revoked. CheckSigningKey must
+// refuse each of the two, as signing a manifest with it would fail.
+func TestCheckSigningKeyRefusesKeyThatCannotSign(t *testing.T) {
+	t.Parallel()
+
+	require.NoError(t, CheckSigningKey(testSigningOptions(t)))
+
+	made := time.Date(2020, 1, 1, 0, 0, 0, 0, time.UTC)
+	expired := newTestKey(t, &packet.Config{
+		Time:            func() time.Time { return made },
+		KeyLifetimeSecs: uint32((24 * time.Hour).Seconds()),
+	})
+
+	revoked := newTestKey(t, nil)
+	require.NoError(t, revoked.RevokeKey(packet.KeyRetired, "", nil))
+
+	for name, key := range map[string]*openpgp.Entity{
+		"expired": expired,
+		"revoked": revoked,
+	} {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			assert.ErrorContains(t, CheckSigningKey(
+				&SigningOptions{SecretKey: armoredSecretKeys(t, key)}),
+				"signing key cannot sign")
+		})
+	}
+}
+
+// TestSigningRefusesVersion6Key signs with an OpenPGP version 6 key, whose
+// fingerprint is 64 hex characters. mfer signs only with version 4 keys.
+func TestSigningRefusesVersion6Key(t *testing.T) {
+	t.Parallel()
+
+	key, err := openpgp.NewEntity("MFER Test Key", "", "test@mfer.test",
+		&packet.Config{V6Keys: true, Algorithm: packet.PubKeyAlgoEd25519})
+	require.NoError(t, err)
+
+	secretKey := armoredSecretKeys(t, key)
+
+	_, err = SecretKeyIsProtected(secretKey)
+	require.ErrorIs(t, err, errNotV4Key)
+
+	b := NewBuilder()
+	b.SetSigningOptions(&SigningOptions{SecretKey: secretKey})
+	require.ErrorIs(t, b.Build(context.Background(), io.Discard), errNotV4Key)
+}
+
+// TestMalformedArmorIsNamed reads a signing key file, a signature and an
+// embedded public key block whose armor is malformed: a header line with
+// no colon, or no blank line after the BEGIN line. Each must fail saying
+// the armor is malformed.
+func TestMalformedArmorIsNamed(t *testing.T) {
+	t.Parallel()
+
+	opts := testSigningOptions(t)
+	manifest := signedTestManifest(t, opts)
+
+	for name, change := range map[string]func([]byte) []byte{
+		"header line with no colon": func(block []byte) []byte {
+			return bytes.Replace(block,
+				[]byte("-----\n"), []byte("-----\nno colon\n"), 1)
+		},
+		"no blank line after the BEGIN line": func(block []byte) []byte {
+			return bytes.Replace(block, []byte("-----\n\n"), []byte("-----\n"), 1)
+		},
+	} {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			_, err := SecretKeyIsProtected(change(opts.SecretKey))
+			require.ErrorIs(t, err, errMalformedArmor)
+
+			for _, changed := range [][]byte{
+				rewriteOuter(t, manifest, func(outer *MFFileOuter) {
+					outer.Signature = change(outer.GetSignature())
+				}),
+				rewriteOuter(t, manifest, func(outer *MFFileOuter) {
+					outer.SigningPubKey = change(outer.GetSigningPubKey())
+				}),
+			} {
+				_, err = NewManifestFromReader(bytes.NewReader(changed))
+				require.ErrorIs(t, err, errMalformedArmor)
+			}
+		})
+	}
+}
+
+func TestVerifySignature(t *testing.T) {
+	t.Parallel()
+
+	key := newTestKey(t, nil)
+	data := []byte("test data to sign and verify")
+
+	var armored, binary bytes.Buffer
+
+	require.NoError(t, openpgp.ArmoredDetachSign(&armored, key,
+		bytes.NewReader(data), nil))
+	require.NoError(t, openpgp.DetachSign(&binary, key, bytes.NewReader(data), nil))
+
+	pubKey, err := armoredPublicKey(key)
+	require.NoError(t, err)
+
+	var binaryPubKey bytes.Buffer
+
+	require.NoError(t, key.Serialize(&binaryPubKey))
+
