Parse age_recipients at config load and never echo the entry (closes #153)

Config.Validate now parses every age_recipients entry with
age.ParseX25519Recipient, so a bad recipient fails at config load instead
of deep in a backup after the snapshot row and tree walk. On failure the
error names the position (age_recipients[N]) and never the value: a
recipient string can itself be a secret key an operator pasted by mistake,
and age's own error quotes its input. An entry starting with
AGE-SECRET-KEY- (compared case-insensitively) gets a specific message.

The remaining parse sites (blobgen.NewWriter, crypto NewEncryptor and
UpdateRecipients), reachable by callers that skip config.Load, likewise
drop the value and age's wrapped error, naming only the position.

test/config.yaml's placeholder second recipient is replaced with a valid
X25519 key so it still loads.

Model: opus-4-8

internal/config/config.go

@@ -22,6 +22,13 @@ import (
 
 const appName = "vaultik"
 
+// secretKeyPrefix marks an age secret (private) key. It is compared
+// case-insensitively so a recipient entry that is actually a private key is
+// caught and never passed to age or echoed back.
+//
+//nolint:gosec // G101: marker for detecting a pasted secret key, not a credential
+const secretKeyPrefix = "AGE-SECRET-KEY-"
+
 // Defaults and validation bounds for tunable settings.
 const (
 	defaultBlobSizeLimit    = Size(10 * 1024 * 1024 * 1024) // 10GB
@@ -38,6 +45,10 @@ var (
 	errNoConfigPath    = errors.New("config path not provided")
 	errNoAgeRecipients = errors.New(
 		"at least one age_recipient is required (generate with: age-keygen)")
+	errRecipientIsSecretKey = errors.New(
+		"an age secret key was given where a public key (age1...) belongs")
+	errRecipientNotX25519 = errors.New(
+		"not a valid recipient; only X25519 age1... public keys are supported")
 	errNoSnapshots = errors.New(
 		"at least one snapshot must be configured (see config.example.yml)")
 	errSnapshotNoPaths   = errors.New("snapshot must have at least one path")
@@ -290,7 +301,9 @@ func Load(path string) (*Config, error) {
 
 // Validate checks if the configuration is valid and complete.
 // It ensures all required fields are present and have valid values:
-//   - At least one age recipient must be specified
+//   - At least one age recipient must be specified, and every recipient must
+//     parse as an X25519 age1... public key (so a bad entry fails at load, not
+//     mid-backup); errors name the position, never the value
 //   - At least one snapshot must be configured with at least one path
 //   - Storage must be configured (either storage_url or s3.* fields)
 //   - Chunk size must be at least 1MB
@@ -305,6 +318,13 @@ func (c *Config) Validate() error {
 		return errNoAgeRecipients
 	}
 
+	for i, recipient := range c.AgeRecipients {
+		err := validateAgeRecipient(recipient)
+		if err != nil {
+			return fmt.Errorf("age_recipients[%d]: %w", i, err)
+		}
+	}
+
 	if len(c.Snapshots) == 0 {
 		return errNoSnapshots
 	}
@@ -342,6 +362,23 @@ func (c *Config) Validate() error {
 	return nil
 }
 
+// validateAgeRecipient parses one age_recipients entry with the age library
+// and returns a value-free error on failure. A recipient string can be
+// sensitive (an operator may paste a secret key by mistake), so neither the
+// entry nor age's own error (which quotes its input) is ever included.
+func validateAgeRecipient(recipient string) error {
+	if strings.HasPrefix(strings.ToUpper(recipient), secretKeyPrefix) {
+		return errRecipientIsSecretKey
+	}
+
+	_, err := age.ParseX25519Recipient(recipient)
+	if err != nil {
+		return errRecipientNotX25519
+	}
+
+	return nil
+}
+
 // validateStorage validates storage configuration.
 // If StorageURL is set, it takes precedence. S3 URLs require credentials.
 // File URLs don't require any S3 configuration.