gofiber/fiber · error

failed to base64-decode value

Error message

failed to base64-decode value: %w

What it means

Returned by DecryptCookie when the incoming cookie value is not valid base64 (StdEncoding). DecryptCookie first base64-decodes the stored value before attempting AES-GCM decryption, so any non-base64 payload fails here. The wrapped %w carries encoding/base64's CorruptInputError, which reports the byte offset of corruption.

Solutions

1. Verify the cookie was produced by EncryptCookie with the same base64 (StdEncoding) variant.
2. Check for proxy/gateway re-encoding of cookie values (URL-encoding, stripping padding).
3. If migrating encoding, clear/rotate old cookies so clients get fresh EncryptCookie output.
4. Log the raw value length and the CorruptInputError offset to pinpoint corruption.

Example fix

// before
plaintext, err := encryptcookie.DecryptCookie(name, c.Cookies(name), key)
// after
raw := c.Cookies(name)
if _, bErr := base64.StdEncoding.DecodeString(raw); bErr != nil {
    c.ClearCookie(name) // stale/tampered cookie; let the user re-auth
    return c.Redirect("/login")
}
plaintext, err := encryptcookie.DecryptCookie(name, raw, key)

if _, err := base64.StdEncoding.DecodeString(value); err != nil {
    // not a valid EncryptCookie payload; treat as missing
    return redirectToLogin()
}

func isStdBase64(s string) bool {
    _, err := base64.StdEncoding.DecodeString(s)
    return err == nil
}

plain, err := encryptcookie.DecryptCookie(name, value, key)
if err != nil {
    c.ClearCookie(name)
    return c.Redirect("/login")
}

When it happens

Trigger: Calling encryptcookie.DecryptCookie(name, value, key) where value is not a valid base64 string — e.g. a tampered cookie, a cookie set by a different library, a URL-safe base64 payload, or a value with whitespace/newlines that StdEncoding rejects.

Common situations: Switching from URLSafe base64 to standard base64 (or vice versa), clients truncating long cookies, proxies stripping '=' padding, rotating the encryption scheme without invalidating old cookies, or decrypting a plaintext cookie set by middleware that does not use EncryptCookie.

Related errors

• failed to base64-decode key
• decode SHA256 password
• decode SHA512 password
• encryption key must be 16, 24, or 32 bytes
• ErrEmptySessionID

AI-assisted analysis of gofiber/fiber@a105acad6c (2026-08-11). Data as JSON: /api/errors/af9178e2ba2161c3. Report an issue: GitHub.

Appendix: source

Thrown at middleware/encryptcookie/utils.go:70

	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)
	}

	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)
	}

	if len(enc) < gcm.NonceSize()+gcm.Overhead() {
		return "", ErrInvalidEncryptedValue
	}

	plaintext, err := gcm.Open(nil, nil, enc, []byte(name))
	if err != nil {

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