+	// Verifying names the key that made the signature, whether the
+	// signature and key are armored or not.
+	signer, err := verifySignature(data, armored.Bytes(), pubKey)
+	require.NoError(t, err)
+	assert.Equal(t, fingerprint(key), signer)
+
+	signer, err = verifySignature(data, binary.Bytes(), binaryPubKey.Bytes())
+	require.NoError(t, err)
+	assert.Equal(t, fingerprint(key), signer)
+
+	// A signature over other data is bad.
+	_, err = verifySignature([]byte("different data"), armored.Bytes(), pubKey)
+	require.Error(t, err)
+
+	// A public key that is not one cannot verify anything.
+	_, err = verifySignature(data, armored.Bytes(), []byte("not a public key"))
+	assert.Error(t, err)
+}
+
+// TestVerifySignatureKeyExpiredSince verifies a signature made in 2020 by
+// a key that expired a day after it was made. The signature is still good.
+func TestVerifySignatureKeyExpiredSince(t *testing.T) {
+	t.Parallel()
+
+	made := time.Date(2020, 1, 1, 0, 0, 0, 0, time.UTC)
+	config := &packet.Config{
+		Time:            func() time.Time { return made },
+		KeyLifetimeSecs: uint32((24 * time.Hour).Seconds()),
+	}
+	key := newTestKey(t, config)
+	data := []byte("signed in 2020")
+
+	var sig bytes.Buffer
+
+	require.NoError(t, openpgp.ArmoredDetachSign(&sig, key, bytes.NewReader(data), config))
+
+	pubKey, err := armoredPublicKey(key)
+	require.NoError(t, err)
+
+	signer, err := verifySignature(data, sig.Bytes(), pubKey)
+	require.NoError(t, err)
+	assert.Equal(t, fingerprint(key), signer)
+}
+
+func TestManifestSignatureVerification(t *testing.T) {
+	t.Parallel()
+
+	// Parse the manifest - signature should be verified during load
+	manifest, err := NewManifestFromReader(bytes.NewReader(
+		signedTestManifest(t, testSigningOptions(t))))
+	require.NoError(t, err)
+	require.NotNil(t, manifest)
+
+	// Signature should be present and valid
+	assert.NotEmpty(t, manifest.pbOuter.GetSignature())
+}
+
+func TestManifestTamperedSignatureFails(t *testing.T) {
+	t.Parallel()
+
+	// Change one character of the signature's base64 body, which starts
+	// after the blank line that ends the armor headers.
+	data := rewriteOuter(t, signedTestManifest(t, testSigningOptions(t)),
+		func(outer *MFFileOuter) {
+			sig := outer.GetSignature()
+			i := bytes.Index(sig, []byte("\n\n")) + len("\n\n") + 20
+
+			sig[i]++
+		})
+
+	// Try to load the tampered manifest - should fail
+	_, err := NewManifestFromReader(bytes.NewReader(data))
+	assert.Error(t, err)
+}
+
+// TestManifestSignedByGPGLoads loads the signed seed of
+// FuzzNewManifestFromReader, a manifest signed with gpg before mfer signed
+// and verified manifests itself.
+func TestManifestSignedByGPGLoads(t *testing.T) {
+	t.Parallel()
+
+	seed, err := os.ReadFile(filepath.Join(
+		"testdata", "fuzz", "FuzzNewManifestFromReader", "signed"))
+	require.NoError(t, err)
+
+	// After its header line the seed holds the manifest as []byte("...").
+	_, quoted, found := strings.Cut(string(seed), "[]byte(")
+	require.True(t, found)
+
+	manifest, err := strconv.Unquote(
+		strings.TrimSuffix(strings.TrimSpace(quoted), ")"))
+	require.NoError(t, err)
+
+	m, err := NewManifestFromReader(strings.NewReader(manifest))
+	require.NoError(t, err)
+	assert.Equal(t, "4F562BFB863FDC6B51B4EE88872A51176CEF23AE",
+		string(m.pbOuter.GetSigner()))
+}
+
+// TestManifestRefusesSecondEmbeddedKey loads manifests whose embedded
