jwtk/jjwt · error · IllegalArgumentException

The algorithm does not support shared secret keys.

Error message

The  algorithm does not support shared secret keys.

What it means

Deprecated Keys.secretKeyFor(SignatureAlgorithm) throws IllegalArgumentException when the given algorithm is not a MAC (shared-secret) algorithm, i.e. an asymmetric algorithm like RS256/ES256 was requested as a secret key.

Solutions

1. Only call secretKeyFor with HMAC algorithms: HS256, HS384, HS512
2. Use Keys.keyPairFor(alg) instead for asymmetric algorithms
3. Prefer the modern API: Jwts.SIG.HS256.key().build() (or the matching SIG instance)
4. Check alg.name().startsWith("HS") before generating a secret key

Example fix

// before
SecretKey key = Keys.secretKeyFor(SignatureAlgorithm.RS256); // throws
// after
KeyPair kp = Keys.keyPairFor(SignatureAlgorithm.RS256); // asymmetric path
SecretKey key = Keys.secretKeyFor(SignatureAlgorithm.HS256); // for HMAC

if (!alg.name().startsWith("HS")) throw new IllegalArgumentException(alg + " is asymmetric; use keyPairFor");

static boolean isHmac(io.jsonwebtoken.SignatureAlgorithm alg) {
    return alg != null && alg.name().startsWith("HS");
}

try {
    SecretKey key = Keys.secretKeyFor(alg);
} catch (IllegalArgumentException e) {
    KeyPair kp = Keys.keyPairFor(alg); // fall back to asymmetric generation
}

When it happens

Trigger: Calling Keys.secretKeyFor(SignatureAlgorithm.RS256) (or ES256, PS256, EdDSA) — any non-HMAC SignatureAlgorithm whose resolved SecureDigestAlgorithm is not a MacAlgorithm.

Common situations: Confusing HMAC vs RSA/ECDSA key generation; loops that iterate all SignatureAlgorithm values and generate keys indiscriminately; legacy code after migrating to JJWT 0.12.x APIs.

Understand the failure class

Background: "is deprecated and will be removed" — deprecation warnings for old API names, keywords, and options, and how to migrate before the removal release — this error's family across 29 libraries.

Related errors

• The algorithm does not support Key Pairs.
• bitLength must be an even multiple of 8
• derivedKeyBitLength may not exceed
• EC JWK x,y coordinates do not exist on elliptic curve
• ECPublicKey's ECPoint does not exist on elliptic curve

AI-assisted analysis of jwtk/jjwt@fb71496164 (2026-09-09). Data as JSON: /api/errors/d64f742a3f403b04. Report an issue: GitHub.

Appendix: source

Thrown at api/src/main/java/io/jsonwebtoken/security/Keys.java:145

     * <td>512 bits (64 bytes)</td>
     * </tr>
     * </table>
     *
     * @param alg the {@code SignatureAlgorithm} to inspect to determine which key length to use.
     * @return a new {@link SecretKey} instance suitable for use with the specified {@link SignatureAlgorithm}.
     * @throws IllegalArgumentException for any input value other than {@link io.jsonwebtoken.SignatureAlgorithm#HS256},
     *                                  {@link io.jsonwebtoken.SignatureAlgorithm#HS384}, or {@link io.jsonwebtoken.SignatureAlgorithm#HS512}
     * @deprecated since 0.12.0.  Use your preferred {@link MacAlgorithm} instance's
     * {@link MacAlgorithm#key() key()} builder method directly.
     */
    @SuppressWarnings("DeprecatedIsStillUsed")
    @Deprecated
    public static SecretKey secretKeyFor(io.jsonwebtoken.SignatureAlgorithm alg) throws IllegalArgumentException {
        Assert.notNull(alg, "SignatureAlgorithm cannot be null.");
        SecureDigestAlgorithm<?, ?> salg = Jwts.SIG.get().get(alg.name());
        if (!(salg instanceof MacAlgorithm)) {
            String msg = "The " + alg.name() + " algorithm does not support shared secret keys.";
            throw new IllegalArgumentException(msg);
        }
        return ((MacAlgorithm) salg).key().build();
    }

    /**
     * <p><b>Deprecation Notice</b></p>
     *
     * <p>As of JJWT 0.12.0, asymmetric key algorithm instances can generate KeyPairs of suitable strength
     * for that specific algorithm by calling their {@code keyPair()} builder method directly. For example:</p>
     *
     * <blockquote><pre>
     * Jwts.SIG.{@link Jwts.SIG#RS256 RS256}.keyPair().build();
     * Jwts.SIG.{@link Jwts.SIG#RS384 RS384}.keyPair().build();
     * Jwts.SIG.{@link Jwts.SIG#RS512 RS512}.keyPair().build();
     * ... etc ...
     * Jwts.SIG.{@link Jwts.SIG#ES512 ES512}.keyPair().build();</pre></blockquote>
     *
     * <p>Call those methods as needed instead of this static {@code keyPairFor} helper method - the returned

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