Reject invalid secret names before any command builds a path (closes #33)

`secret rm ..` resolved to the vault directory and deleted the whole
vault; `secret rm .` and `secret rm ""` deleted every secret. rm, mv,
the version commands, encrypt and decrypt built paths from the name
without checking it; import checked it only after reading the source
file.

vault.ValidateSecretName wraps the existing name rule; its error and
README.md state the rule. Each of those commands calls it on the name
as given, before building any path; MoveSecret checks both names once,
for every form of the move, before switching the current vault.
AddSecret, GetSecretVersion and GetSecretObject use it too.

The regression test copies two in-memory vaults for each rejected
command and requires the exact error and an unchanged state directory.

Model: opus-5-5

internal/vault/errors.go

@@ -29,10 +29,11 @@ var (
 	// ErrNilValueBuffer indicates a nil value buffer was supplied.
 	ErrNilValueBuffer = errors.New("value buffer is nil")
 
-	// ErrInvalidSecretName indicates a secret name that does not match
-	// the allowed pattern [a-z0-9.\-_/]+. Composed as
-	// "invalid secret name '<name>': must match pattern [a-z0-9.\-_/]+",
-	// or as "invalid secret name: <name>" by GetSecretObject.
+	// ErrInvalidSecretName indicates a secret name that breaks the naming
+	// rule: only letters, digits, '.', '-', '_' and '/'; not empty; no
+	// leading '.' or '/', no trailing '/', no '//', no '..' path segment.
+	// Composed by ValidateSecretName as
+	// "invalid secret name '<name>': <the rule>".
 	ErrInvalidSecretName = errors.New("invalid secret name")
 
 	// ErrSecretExists indicates the secret already exists and --force

internal/vault/secrets.go

@@ -79,6 +79,7 @@ func (v *Vault) ListSecrets() ([]string, error) {
 // - No leading or trailing slashes
 // - No double slashes
 // - No names starting with dots
+// - No ".." path segments
 func isValidSecretName(name string) bool {
 	if name == "" {
 		return false
@@ -110,6 +111,22 @@ func isValidSecretName(name string) bool {
 	return matched
 }
 
+// ValidateSecretName returns an error wrapping ErrInvalidSecretName when
+// name is not a valid secret name. Call it on the name exactly as the user
+// gave it, before building any path from it.
+func ValidateSecretName(name string) error {
+	if !isValidSecretName(name) {
+		return fmt.Errorf(
+			"%w '%s': only letters, digits, '.', '-', '_' and '/' are allowed, "+
+				"and a name must not be empty, start with '.' or '/', end with '/', "+
+				"contain '//', or have '..' as a path segment",
+			ErrInvalidSecretName, name,
+		)
+	}
+
+	return nil
+}
+
 // AddSecret adds a secret to this vault
 func (v *Vault) AddSecret(name string, value *memguard.LockedBuffer, force bool) error {
 	if value == nil {
@@ -124,13 +141,11 @@ func (v *Vault) AddSecret(name string, value *memguard.LockedBuffer, force bool)
 	)
 
 	// Validate secret name
-	if !isValidSecretName(name) {
+	err := ValidateSecretName(name)
+	if err != nil {
 		secret.Debug("Invalid secret name provided", "secret_name", name)
 
-		return fmt.Errorf(
-			"%w '%s': must match pattern [a-z0-9.\\-_/]+",
-			ErrInvalidSecretName, name,
-		)
+		return err
 	}
 
 	secret.Debug("Secret name validation passed", "secret_name", name)
@@ -358,8 +373,9 @@ func (v *Vault) UnlockVault() (*age.X25519Identity, error) {
 
 // GetSecretObject retrieves a Secret object with metadata loaded from this vault
 func (v *Vault) GetSecretObject(name string) (*secret.Secret, error) {
-	if !isValidSecretName(name) {
-		return nil, fmt.Errorf("%w: %s", ErrInvalidSecretName, name)
+	err := ValidateSecretName(name)
+	if err != nil {
+		return nil, err
 	}
 
 	// First check if the secret exists by checking for the metadata file
@@ -628,13 +644,11 @@ func (v *Vault) updatePreviousVersion(
 // version exist, and resolves an empty version to the current one.
 func (v *Vault) resolveSecretVersion(name, version string) (string, error) {
 	// Validate secret name to prevent path traversal
-	if !isValidSecretName(name) {
+	err := ValidateSecretName(name)
+	if err != nil {
 		secret.Debug("Invalid secret name provided", "secret_name", name)
 
-		return "", fmt.Errorf(
-			"%w '%s': must match pattern [a-z0-9.\\-_/]+",
-			ErrInvalidSecretName, name,
-		)
+		return "", err
 	}
 
 	// Get vault directory