+// public key block holds another key besides the key that signed it: as a
+// public key, as a secret key with no user ID, which openpgp.ReadKeyRing
+// skips, and written as a subkey packet at the start of the block, which
+// openpgp.ReadKeyRing reads as a primary key. Loading must refuse each,
+// although the signature is good and the signer field names the key that
+// made it.
+func TestManifestRefusesSecondEmbeddedKey(t *testing.T) {
+	t.Parallel()
+
+	other := newTestKey(t, nil)
+	signer := newTestKey(t, nil)
+	manifest := signedTestManifest(t,
+		&SigningOptions{SecretKey: armoredSecretKeys(t, signer)})
+
+	otherWithoutUserID := newTestKey(t, nil)
+	otherWithoutUserID.Identities = map[string]*openpgp.Identity{}
+
+	otherAsSubkey := newTestKey(t, nil)
+	otherAsSubkey.PrimaryKey.IsSubkey = true
+
+	for name, block := range map[string][]byte{
+		"public key":                 armoredPublicKeys(t, other, signer),
+		"secret key without user ID": armoredSecretKeys(t, signer, otherWithoutUserID),
+		"subkey packet first":        armoredPublicKeys(t, otherAsSubkey, signer),
+	} {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			embedded := rewriteOuter(t, manifest, func(outer *MFFileOuter) {
+				outer.SigningPubKey = block
+			})
+
+			_, err := NewManifestFromReader(bytes.NewReader(embedded))
+			require.ErrorIs(t, err, errKeyCount)
+		})
+	}
+}
+
+// TestManifestRefusesSecondEmbeddedKeyWithoutUserID loads a manifest whose
+// embedded public key block holds, before the key that signed it, another
+// key with no user ID, which openpgp.ReadKeyRing skips. Loading must
+// refuse it: the block holds two keys.
+func TestManifestRefusesSecondEmbeddedKeyWithoutUserID(t *testing.T) {
+	t.Parallel()
+
+	other := newTestKey(t, nil)
+	other.Identities = map[string]*openpgp.Identity{}
+
+	signer := newTestKey(t, nil)
+
+	manifest := rewriteOuter(t, signedTestManifest(t,
+		&SigningOptions{SecretKey: armoredSecretKeys(t, signer)}),
+		func(outer *MFFileOuter) {
+			outer.SigningPubKey = armoredPublicKeys(t, other, signer)
+		})
+
+	_, err := NewManifestFromReader(bytes.NewReader(manifest))
+	require.ErrorIs(t, err, errKeyCount)
+}
+
+// dsaKeyPacket returns a public key packet holding a DSA key, or a public
+// subkey packet when isSubkey. Its numbers are not a working key: loading
+// must refuse the packet before it uses them.
+func dsaKeyPacket(t *testing.T, isSubkey bool) []byte {
+	t.Helper()
+
+	key := packet.NewDSAPublicKey(time.Now(), &dsa.PublicKey{
+		P: big.NewInt(23), Q: big.NewInt(11), G: big.NewInt(4), Y: big.NewInt(8),
+	})
+	key.IsSubkey = isSubkey
+
+	var buf bytes.Buffer
+
+	require.NoError(t, key.Serialize(&buf))
+
+	return buf.Bytes()
+}
+
+// TestManifestRefusesDSAKey loads manifests whose embedded public key
+// block holds a DSA key: alone, or as a subkey after the key that signed
+// the manifest. openpgp.ReadKeyRing checks a key's self-signatures, which
+// for a DSA key with very large numbers takes minutes each. Loading must
+// refuse each block before that.
+func TestManifestRefusesDSAKey(t *testing.T) {
+	t.Parallel()
+
+	signer := newTestKey(t, nil)
+	manifest := signedTestManifest(t,
+		&SigningOptions{SecretKey: armoredSecretKeys(t, signer)})
+
+	var signerKey bytes.Buffer
+
+	require.NoError(t, signer.Serialize(&signerKey))
+
+	for name, block := range map[string][]byte{
+		"DSA key":    dsaKeyPacket(t, false),
