Fix missing-key errors for restore and deep verify to name both key sources (closes #401)

The missing-key error for restore and deep verify is one value, errDecryptionKeyRequired, whose text names both places the key can be set: the VAULTIK_AGE_SECRET_KEY environment variable first, then the age_secret_key config file field.

Model: deepseek-v4-pro

internal/vaultik/restore.go

@@ -31,10 +31,9 @@ var (
 	errPathsNotInSnapshot    = errors.New("requested path(s) not in the snapshot")
 	errFilesFailedVerify     = errors.New("files failed verification")
 	errDecryptionKeyRequired = errors.New(
-		"decryption key required for restore\n\n" +
+		"age secret key is missing; set it in the " +
-			"Set the VAULTIK_AGE_SECRET_KEY environment variable to your " +
+			"VAULTIK_AGE_SECRET_KEY environment variable or the " +
-			"age private key file:\n" +
+			"age_secret_key config file field")
-			"  export VAULTIK_AGE_SECRET_KEY=\"$(cat vaultik_backup_private_key.txt)\"")
 	// errInvalidAgeSecretKey is returned when the configured key does not
 	// parse as any age identity. It names the source but never the value,
 	// which is secret, so the message is safe to print and log.

internal/vaultik/restore_identity_test.go

@@ -106,3 +106,23 @@ func TestRestoreIdentitiesMissingKey(t *testing.T) {
 	_, err := v.restoreIdentities()
 	require.ErrorIs(t, err, errDecryptionKeyRequired)
 }
+
+// TestMissingKeyErrorsNameBothSources asserts restore and deep verify
+// return the same missing-key error, whose text names both places the key
+// can be set: the VAULTIK_AGE_SECRET_KEY environment variable first since
+// it is preferred, then the age_secret_key config file field.
+func TestMissingKeyErrorsNameBothSources(t *testing.T) {
+	t.Parallel()
+
+	const want = "age secret key is missing; set it in " +
+		"the VAULTIK_AGE_SECRET_KEY environment variable or the " +
+		"age_secret_key config file field"
+
+	v := &Vaultik{Config: &config.Config{}}
+
+	_, err := v.restoreIdentities()
+	require.EqualError(t, err, want)
+
+	err = v.RunDeepVerify("any-snapshot", &VerifyOptions{})
+	require.EqualError(t, err, want)
+}

internal/vaultik/vaultik.go

@@ -162,11 +162,6 @@ func (v *Vaultik) StartOperation(fn func()) func(context.Context) bool {
 	}
 }
 
-// CanDecrypt returns true if this Vaultik instance has decryption capabilities
-func (v *Vaultik) CanDecrypt() bool {
-	return v.Config.AgeSecretKey != ""
-}
-
 // GetFilesystem returns the filesystem instance used by Vaultik
 //
 //nolint:ireturn // afero.Fs is the filesystem abstraction by design

internal/vaultik/verify.go

@@ -22,9 +22,7 @@ import (
 
 // Sentinel errors for snapshot verification failures.
 var (
-	errVerificationFailed = errors.New("verification failed")
+	errVerificationFailed  = errors.New("verification failed")
-	errSecretKeyRequired  = errors.New(
-		"VAULTIK_AGE_SECRET_KEY not set; required for deep verification")
 	errChunksOutOfOrder    = errors.New("chunks out of order")
 	errChunkHashMismatch   = errors.New("chunk hash mismatch")
 	errNegativeChunkLength = errors.New("chunk length is negative")
@@ -106,13 +104,9 @@ func (v *Vaultik) RunDeepVerify(snapshotID string, opts *VerifyOptions) error {
 		Mode:       "deep",
 	}
 
-	if !v.CanDecrypt() {
-		return v.deepVerifyFailure(result, opts,
-			errSecretKeyRequired.Error(), errSecretKeyRequired)
-	}
-
 	// Parse the age secret key once, the same way restore does, and reuse
-	// the identities for the database and every blob.
+	// the identities for the database and every blob. A missing key fails
+	// here, before anything is downloaded.
 	identities, err := v.restoreIdentities()
 	if err != nil {
 		return v.deepVerifyFailure(result, opts, err.Error(), err)