How do I decode a JWT token using an RSA public key in PEM format?

My Java application is receiving a JWT. I have the public key in PEM format:

I know I can use a JwtConsumer to verify and decode the JWT:

JwtConsumer jwtConsumer = new JwtConsumerBuilder()
    .setRequireExpirationTime()
    .setVerificationKey(publicKey) // what do I pass here?
    .build();

But how do I convert my PEM file into a format understood by .setVerificationKey()?

Solution

With v0.5.0 there is some utility support for dealing with the PEM encoded public keys, RsaKeyUtil.fromPemEncoded(String pem), which can maybe simplify things for you a bit. You can also get claim values directly from the JwtClaims object, which might also simplify. Here's your example with those slight modifications:

    String jwt = "eyJhbGciOiJSUzI1NiJ9.eyJ1c2VybmFtZSI6Ik1DaGFtYmU0IiwiZXhwIjoxNDU2OTEwODgzLCJzY29wZSI6WyJvcGVuaWQiLCJwMnAiLCJociIsImRhcyIsIm1lIl0sImNsaWVudF9pZCI6Im1vYmlsZSIsImp0aSI6ImNZcHBMYXltVzlmNXFBZk4ifQ.QqZI9vV8IznTjN-GtUSCri9-6HH6Yl1Oae6K8-d2yjQ4fysF5d3wStdL2kMazl7xeqbtSIsw-F5Aol9eHdGAu54b9IyBEM_QIasy0lnT8xFk0Zi36NJ-7yhl_89f6SB6TGimM59xUvzXxuAw3FzWM6TbiptInrCL2TXkhS69Gng-ANPeiSITUX5A1TDInssds6ZoSb7IOUMtxPGfrbO9sBjx8aJlIu9igkqk4OX5xBmxLp3icoo98I5v9Wt_Huu7eWKBfOskMSEav4X_m5_phbAZJ_F8nWRmcxk6O7hCQdawzegnhMxP2IPIhwlWRNX_8WxkNErq2fJgdazDf8pS_Q";

    // read public key from a file or config or something
    String publicKeyPEM =
            "-----BEGIN PUBLIC KEY-----\n" +
            "MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAixn0CGu8/M4txn4pdp8K\n" +
            "m8RQfVa+cHX25/a5sPmzP49u7YlQsRvtOexzgdwDcfUJm3hHMZcbZBtrHKsS8q4Q\n" +
            "QtGQioyVml8EaLuFNFYisaIEldVyRbXFG54FNp03vSU9ImS/cOiM9swo+1w5JgWO\n" +
            "F9efy7JO40LA9E7lv64COUYjFhrn+HRZuKoblL19+Sj49FyXexAUS29UM9PfIdY6\n" +
            "ar1FA8cxzPqW7EkXZ0Mua3IzNnYcjMvUL9TJwoLAAz9S1Tv4Is5jupy9UXkuJ4r8\n" +
            "Jx9DqI3Q3ur0VekYSd5tnTI4K+no9ABCFVv7+6Q45Ec2eB0xMwlqI+phcGhGMVCX\n" +
            "1QIDAQAB\n" +
            "-----END PUBLIC KEY-----";

    RsaKeyUtil rsaKeyUtil = new RsaKeyUtil();
    PublicKey publicKey = rsaKeyUtil.fromPemEncoded(publicKeyPEM);

    // create a JWT consumer
    JwtConsumer jwtConsumer = new JwtConsumerBuilder()
            .setRequireExpirationTime()
            .setVerificationKey(publicKey)
            .build();

    // validate and decode the jwt
    JwtClaims jwtDecoded = jwtConsumer.processToClaims(jwt);
    String username = jwtDecoded.getStringClaimValue("username"); // "MChambe4"

    // ensure the required scope is claimed
    String requiredScope = "das";
    List<String> scopes = jwtDecoded.getStringListClaimValue("scope");
    if (!scopes.stream().anyMatch(scope -> scope.equals(requiredScope))) {
        throw new Exception("Required scope is not claimed: " + requiredScope);
    }
}