+		"DSA subkey": slices.Concat(signerKey.Bytes(), dsaKeyPacket(t, true)),
+	} {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			embedded := rewriteOuter(t, manifest, func(outer *MFFileOuter) {
+				outer.SigningPubKey = block
+			})
+
+			_, err := NewManifestFromReader(bytes.NewReader(embedded))
+			require.ErrorIs(t, err, errDSAKey)
+		})
+	}
+}
+
+// elGamalSecretSubkeyPacket returns a secret subkey packet holding an
+// ElGamal key. Its numbers are not a working key: loading must refuse the
+// packet before it uses them.
+func elGamalSecretSubkeyPacket(t *testing.T) []byte {
+	t.Helper()
+
+	key := packet.NewElGamalPrivateKey(time.Now(), &elgamal.PrivateKey{
+		P: big.NewInt(23), G: big.NewInt(4), Y: big.NewInt(8), X: big.NewInt(3),
+	})
+	key.IsSubkey = true
+
+	var buf bytes.Buffer
+
+	require.NoError(t, key.Serialize(&buf))
+
+	return buf.Bytes()
+}
+
+// TestManifestRefusesSecretKey loads manifests whose embedded public key
+// block holds a secret key: the key that signed the manifest with its
+// secret key, or its public key followed by an ElGamal secret subkey.
+// openpgp.ReadKeyRing checks the numbers of every secret key it reads,
+// which for an ElGamal key with a very large prime takes minutes. Loading
+// must refuse each block before that.
+func TestManifestRefusesSecretKey(t *testing.T) {
+	t.Parallel()
+
+	signer := newTestKey(t, nil)
+	manifest := signedTestManifest(t,
+		&SigningOptions{SecretKey: armoredSecretKeys(t, signer)})
+
+	var signerKey bytes.Buffer
+
+	require.NoError(t, signer.Serialize(&signerKey))
+
+	for _, block := range [][]byte{
+		armoredSecretKeys(t, signer),
+		slices.Concat(signerKey.Bytes(), elGamalSecretSubkeyPacket(t)),
+	} {
+		embedded := rewriteOuter(t, manifest, func(outer *MFFileOuter) {
+			outer.SigningPubKey = block
+		})
+
+		_, err := NewManifestFromReader(bytes.NewReader(embedded))
+		require.ErrorIs(t, err, errSecretKey)
+	}
+}
+
+// TestManifestRefusesTwoSignatures loads a manifest whose signature field
+// holds its good signature twice, not armored. Loading must refuse it.
+func TestManifestRefusesTwoSignatures(t *testing.T) {
+	t.Parallel()
+
+	manifest := rewriteOuter(t, signedTestManifest(t, testSigningOptions(t)),
+		func(outer *MFFileOuter) {
+			sig, err := dearmor(outer.GetSignature())
+			require.NoError(t, err)
+
+			outer.Signature = slices.Concat(sig, sig)
+		})
+
+	_, err := NewManifestFromReader(bytes.NewReader(manifest))
+	require.ErrorIs(t, err, errNotOneSignature)
+}
+
+// TestManifestRefusesFieldNotOneArmoredBlock loads manifests whose
+// signature or embedded public key block holds its good armored block with
+// something else: a second armored block, text after the END line, or many
+// END lines before the block. Decoding the block once for each END line
+// before it would take time and memory that grow with the square of the
+// field's size. Loading must refuse each.
+func TestManifestRefusesFieldNotOneArmoredBlock(t *testing.T) {
+	t.Parallel()
+
+	manifest := signedTestManifest(t, testSigningOptions(t))
+
+	for name, change := range map[string]func([]byte) []byte{
+		"second armored block": func(block []byte) []byte {
+			return joinArmored(block, block)
+		},
+		"text after the END line": func(block []byte) []byte {
+			return slices.Concat(block, []byte("\nmore text\n"))
+		},
+		"END lines before the block": func(block []byte) []byte {
+			return slices.Concat(
