JJWT (io.jsonwebtoken)

Facts and the checks behind them

Nothing below is copied from documentation. The check content/libraries/verify/jjwt/run.sh ran JJWT (io.jsonwebtoken) 0.13.0 in the container image maven:3.9.16-eclipse-temurin-25 on October 20, 2026; the page must match what it printed, and each fact lists those lines.

• JWS algorithms

  HS256HS384HS512RS256RS384RS512PS256PS384PS512ES256ES384ES512EdDSA

  HS, RS, PS and ES at 256/384/512 and EdDSA signed and verified; ES256K, Ed25519 and Ed448 are not in Jwts.SIG.

  Recorded by the check

  fact jws-algorithms = HS256, HS384, HS512, RS256, RS384, RS512, PS256, PS384, PS512, ES256, ES384, ES512, EdDSA
  evidence jws-algorithms: ES256K: not in Jwts.SIG
  evidence jws-algorithms: Ed25519: not in Jwts.SIG
  evidence jws-algorithms: Ed448: not in Jwts.SIG

• ES256 signature encoding

  raw

  ES256 signatures are 64 bytes of raw r‖s.

  Recorded by the check

  fact jws-ecdsa-encoding = raw

• DER signature inside a JWS

  rejected

  A DER-encoded ES256 signature fails with Provided signature is 576 bits (72 bytes) but ES256 signatures must be exactly 512 bits (64 bytes).

  Recorded by the check

  fact jws-der-signature = rejected
  evidence jws-der-signature: SignatureException: Unable to verify Elliptic Curve signature using provided ECPublicKey: Provided signature is 576 bits (72 bytes) but ES256 signatures must be exactly 512 bits (64 bytes) per [RFC 7518, Section 3.4 (validation)](https://www.rfc-editor.org/rfc/rfc7518.html#section-3.4).

• Algorithm allow-list

  from-key

  The verification key determines the algorithm: an RS256 token against an EC public key failed with UnsupportedJwtException.

  Recorded by the check

  fact jwt-algorithm-allowlist = from-key
  evidence jwt-algorithm-allowlist: RS256 token, verifyWith(EC public key): UnsupportedJwtException: The parsed JWT indicates it was signed with the 'RS256' signature algorithm, but the provided sun.security.ec.ECPublicKeyImpl key may not be used to verify RS256 signatures.  Because the specified key reflects a specific and expected algorithm, and the JWT does not reflect this algorithm, it is likely that the JWT was not expected and therefore should not be trusted.  Another possibility is that the parser was provided the incorrect signature verification key, but this cannot be assumed for security reasons.

• Unsigned tokens (alg none)

  opt-in

  parseSignedClaims rejects unsigned tokens; Jwts.parser().unsecured() with parseUnsecuredClaims reads them.

  Recorded by the check

  fact jwt-alg-none = opt-in
  evidence jwt-alg-none: parseSignedClaims with the EC key: UnsupportedJwtException: Unsecured JWSs (those with an 'alg' (Algorithm) header value of 'none') are disallowed by default as mandated by https://www.rfc-editor.org/rfc/rfc7518.html#section-3.6. If you wish to allow them to be parsed, call the JwtParserBuilder.unsecured() method, but please read the security considerations covered in that method's JavaDoc before doing so. Header: {alg=none, typ=JWT}
  evidence jwt-alg-none: parser().unsecured().parseUnsecuredClaims: verified

• Public key used as an HMAC secret

  rejected

  An HS256 token keyed with the PEM text was rejected against the EC public key, but verified with Keys.hmacShaKeyFor(pemBytes).

  Recorded by the check

  fact jwt-hmac-public-key = rejected
  evidence jwt-hmac-public-key: verifyWith(EC public key): UnsupportedJwtException: The parsed JWT indicates it was signed with the 'HS256' signature algorithm, but the provided sun.security.ec.ECPublicKeyImpl key may not be used to verify HS256 signatures.  Because the specified key reflects a specific and expected algorithm, and the JWT does not reflect this algorithm, it is likely that the JWT was not expected and therefore should not be trusted.  Another possibility is that the parser was provided the incorrect signature verification key, but this cannot be assumed for security reasons.
  evidence jwt-hmac-public-key: verifyWith(Keys.hmacShaKeyFor(PEM bytes)): verified

• Curve checked against alg

  enforced

  Signing ES256 with a P-384 key fails: the 'ES256' algorithm requires EC Keys with 256 bits.

  Recorded by the check

  fact jwt-ecdsa-curve-check = enforced
  evidence jwt-ecdsa-curve-check: SignatureException: Unable to compute ES256 signature. Cause: The provided Elliptic Curve signing key size (aka order bit length) is 384 bits (48 bytes), but the 'ES256' algorithm requires EC Keys with 256 bits (32 bytes) per [RFC 7518, Section 3.4](https://www.rfc-editor.org/rfc/rfc7518.html#section-3.4).

• Default clock skew

  0s

  No clock skew by default: a token expired three seconds earlier was rejected.

  Recorded by the check

  fact jwt-clock-skew = 0s

• Expiry claim required

  no

  A token without exp parsed successfully.

  Recorded by the check

  fact jwt-exp-required = no

• Short HMAC secret

  rejected

  Keys.hmacShaKeyFor rejected a 16-byte key with WeakKeyException.

  Recorded by the check

  fact jwt-hmac-short-secret = rejected
  evidence jwt-hmac-short-secret: Keys.hmacShaKeyFor(16 bytes): WeakKeyException: The specified key byte array is 128 bits which is not secure enough for any JWT HMAC-SHA algorithm.  The JWT JWA Specification (RFC 7518, Section 3.2) states that keys used with HMAC-SHA algorithms MUST have a size >= 256 bits (the key size must be greater than or equal to the hash output size).  Consider using the Jwts.SIG.HS256.key() builder (or HS384.key() or HS512.key()) to create a key guaranteed to be secure enough for your preferred HMAC-SHA algorithm.  See https://tools.ietf.org/html/rfc7518#section-3.2 for more information.

Usage

import io.jsonwebtoken.Claims;
import io.jsonwebtoken.Jwts;
import java.security.KeyPair;
import java.util.Date;

public class Usage {
    public static void main(String[] args) {
        KeyPair pair = Jwts.SIG.ES256.keyPair().build();
        String token = Jwts.builder()
            .subject("user-42")
            .issuer("https://auth.example.com")
            .expiration(new Date(System.currentTimeMillis() + 600_000))
            .signWith(pair.getPrivate(), Jwts.SIG.ES256)
            .compact();

        // The key decides which algorithms are acceptable; require iss and exp explicitly
        // and allow some clock skew (the default is none).
        Claims claims = Jwts.parser()
            .verifyWith(pair.getPublic())
            .requireIssuer("https://auth.example.com")
            .clockSkewSeconds(30)
            .build()
            .parseSignedClaims(token)
            .getPayload();
        System.out.println("sub: " + claims.getSubject() + ", has exp: " + (claims.getExpiration() != null));
    }
}