'How to verify JWT signature using a token and public key in Java

I have a token in the form of a string and I downloaded the public cert and created a public key out of it as follows.

But I'm not sure how proceed for verification with just this much info.

I found solutions for C# and .NET but not for Java. Please note I don't have the jks file or private key.

    FileInputStream fin = new FileInputStream("d://public.crt");
    CertificateFactory f = CertificateFactory.getInstance("X.509");
    X509Certificate certificate = (X509Certificate)f.generateCertificate(fin);
    PublicKey pk = certificate.getPublicKey();
javajwt


Solution 1:[1]

To verify a JWT in Java using Auth0 library (com.auth0:java-jwt):

  1. Retrieve the algorithm the key has been signed with, for example:

    // Load your public key from a file
final PublicKey ecdsa256PublicKey = getPublicKey(...);
final Algorithm algorithm = Algorithm.ECDSA256((ECPublicKey) ecdsa256PublicKey, null);

  2. Verify its signature using the corresponding algorithm:

    final DecodedJWT decodedJWT = JWT.decode("J.W.T[...]");
// Will throw a SignatureVerificationException if the token's signature is invalid
algorithm.verify(decodedJWT);

Solution 2:[2]

I did something like this to verify JWT

try {
        DecodedJWT decodedJWT = JWT.decode(jwt); // your string
        JwkProvider provider =  new JwkProviderBuilder(new URL("JWKS URL")).build();
        Jwk jwk = provider.get(decodedJWT.getKeyId());
        Algorithm algorithm = Algorithm.RSA256((RSAPublicKey) jwk.getPublicKey(), null);

        Verification verifier = JWT.require(algorithm);
        verifier.build().verify(decodedJWT);
    } catch (JWTVerificationException | JwkException | MalformedURLException e) {
        // throw your exception
    }

JwkProviderBuilder can be expensive, so if you are using Spring, you can extract it as another method and annotate it with @PostConstruct to optimise.

Solution 3:[3]

A working example with RSA key is as given below:

/* Verification of JWT */
try {
    String token = "some-token";
    String publicKey = "some-key";
    
    //Convert public key string to RSAPublicKey
    byte[] publicKeyByteArr = Base64.getDecoder().decode(publicKey);
    KeyFactory keyFactory = KeyFactory.getInstance("RSA");
    RSAPublicKey rsaPublicKey = (RSAPublicKey) keyFactory.generatePublic(new X509EncodedKeySpec(publicKeyByteArr));

    //If the token has an invalid signature, JWTVerificationException will raise.
    Algorithm algorithm = Algorithm.RSA256(rsaPublicKey, null);
    JWTVerifier verifier = JWT.require(algorithm)
                //.withIssuer("auth0")
                .build(); //Reusable verifier instance
    DecodedJWT jwt = verifier.verify(token);

}catch(InvalidKeySpecException | NoSuchAlgorithmException | JWTVerificationException e) {
    logger.info("JWT verification is failed");
    throw new ResponseStatusException(HttpStatus.UNAUTHORIZED);
}

It's obvious but please note that token and publicKey are arbitrary.

Sources

This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.

Source: Stack Overflow

Solution Source
Solution 1 Florian Lopes
Solution 2
Solution 3 Sezerb

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