+				[]byte(strings.Repeat(armorEnd+"\n", 1000)), block)
+		},
+	} {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			for _, changed := range [][]byte{
+				rewriteOuter(t, manifest, func(outer *MFFileOuter) {
+					outer.Signature = change(outer.GetSignature())
+				}),
+				rewriteOuter(t, manifest, func(outer *MFFileOuter) {
+					outer.SigningPubKey = change(outer.GetSigningPubKey())
+				}),
+			} {
+				_, err := NewManifestFromReader(bytes.NewReader(changed))
+				require.ErrorIs(t, err, errNotOneArmoredBlock)
+			}
+		})
+	}
+}
+
+// TestManifestSignedWithSubkey signs with a key that has a signing subkey,
+// which signs in place of the primary key. The manifest must load, with
+// the primary key's fingerprint as signer.
+func TestManifestSignedWithSubkey(t *testing.T) {
+	t.Parallel()
+
+	key := newTestKey(t, nil)
+	require.NoError(t, key.AddSigningSubkey(
+		&packet.Config{Algorithm: packet.PubKeyAlgoEdDSA}))
+
+	m, err := NewManifestFromReader(bytes.NewReader(signedTestManifest(t,
+		&SigningOptions{SecretKey: armoredSecretKeys(t, key)})))
+	require.NoError(t, err)
+	assert.Equal(t, fingerprint(key), string(m.pbOuter.GetSigner()))
+
+	block, err := armor.Decode(bytes.NewReader(m.pbOuter.GetSignature()))
+	require.NoError(t, err)
+
+	p, err := packet.Read(block.Body)
+	require.NoError(t, err)
+
+	sig, ok := p.(*packet.Signature)
+	require.True(t, ok)
+
+	subkey := key.Subkeys[len(key.Subkeys)-1].PublicKey
+	assert.Equal(t, subkey.KeyId, *sig.IssuerKeyId,
+		"the signing subkey made the signature")
+}
+
+// TestManifestRefusesSignerOtherThanSigningKey loads a manifest whose
+// signer field names a key other than the one that made the signature.
+func TestManifestRefusesSignerOtherThanSigningKey(t *testing.T) {
+	t.Parallel()
+
+	manifest := rewriteOuter(t, signedTestManifest(t, testSigningOptions(t)),
+		func(outer *MFFileOuter) {
+			outer.Signer = []byte(strings.Repeat("A", len(outer.GetSigner())))
+		})
+
+	_, err := NewManifestFromReader(bytes.NewReader(manifest))
+	require.ErrorIs(t, err, errSignerNotSigningKey)
+}
+
+func TestBuilderWithoutSigning(t *testing.T) {
+	t.Parallel()
+
+	// Create a builder without signing options
+	b := NewBuilder()
+
+	// Add a test file
+	content := []byte("test file content")
+	reader := bytes.NewReader(content)
+	_, err := b.AddFile("test.txt", FileSize(len(content)), ModTime{}, 0, reader, nil)
+	require.NoError(t, err)
+
+	// Build the manifest
+	var buf bytes.Buffer
+
+	err = b.Build(context.Background(), &buf)
+	require.NoError(t, err)
+
+	// Parse the manifest and verify signature fields are empty
+	manifest, err := NewManifestFromReader(&buf)
+	require.NoError(t, err)
+	require.NotNil(t, manifest.pbOuter)
+
+	assert.Empty(t, manifest.pbOuter.GetSignature(),
+		"signature should be empty when not signing")
+	assert.Empty(t, manifest.pbOuter.GetSigner(),
+		"signer should be empty when not signing")
+	assert.Empty(t, manifest.pbOuter.GetSigningPubKey(),
+		"signing public key should be empty when not signing")
+}
+
+// TestBuildPassesContextToSigning checks that Build does not sign once the
+// context given to it has ended.
+func TestBuildPassesContextToSigning(t *testing.T) {
+	t.Parallel()
+
+	b := NewBuilder()
+	b.SetSigningOptions(&SigningOptions{SecretKey: []byte("any")})
+
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	require.ErrorIs(t, b.Build(ctx, io.Discard), context.Canceled)
+}

