jwtk/jjwt · error · WeakKeyException

The specified SecretKey is not strong enough to be used…

Error message

The specified SecretKey is not strong enough to be used with JWT HMAC signature algorithms.  The JWT specification requires HMAC keys to be >= 256 bits long.  The specified key is ${bitLength} bits.  See https://tools.ietf.org/html/rfc7518#section-3.2 for more information.

What it means

Error "The specified SecretKey is not strong enough to be used with JWT HMAC signature algorithms. The JWT specification requires HMAC keys to be >= 256 bits long. The specified key is ${bitLength} bits. See https://tools.ietf.org/html/rfc7518#section-3.2 for more information." thrown in jwtk/jjwt.

Solutions

1. Create a strong HMAC key with Keys.secretKeyFor(SignatureAlgorithm.HS256) or larger (384/512 bits).
2. If the existing secret is too short, derive a longer key (e.g. via a KDF) to reach >=256 bits rather than padding naively.
3. For testing only, use a fixed >=32-byte secret; do not weaken production requirements.

When it happens

Trigger: Thrown at api/src/main/java/io/jsonwebtoken/SignatureAlgorithm.java:607 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of jwtk/jjwt@fb71496164 (2026-09-09). Data as JSON: /api/errors/3bff82f1a056a714. Report an issue: GitHub.

Appendix: source

Thrown at api/src/main/java/io/jsonwebtoken/SignatureAlgorithm.java:607

            try {
                encoded = secretKey.getEncoded();
                bitLength = io.jsonwebtoken.lang.Arrays.length(encoded) * Byte.SIZE;
            } finally {
                Arrays.fill(encoded, (byte) 0);
            }

            for (SignatureAlgorithm alg : PREFERRED_HMAC_ALGS) {
                // ensure compatibility check is based on key length. See https://github.com/jwtk/jjwt/issues/381
                if (bitLength >= alg.minKeyLength) {
                    return alg;
                }
            }

            String msg = "The specified SecretKey is not strong enough to be used with JWT HMAC signature " +
                    "algorithms.  The JWT specification requires HMAC keys to be >= 256 bits long.  The specified " +
                    "key is " + bitLength + " bits.  See https://tools.ietf.org/html/rfc7518#section-3.2 for more " +
                    "information.";
            throw new WeakKeyException(msg);
        }

        if (key instanceof RSAKey) {

            RSAKey rsaKey = (RSAKey) key;
            int bitLength = rsaKey.getModulus().bitLength();

            if (bitLength >= 4096) {
                RS512.assertValidSigningKey(key);
                return RS512;
            } else if (bitLength >= 3072) {
                RS384.assertValidSigningKey(key);
                return RS384;
            } else if (bitLength >= RS256.minKeyLength) {
                RS256.assertValidSigningKey(key);
                return RS256;
            }

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