gofiber/fiber · error

failed to create GCM mode

Error message

failed to create GCM mode: %w

What it means

Returned by EncryptCookie when cipher.NewGCMWithRandomNonce fails after the AES block cipher was created. The Go stdlib only errors here if the block size is unsupported; AES blocks are always 128-bit so this branch is effectively unreachable for valid AES keys. Because decodeKey already enforces 16/24/32-byte lengths, hitting it implies an unexpected crypto/cipher runtime state or a corrupted binary.

Solutions

1. Confirm the error is not actually from decodeKey or aes.NewCipher (read the wrapped %w chain) — those are the real-world causes.
2. Ensure you are using the unmodified Go standard library crypto/aes (no vendored forks).
3. Regenerate the key with encryptcookie.GenerateKey(32) to rule out key-shape corruption.
4. If genuinely hit, file a Go stdlib issue; this branch is not actionable from application code.

Example fix

// before
key := "some-hardcoded-value"
enc, err := encryptcookie.EncryptCookie(name, value, key)
// after
key := encryptcookie.GenerateKey(32) // valid base64 AES-256 key
enc, err := encryptcookie.EncryptCookie(name, value, key)
if err != nil {
    return fmt.Errorf("encrypt cookie: %w", err)
}

if err := encryptcookie.KeyValidator(key); err != nil { return fmt.Errorf("boot: %w", err) }

enc, err := encryptcookie.EncryptCookie(name, value, key)
if err != nil {
    if errors.Is(err, cipher.NewGCMWithRandomNonceError) { /* unreachable in practice */ }
    return fmt.Errorf("encrypt cookie: %w", err)
}

When it happens

Trigger: Calling encryptcookie.EncryptCookie(name, value, key) with a key that passed base64+length validation but for which cipher.NewGCMWithRandomNonce(block) returns a non-nil error. In practice this never fires with stdlib AES; it would only surface if a custom/modified aes.NewCipher returned a block with a non-standard block size.

Common situations: Developers see this only as a defensive guard. It can appear after swapping the stdlib crypto/aes for a faulty fork, on exotic GOOS/GOARCH builds with broken crypto assembly, or when fuzzing the cipher layer. Standard deployments never encounter it.

Related errors

• failed to decrypt ciphertext
• encryption key must be 16, 24, or 32 bytes
• failed to base64-decode key
• failed to create AES cipher
• decode SHA256 password: invalid length

AI-assisted analysis of gofiber/fiber@a105acad6c (2026-08-11). Data as JSON: /api/errors/a1c9bb2c10b7a694. Report an issue: GitHub.

Appendix: source

Thrown at middleware/encryptcookie/utils.go:54

	_, err := decodeKey(key)
	return err
}

// EncryptCookie Encrypts a cookie value with specific encryption key
func EncryptCookie(name, value, key string) (string, error) {
	keyDecoded, err := decodeKey(key)
	if err != nil {
		return "", err
	}

	block, err := aes.NewCipher(keyDecoded)
	if err != nil {
		return "", fmt.Errorf("failed to create AES cipher: %w", err)
	}

	gcm, err := cipher.NewGCMWithRandomNonce(block)
	if err != nil {
		return "", fmt.Errorf("failed to create GCM mode: %w", err)
	}

	ciphertext := gcm.Seal(nil, nil, []byte(value), []byte(name))
	return base64.StdEncoding.EncodeToString(ciphertext), nil
}

// DecryptCookie Decrypts a cookie value with specific encryption key
func DecryptCookie(name, value, key string) (string, error) {
	keyDecoded, err := decodeKey(key)
	if err != nil {
		return "", err
	}

	enc, err := base64.StdEncoding.DecodeString(value)
	if err != nil {
		return "", fmt.Errorf("failed to base64-decode value: %w", err)
	}

View on GitHub (pinned to a105acad6c)