mfer/scanner.go

@@ -2,6 +2,7 @@ package mfer
 
 import (
 	"context"
+	"fmt"
 	"io"
 	"io/fs"
 	"os"
@@ -57,7 +58,7 @@ type ScannerOptions struct {
 	IncludePermissions bool
 	// Fs is the filesystem to use, defaults to OsFs if nil.
 	Fs afero.Fs
-	// SigningOptions holds GPG signing options (nil = no signing).
+	// SigningOptions holds the key to sign with (nil = no signing).
 	SigningOptions *SigningOptions
 	// Seed, if set, derives a deterministic UUID from this seed.
 	Seed string
@@ -79,6 +80,7 @@ type FileEntry struct {
 type Scanner struct {
 	mu         sync.RWMutex
 	files      []*FileEntry
+	paths      map[RelFilePath]AbsFilePath // the file at each path in files
 	totalBytes FileSize                    // cached sum of all file sizes
 	options    *ScannerOptions
 	fs         afero.Fs
@@ -103,6 +105,7 @@ func NewScannerWithOptions(opts *ScannerOptions) *Scanner {
 
 	s := &Scanner{
 		files:   make([]*FileEntry, 0),
+		paths:   make(map[RelFilePath]AbsFilePath),
 		options: opts,
 		fs:      fs,
 	}
@@ -136,28 +139,20 @@ func (s *Scanner) EnumerateFile(filePath string) error {
 	return s.enumerateFileWithInfo(filepath.Base(abs), basePath, info, nil)
 }
 
-// EnumeratePath walks a directory path and adds all files to the scanner.
+// EnumeratePath adds inputPath, a directory or a file, to the scanner as
+// EnumeratePaths adds each of its paths.
 // If progress is non-nil, status updates are sent as files are discovered.
 // The progress channel is closed when the method returns.
 func (s *Scanner) EnumeratePath(
 	inputPath string,
 	progress chan<- EnumerateStatus,
 ) error {
-	if progress != nil {
-		defer close(progress)
-	}
-
-	abs, err := filepath.Abs(inputPath)
-	if err != nil {
-		return err
-	}
-
-	afs := afero.NewReadOnlyFs(afero.NewBasePathFs(s.fs, abs))
-
-	return s.enumerateFS(afs, abs, progress)
+	return s.EnumeratePaths(progress, inputPath)
 }
 
-// EnumeratePaths walks multiple directory paths and adds all files to the scanner.
+// EnumeratePaths adds to the scanner the files under each directory path,
+// listed by their paths under it, and each file path, listed by its name
+// as EnumerateFile lists it.
 // If progress is non-nil, status updates are sent as files are discovered.
 // The progress channel is closed when the method returns.
 func (s *Scanner) EnumeratePaths(
@@ -174,9 +169,27 @@ func (s *Scanner) EnumeratePaths(
 			return err
 		}
 
-		afs := afero.NewReadOnlyFs(afero.NewBasePathFs(s.fs, abs))
+		info, err := s.fs.Stat(abs)
+		if err != nil {
+			return err
+		}
 
+		if info.IsDir() {
+			// The walk does not follow a symlink at its top, so a directory
+			// named through one is resolved first. If that fails, the
+			// directory is walked as named and the walk reports the problem.
+			resolved, evalErr := filepath.EvalSymlinks(abs)
+			if evalErr == nil {
+				abs = resolved
+			}
+
+			afs := afero.NewReadOnlyFs(afero.NewBasePathFs(s.fs, abs))
 			err = s.enumerateFS(afs, abs, progress)
+		} else {
+			err = s.enumerateFileWithInfo(
+				filepath.Base(abs), filepath.Dir(abs), info, progress)
+		}
+
 		if err != nil {
 			return err
 		}
@@ -478,6 +491,18 @@ func (s *Scanner) enumerateFileWithInfo(
 	}
 
 	s.mu.Lock()
+
+	// Each path is relative to the input path it was found under, so files
+	// under two input paths can share one.
+	first, ok := s.paths[entry.Path]
+	if ok {
+		s.mu.Unlock()
+
+		return fmt.Errorf("%w %q: %s and %s",
+			errDuplicatePath, entry.Path, first, entry.AbsPath)
+	}
+
+	s.paths[entry.Path] = entry.AbsPath
 	s.files = append(s.files, entry)
 	s.totalBytes += entry.Size
 	filesFound := FileCount(len(s.files))

mfer/scanner_test.go

@@ -118,6 +118,27 @@ func TestScannerEnumeratePathWithProgress(t *testing.T) {
 	assert.Equal(t, FileSize(6), final.BytesFound)
 }
 
