README.md

@@ -554,7 +554,11 @@ secret decrypt encryption/mykey --input document.txt.age --output document.txt
 ### Vault Management
 
 - **Derivation Index**: Each vault uses a unique derivation index from the
-  mnemonic, and thus a unique key pair
+  mnemonic, and thus a unique key pair; creating a vault (`secret init` and
+  `secret vault create`) fails, with an error naming the file, when any vault's
+  `vault-metadata.json` exists but cannot be read or parsed; a vault directory
+  with no `vault-metadata.json` is skipped, so the index it uses can be given to
+  a new vault
 - **Public Key Hash**: Double SHA-256 hash of the vault's public key; the same
   hash of the index-0 public key identifies vaults from the same mnemonic
 - **Automatic Key Derivation**: When creating vaults with a mnemonic, keys are

internal/vault/metadata.go

@@ -4,7 +4,9 @@ import (
 	"crypto/sha256"
 	"encoding/hex"
 	"encoding/json"
+	"errors"
 	"fmt"
+	"os"
 	"path/filepath"
 
 	"github.com/spf13/afero"
@@ -33,7 +35,10 @@ func ComputeDoubleSHA256(data []byte) string {
 }
 
 // GetNextDerivationIndex finds the next available derivation index for a given mnemonic
-// by deriving the public key for index 0 and using its hash to identify related vaults
+// by deriving the public key for index 0 and using its hash to identify related vaults.
+// It fails when a vault's metadata file exists but cannot be read or parsed: the
+// index that vault uses is then unknown, and handing it out again would give a
+// second vault the same long-term key.
 func GetNextDerivationIndex(
 	fs afero.Fs, stateDir string, mnemonic string,
 ) (uint32, error) {
@@ -72,21 +77,23 @@ func GetNextDerivationIndex(
 			continue
 		}
 
-		// Try to read vault metadata
 		metadataPath := filepath.Join(vaultsDir, entry.Name(), "vault-metadata.json")
 
 		metadataBytes, err := afero.ReadFile(fs, metadataPath)
-		if err != nil {
-			// Skip vaults without metadata
+		if errors.Is(err, os.ErrNotExist) {
+			// Skip directories without metadata
 			continue
 		}
 
+		if err != nil {
+			return 0, fmt.Errorf("failed to read vault metadata %s: %w", metadataPath, err)
+		}
+
 		var metadata Metadata
 
 		err = json.Unmarshal(metadataBytes, &metadata)
 		if err != nil {
-			// Skip vaults with invalid metadata
-			continue
+			return 0, fmt.Errorf("failed to parse vault metadata %s: %w", metadataPath, err)
 		}
 
 		// Check if this vault uses the same mnemonic by comparing family hashes

internal/vault/metadata_test.go

@@ -1,11 +1,15 @@
 package vault_test
 
 import (
+	"errors"
 	"path/filepath"
 	"strings"
 	"testing"
 
 	"github.com/spf13/afero"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"sneak.berlin/go/secret/internal/secret"
 	"sneak.berlin/go/secret/internal/vault"
 	"sneak.berlin/go/secret/pkg/agehd"
 )
@@ -163,6 +167,88 @@ func testGetNextDerivationIndex(t *testing.T, fs afero.Fs) {
 	}
 }
 
+// errMetadataUnreadable is returned by the test filesystem in place of a
+// successful open of one vault's metadata file.
+var errMetadataUnreadable = errors.New("input/output error")
+
+// metadataReadFailFs fails every open of the file at unreadablePath. The
+// file still exists, so checking for it succeeds and only reading it fails.
+type metadataReadFailFs struct {
+	afero.Fs
+
+	unreadablePath string
+}
+
+//nolint:ireturn // afero.File is the interface required by afero.Fs
+func (f *metadataReadFailFs) Open(name string) (afero.File, error) {
+	if name == f.unreadablePath {
+		return nil, errMetadataUnreadable
+	}
+
+	return f.Fs.Open(name)
+}
+
+// TestCreateVaultWithCorruptMetadataFails checks that creating a vault from
+// a mnemonic fails, naming the file and leaving no new vault, when another
+// vault's metadata file exists but cannot be parsed or read. That vault's
+// derivation index is then unknown, so the new vault could get the same one
+// and the same key.
+func TestCreateVaultWithCorruptMetadataFails(t *testing.T) {
+	t.Parallel()
+
+	vaultsDir := filepath.Join(testStateDir, "vaults.d")
+	metadataPath := filepath.Join(vaultsDir, "work", "vault-metadata.json")
+
+	tests := []struct {
+		name string
+		// breakMetadata makes the work vault's metadata file unusable on
+		// base and returns the filesystem to create the second vault on.
+		breakMetadata func(t *testing.T, base afero.Fs) afero.Fs
+	}{
+		{
+			name: "parsing the file fails",
+			breakMetadata: func(t *testing.T, base afero.Fs) afero.Fs {
+				t.Helper()
+
+				err := afero.WriteFile(base, metadataPath, []byte("{"),
+					secret.FilePerms)
+				require.NoError(t, err)
+
+				return base
+			},
+		},
+		{
+			name: "reading the file fails",
+			breakMetadata: func(_ *testing.T, base afero.Fs) afero.Fs {
+				return &metadataReadFailFs{Fs: base, unreadablePath: metadataPath}
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			base := afero.NewMemMapFs()
+
+			_, err := vault.CreateVault(base, testStateDir, "work",
+				testMnemonicBuffer(t), nil)
+			require.NoError(t, err)
+
+			fs := tt.breakMetadata(t, base)
+
+			_, err = vault.CreateVault(fs, testStateDir, "other",
+				testMnemonicBuffer(t), nil)
+			require.ErrorContains(t, err, metadataPath)
+
+			entries, err := afero.ReadDir(base, vaultsDir)
+			require.NoError(t, err)
+			require.Len(t, entries, 1)
+			assert.Equal(t, "work", entries[0].Name())
+		})
+	}
+}
+
 func testMetadataPersistence(t *testing.T, fs afero.Fs) {
 	t.Helper()