Hash the plaintext, not the encrypted bytes, in verify --deep (closes #131)

Deep verification's final blob-integrity check hashed the encrypted
downloaded bytes with a single SHA256 and compared that to the blob's
remote name, which is the double SHA256 of the plaintext
(blobgen.Writer.Sum256). The two can never be equal, so verify --deep
reported every healthy blob as "blob hash mismatch". The per-chunk and
blob-existence checks were correct; only this final comparison was wrong.

It now hashes the decompressed plaintext as chunk verification streams it
and compares its double SHA256 to the blob name.

Added a test that backs up a real snapshot, deep-verifies it (which fails
before this fix), then flips a byte in one stored blob and confirms deep
verification then fails.

Model: opus-4-8

TODO.md

@@ -25,6 +25,16 @@ release" is exactly the contradiction
 
 # Completed Steps
 
+- 2026-09-21: Fixed `verify --deep` reporting healthy snapshots as
+  corrupt. Its final blob-integrity check hashed the encrypted
+  downloaded bytes with a single SHA256 and compared that to the blob
+  ID, which is the double SHA256 of the plaintext, so the two could
+  never match. It now hashes the decompressed plaintext and compares the
+  double SHA256. Added a test that backs up a real snapshot, deep-verifies
+  it, then flips a byte in one stored blob and confirms deep verification
+  then fails
+  ([issue #131](https://git.eeqj.de/sneak/vaultik/issues/131)).
+
 - 2026-09-21: Made `snapshot create` VACUUM the per-snapshot metadata
   database through the `modernc.org/sqlite` driver instead of shelling
   out to the external `sqlite` command-line binary (issue #120). A

internal/vaultik/deep_verify_test.go

@@ -0,0 +1,108 @@
+package vaultik_test
+
+import (
+	"context"
+	"io"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/spf13/afero"
+	"github.com/stretchr/testify/require"
+	"sneak.berlin/go/vaultik/internal/log"
+	"sneak.berlin/go/vaultik/internal/ui"
+	"sneak.berlin/go/vaultik/internal/vaultik"
+)
+
+// TestDeepVerifyAcceptsHealthyAndRejectsCorruptBlob backs up a real
+// snapshot with the on-disk storage backend, runs deep verification on
+// it, then flips a byte inside one stored blob and runs deep
+// verification again. A healthy snapshot must pass; a corrupted blob
+// must fail. The healthy case is the regression guard: deep
+// verification used to hash the encrypted blob bytes and compare them
+// to the blob's ID (the double SHA256 of the plaintext), so it reported
+// every healthy blob as corrupt.
+func TestDeepVerifyAcceptsHealthyAndRejectsCorruptBlob(t *testing.T) {
+	log.Initialize(log.Config{})
+	t.Parallel()
+
+	fs := afero.NewOsFs()
+	tempDir := t.TempDir()
+
+	dataDir := filepath.Join(tempDir, "source")
+	storeDir := filepath.Join(tempDir, "remote")
+	dbPath := filepath.Join(tempDir, "index.sqlite")
+
+	chunkSize := int64(64 * 1024)
+	maxBlobSize := int64(512 * 1024)
+
+	// One file large enough to span several chunks within a single blob.
+	require.NoError(t, fs.MkdirAll(dataDir, 0o755))
+	require.NoError(t, afero.WriteFile(fs,
+		filepath.Join(dataDir, "data.bin"),
+		bytesPattern("deep-", int(chunkSize*3)), 0o644))
+
+	ctx := context.Background()
+
+	// runFileStorageBackup writes a real snapshot to storeDir and closes
+	// the source index, so verification runs from remote bytes only.
+	cfg, storer, snapshotID := runFileStorageBackup(
+		ctx, t, fs, dataDir, storeDir, dbPath, chunkSize, maxBlobSize)
+
+	newVerifier := func() *vaultik.Vaultik {
+		v := &vaultik.Vaultik{
+			Config:  cfg,
+			Storage: storer,
+			Fs:      fs,
+			Stdout:  io.Discard,
+			Stderr:  io.Discard,
+			UI:      ui.NewWithColor(io.Discard, false),
+		}
+		v.SetContext(ctx)
+
+		return v
+	}
+
+	require.NoError(t,
+		newVerifier().RunDeepVerify(snapshotID, &vaultik.VerifyOptions{Deep: true}),
+		"deep verify should pass on a healthy snapshot")
+
+	// Flip a byte inside one blob without changing its length, so the
+	// blob-existence and size checks still pass and verification reaches
+	// the blob-content stage.
+	corruptOneBlob(t, fs, filepath.Join(storeDir, "blobs"))
+
+	require.Error(t,
+		newVerifier().RunDeepVerify(snapshotID, &vaultik.VerifyOptions{Deep: true}),
+		"deep verify should fail on a corrupted blob")
+}
+
+// corruptOneBlob flips a middle byte of the first blob file found under
+// blobsDir, leaving the file length unchanged.
+func corruptOneBlob(t *testing.T, fs afero.Fs, blobsDir string) {
+	t.Helper()
+
+	var blobPath string
+
+	err := afero.Walk(fs, blobsDir,
+		func(path string, info os.FileInfo, err error) error {
+			if err != nil {
+				return err
+			}
+
+			if blobPath == "" && !info.IsDir() {
+				blobPath = path
+			}
+
+			return nil
+		})
+	require.NoError(t, err)
+	require.NotEmpty(t, blobPath, "expected at least one blob on disk")
+
+	data, err := afero.ReadFile(fs, blobPath)
+	require.NoError(t, err)
+	require.NotEmpty(t, data)
+
+	data[len(data)/2] ^= 0xff
+	require.NoError(t, afero.WriteFile(fs, blobPath, data, 0o644))
+}