+// TestScannerEnumeratePathFile gives EnumeratePath a file: it is listed
+// by its name, as EnumerateFile lists it, and the manifest can be built.
+func TestScannerEnumeratePathFile(t *testing.T) {
+	t.Parallel()
+
+	fs := afero.NewMemMapFs()
+	require.NoError(t, fs.MkdirAll("/dir", 0o755))
+	require.NoError(t, afero.WriteFile(fs, "/dir/one.txt", []byte("1"), 0o644))
+
+	s := NewScannerWithOptions(&ScannerOptions{Fs: fs})
+	require.NoError(t, s.EnumeratePath("/dir/one.txt", nil))
+
+	var buf bytes.Buffer
+	require.NoError(t, s.ToManifest(context.Background(), &buf, nil))
+
+	m, err := NewManifestFromReader(&buf)
+	require.NoError(t, err)
+	require.Len(t, m.Files(), 1)
+	assert.Equal(t, "one.txt", m.Files()[0].GetPath())
+}
+
 func TestScannerEnumeratePaths(t *testing.T) {
 	t.Parallel()
 
@@ -134,6 +155,28 @@ func TestScannerEnumeratePaths(t *testing.T) {
 	assert.Equal(t, FileCount(2), s.FileCount())
 }
 
+// TestScannerEnumeratePathsFile gives EnumeratePaths a directory and a
+// file: the file is listed by its name, as EnumerateFile lists it.
+func TestScannerEnumeratePathsFile(t *testing.T) {
+	t.Parallel()
+
+	fs := afero.NewMemMapFs()
+	require.NoError(t, fs.MkdirAll("/dir/sub", 0o755))
+	require.NoError(t, fs.MkdirAll("/other", 0o755))
+	require.NoError(t, afero.WriteFile(fs, "/dir/sub/one.txt", []byte("1"), 0o644))
+	require.NoError(t, afero.WriteFile(fs, "/other/two.txt", []byte("2"), 0o644))
+
+	s := NewScannerWithOptions(&ScannerOptions{Fs: fs})
+	require.NoError(t, s.EnumeratePaths(nil, "/dir", "/other/two.txt"))
+
+	paths := make([]RelFilePath, 0, s.FileCount())
+	for _, f := range s.Files() {
+		paths = append(paths, f.Path)
+	}
+
+	assert.Equal(t, []RelFilePath{"sub/one.txt", "two.txt"}, paths)
+}
+
 func TestScannerExcludeDotfiles(t *testing.T) {
 	t.Parallel()
 

mfer/serialize.go

@@ -7,9 +7,11 @@ import (
 	"errors"
 	"fmt"
 	"math"
+	"strings"
 	"time"
 	"uuid"
 
+	"github.com/ProtonMail/go-crypto/openpgp"
 	"github.com/klauspost/compress/zstd"
 	"google.golang.org/protobuf/proto"
 )
@@ -20,11 +22,9 @@ const MAGIC string = "ZNAVSRFG"
 var (
 	// errInnerNotSet is returned by generate when the inner manifest is
 	// missing.
-	errInnerNotSet = errors.New("internal error: pbInner not set")
+	errInnerNotSet = errors.New("inner message not set")
 	// errInternal is returned by generateOuter for the same condition.
-	// The two messages differ, and both are load-bearing for callers that
-	// match on text, so they are kept distinct.
-	errInternal = errors.New("internal error")
+	errInternal = errors.New("inner message not set")
 )
 
 // nanosecondsInt32 converts t's nanosecond component to int32.
@@ -65,14 +65,14 @@ func (m *manifest) generate(ctx context.Context) error {
 
 	dat, err := proto.MarshalOptions{Deterministic: true}.Marshal(m.pbOuter)
 	if err != nil {
-		return fmt.Errorf("serialize: marshal outer: %w", err)
+		return fmt.Errorf("marshal outer message: %w", err)
 	}
 
 	m.output = bytes.NewBufferString(MAGIC)
 
