gofiber/fiber · error

failed to create AES cipher

Error message

failed to create AES cipher: %w

What it means

Returned by EncryptCookie (and DecryptCookie) when aes.NewCipher fails on the decoded key. Because decodeKey already validated the length to be 16/24/32 bytes, aes.NewCipher should not fail in practice — it can only fail on a non-positive key length, which the prior guard excludes. Encountering it indicates an unexpected crypto/library state or a key whose length validation was bypassed.

Solutions

1. Confirm the key reaches EncryptCookie via the public API and decodeKey (which enforces length) — do not call aes.NewCipher directly with user input.
2. Regenerate the key with GenerateKey to rule out malformed key material.
3. Rebuild with a stock Go toolchain to rule out a corrupted crypto/aes.
4. Treat as a bug if reproduced with a valid 16/24/32-byte key through the public API.

// Validate the key material end-to-end at startup by performing a roundtrip.
func validateKeyRoundtrip(key string) error {
    enc, err := encryptcookie.EncryptCookie("probe", "v", key)
    if err != nil {
        return fmt.Errorf("encrypt roundtrip failed: %w", err)
    }
    if _, err := encryptcookie.DecryptCookie("probe", enc, key); err != nil {
        return fmt.Errorf("decrypt roundtrip failed: %w", err)
    }
    return nil
}

// Practically unreachable; guard at the boundary by validating the key once
// at boot rather than per request.
if err := validateKeyRoundtrip(cfg.Key); err != nil {
    log.Fatal("encryptcookie key invalid:", err)
}

When it happens

Trigger: Only reachable if aes.NewCipher returns an error for a key that decodeKey accepted — i.e. effectively unreachable under the documented contract. Hypothetically: a malformed build, a forked/patched decodeKey that skipped length validation, or a corrupted crypto/aes package.

Common situations: Not encountered in normal operation. If seen, suspect a code path that calls aes.NewCipher with an unvalidated key (bypassing decodeKey), or an exotic platform/toolchain issue. Treat as a defense-in-depth guard rather than an operational error.

Related errors

• failed to base64-decode key
• cache: unexpected entry type %T for key
• cache: unexpected raw entry type %T for key
• csrf: unexpected value type %T in storage
• cache: failed to delete key

AI-assisted analysis of gofiber/fiber@a105acad6c (2026-08-11). Data as JSON: /api/errors/6bb342649d1f142b. Report an issue: GitHub.

Appendix: source

Thrown at middleware/encryptcookie/utils.go:49

	return keyDecoded, nil
}

// validateKey checks if the provided base64-encoded key is of valid length.
func validateKey(key string) error {
	_, 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
	}

View on GitHub (pinned to a105acad6c)