internal/vaultik/verify.go

@@ -344,12 +344,8 @@ func (v *Vaultik) verifyBlob(blobInfo snapshot.BlobInfo, db *sql.DB) error {
 		return fmt.Errorf("failed to get decryptor: %w", err)
 	}
 
-	// Hash the encrypted blob data as it streams through to decryption
-	blobHasher := sha256.New()
-	teeReader := io.TeeReader(reader, blobHasher)
-
-	// Decrypt blob (reading through teeReader to hash encrypted data)
-	decryptedReader, err := decryptor.DecryptStream(teeReader)
+	// Decrypt blob
+	decryptedReader, err := decryptor.DecryptStream(reader)
 	if err != nil {
 		return fmt.Errorf("failed to decrypt: %w", err)
 	}
@@ -361,12 +357,19 @@ func (v *Vaultik) verifyBlob(blobInfo snapshot.BlobInfo, db *sql.DB) error {
 	}
 	defer decompressor.Close()
 
-	chunkCount, err := v.verifyBlobChunks(db, blobInfo.Hash, decompressor)
+	// A blob's hash — its remote name — is the double SHA256 of its
+	// decompressed plaintext (see blobgen.Writer.Sum256), not of the
+	// encrypted bytes. Hash the plaintext as chunk verification streams
+	// it, then compare on completion.
+	plaintextHasher := sha256.New()
+	hashedStream := io.TeeReader(decompressor, plaintextHasher)
+
+	chunkCount, err := v.verifyBlobChunks(db, blobInfo.Hash, hashedStream)
 	if err != nil {
 		return err
 	}
 
-	err = v.verifyBlobFinalIntegrity(decompressor, blobHasher, blobInfo.Hash)
+	err = v.verifyBlobFinalIntegrity(hashedStream, plaintextHasher, blobInfo.Hash)
 	if err != nil {
 		return err
 	}
@@ -470,14 +473,13 @@ func (v *Vaultik) verifyBlobChunks(
 }
 
 // verifyBlobFinalIntegrity checks that no trailing data exists in the
-// decompressed stream and that the encrypted blob hash matches the
-// expected value.
+// decompressed stream and that the blob hash matches the expected value.
 func (v *Vaultik) verifyBlobFinalIntegrity(
-	decompressor io.Reader, blobHasher hash.Hash, expectedHash string,
+	plaintext io.Reader, plaintextHasher hash.Hash, expectedHash string,
 ) error {
 	// Verify no remaining data in blob - if the chunk list is accurate,
 	// the blob should be fully consumed.
-	remaining, err := io.Copy(io.Discard, decompressor)
+	remaining, err := io.Copy(io.Discard, plaintext)
 	if err != nil {
 		return fmt.Errorf("failed to check for remaining blob data: %w", err)
 	}
@@ -486,8 +488,11 @@ func (v *Vaultik) verifyBlobFinalIntegrity(
 		return fmt.Errorf("%w: %d bytes", errTrailingBlobData, remaining)
 	}
 
-	// Verify blob hash matches the encrypted data we downloaded
-	calculatedBlobHash := hex.EncodeToString(blobHasher.Sum(nil))
+	// The blob hash is the double SHA256 of its plaintext content.
+	firstHash := plaintextHasher.Sum(nil)
+	secondHash := sha256.Sum256(firstHash)
+	calculatedBlobHash := hex.EncodeToString(secondHash[:])
+
 	if calculatedBlobHash != expectedHash {
 		return fmt.Errorf("%w: calculated %s, expected %s",
 			errBlobHashMismatch, calculatedBlobHash, expectedHash)