fail to verify jwt signature in Java

Question

I am trying to verify a JWT token by verifying its signature. But during the verification I get an error

java.security.SignatureException: Signature length not correct: got 342 but was expecting 256

. I assume that signature is the encrypted sha256 hash of the data. In my case data is base64(header)+"."+base64(body). Here is my code:

static String certificate = "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";
static String signedData = "Z5MwwjtXdypMQGNwmNNuCVmRcDVT24EgtwoDWalF4icxwz7jyB99Yg3262D7OsERewv4cOfdEz3bbOF-iG7YWXeSC9YZeO1tGapqlc8FRtAergSUZC7BcbFEx75MfSy7qLWYTOfdpJesQ23rOzjF7KdrAMJC_Y0T_r6RuBcZVyfT4P55kICETYyv7bBDXc9V8BJUf-QHDu6DaH7u6PSyeOmdzFInI_LwySnMlr3VahoUfUpJmauU8yHQUnFakJBgrMBe1Au9tS-HxtDVnHmoHQw8xGXsVQnEOa1aPAcVWy0v7hILUSmWNAG3IZ0JwUztQitgtnTTzXDszUxTbJ4YlQ";

static String data = "eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiIsIng1dCI6Ik1uQ19WWmNBVGZNNXBPWWlKSE1iYTlnb0VLWSIsImtpZCI6Ik1uQ19WWmNBVGZNNXBPWWlKSE1iYTlnb0VLWSJ9.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";

public static void main(String[] args) throws UnsupportedEncodingException {
    // TODO Auto-generated method stub
    String certString = "-----BEGIN CERTIFICATE-----\r\n" + certificate + "\r\n-----END CERTIFICATE-----";
    Security.addProvider(new org.bouncycastle.jce.provider.BouncyCastleProvider());
        CertificateFactory cf;
        try {
            cf = CertificateFactory.getInstance("X.509");
            InputStream stream = new ByteArrayInputStream(certString.getBytes()); //StandardCharsets.UTF_8
            java.security.cert.Certificate cert = cf.generateCertificate(stream);
            PublicKey pk = cert.getPublicKey();


            //verifying content with signature and content :
            Signature sig = Signature.getInstance("SHA256withRSA");
            sig.initVerify( pk );
            sig.update(data.getBytes());

            Boolean ret = sig.verify(signedData.getBytes());
        } catch (CertificateException | SignatureException | NoSuchAlgorithmException |  InvalidKeyException e) {
            e.printStackTrace();
        }

score 3 · Answer 1 · edited Oct 07 '21 at 10:53

The signature part of a JWT is base64url encoded (see JWS §7.1) so you'll need to properly decode it before verification.

Replace the line Boolean ret = sig.verify(signedData.getBytes()); with something like the following (using the Base64 decoder from Apache Commons Codec),

 byte[] signature = org.apache.commons.codec.binary.Base64.decodeBase64(signedData);
 Boolean ret = sig.verify(signature);

And the signature on that JWT should verify for you.

Having said that, I'd very much recommend using a library for JWT processing. A decent library will handle stuff like that for you and can provide more JWT functionality too.

For example, the following using the jose4j JWT library could replace the stuff from the code in the question after getting the public key from the certificate. This not only verifies the signature per the spec but also validates the claims in the JWT to make sure it's still valid, was issued to you, etc..

 JwtConsumer jwtConsumer = new JwtConsumerBuilder()
    .setVerificationKey(pk)
    .setRequireExpirationTime()
    .setExpectedAudience("https://citrixp.com:8443/")
    .setExpectedIssuer("https://sts.windows.net/dd9b6a3e-29d1-4254-a746-e02941444517/")
    .build();

 JwtClaims claims = jwtConsumer.processToClaims(data + "." + signedData);

 System.out.println("Subject: " + claims.getSubject());
 System.out.println("UPN: " + claims.getStringClaimValue("upn"));  // or whatever, etc....

That JWT expired a few days so the processToClaims will throw an exception, which is what you want. You can add a .setEvaluationTime(NumericDate.fromSeconds(1442626055)) when building the JwtConsumer to get it to work for the given JWT though.

fail to verify jwt signature in Java

1 Answers1