 	_, err = m.output.Write(dat)
 	if err != nil {
-		return fmt.Errorf("serialize: write output: %w", err)
+		return fmt.Errorf("write outer message: %w", err)
 	}
 
 	return nil
@@ -95,7 +95,7 @@ func (m *manifest) generateOuter(ctx context.Context) error {
 
 	innerData, err := proto.MarshalOptions{Deterministic: true}.Marshal(m.pbInner)
 	if err != nil {
-		return fmt.Errorf("serialize: marshal inner: %w", err)
+		return fmt.Errorf("marshal inner message: %w", err)
 	}
 
 	// Compress the inner data
@@ -103,12 +103,12 @@ func (m *manifest) generateOuter(ctx context.Context) error {
 
 	zw, err := zstd.NewWriter(idc, zstd.WithEncoderLevel(zstd.SpeedBestCompression))
 	if err != nil {
-		return fmt.Errorf("serialize: create compressor: %w", err)
+		return fmt.Errorf("create compressor: %w", err)
 	}
 
 	_, err = zw.Write(innerData)
 	if err != nil {
-		return fmt.Errorf("serialize: compress: %w", err)
+		return fmt.Errorf("compress inner message: %w", err)
 	}
 
 	_ = zw.Close()
@@ -120,7 +120,7 @@ func (m *manifest) generateOuter(ctx context.Context) error {
 
 	_, err = h.Write(compressedData)
 	if err != nil {
-		return fmt.Errorf("serialize: hash write: %w", err)
+		return fmt.Errorf("hash inner message: %w", err)
 	}
 
 	sha256Hash := h.Sum(nil)
@@ -135,42 +135,48 @@ func (m *manifest) generateOuter(ctx context.Context) error {
 	}
 
 	// Sign the manifest if signing options are provided
-	if m.signingOptions != nil && m.signingOptions.KeyID != "" {
+	if m.signingOptions != nil {
 		return m.signOuter(ctx)
 	}
 
 	return nil
 }
 
-// signOuter signs the outer message with the configured GPG key and
-// embeds the signature, signer fingerprint, and public key. It signs with
-// and exports the key by its fingerprint, so that a key ID matching more
-// than one key cannot embed a key other than the one that signed.
+// signOuter signs the outer message with the secret key in the signing
+// options and embeds the signature, the key's fingerprint and its public
+// key.
 func (m *manifest) signOuter(ctx context.Context) error {
+	// Unlocking a protected key can take a while; do not start once ctx
+	// has ended.
+	err := ctx.Err()
+	if err != nil {
+		return err
+	}
+
 	sigString, err := m.signatureString()
 	if err != nil {
-		return fmt.Errorf("failed to generate signature string: %w", err)
+		return fmt.Errorf("build signature string: %w", err)
 	}
 
-	fingerprint, err := gpgGetKeyFingerprint(ctx, m.signingOptions.KeyID)
+	key, err := readSigningKey(m.signingOptions)
 	if err != nil {
-		return fmt.Errorf("failed to get key fingerprint: %w", err)
+		return err
 	}
 
-	m.pbOuter.Signer = fingerprint
+	var sig bytes.Buffer
 
-	sig, err := gpgSign(ctx, []byte(sigString), GPGKeyID(fingerprint))
+	err = openpgp.ArmoredDetachSign(&sig, key, strings.NewReader(sigString), nil)
 	if err != nil {
-		return fmt.Errorf("failed to sign manifest: %w", err)
+		return fmt.Errorf("sign manifest: %w", err)
 	}
 
-	m.pbOuter.Signature = sig
-
-	pubKey, err := gpgExportPublicKey(ctx, GPGKeyID(fingerprint))
+	pubKey, err := armoredPublicKey(key)
 	if err != nil {
-		return fmt.Errorf("failed to export public key: %w", err)
+		return err
 	}
 
+	m.pbOuter.Signature = sig.Bytes()
+	m.pbOuter.Signer = []byte(fingerprint(key))
 	m.pbOuter.SigningPubKey = pubKey
 
 	return nil