Verifying JWT Signature using public key endpoint

Question

I'm wanting to verify the signature of some JWTs from Microsoft. I'm using Spring-Boot, the JJWT library and following endpoint: https://login.microsoftonline.com/common/discovery/v2.0/keys

The endpoint returns an array of JSON public keys. Here is one example from the array.

 {
            "kty": "RSA",
            "use": "sig",
            "kid": "9FXDpbfMFT2SvQuXh846YTwEIBw",
            "x5t": "9FXDpbfMFT2SvQuXh846YTwEIBw",
            "n": "kvt1VmR4nwkNM8jMU0wmj2gSS8NznbOt2pZI6Z7HQT_esF7W19GZR7Y72Xo1i5zXRDM9o3GeTIjBrnr3yy41Q_EaUQ7C-b-Hmg94Vy7EBZyBhi_mznz0dYWs2MIXwR86Nni9TmgTXvjgTPF2YGJoZt4TwcMFefW8rijCVyNrCBA0XspDouNJavvG0BEMXYigoThFjLRXS5U3h4BDfNZFZZS3dyliNOXfgRn2k7oITz8h_ueiPvmDRFh38AeQgx1cELhKWc3P5ugtttraSwgH7nP2NUguO9nCrHuL6TZ-KWpmRWZqwH-jYKFQVt3CDpzwNM6XJL-oHbl1x-gI3YYX5w",
            "e": "AQAB",
            "x5c": [
                "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"
            ],
            "issuer": "https://login.microsoftonline.com/{tenantid}/v2.0"
        }

In JJWT I've implemented the SigningKeyResolver interface and I am required to return an instance of RSAPublicKey to do the verification. The issue I'm having is creating that Key correctly from the JSON.

Do I start with the Modulus and Exponent?

BigInteger modulus = new BigInteger(1, Base64.decodeBase64(jsonKey.getN()));
BigInteger exponent = new BigInteger(1, Base64.decodeBase64(jsonKey.getE()));
publicKey = KeyFactory.getInstance("RSA").generatePublic(new RSAPublicKeySpec(modulus, exponent));

Do I start with the x5c, generate an X509Certificate object and pull the PublicKey from there?

CertificateFactory factory = CertificateFactory.getInstance("X.509");
X509Certificate cert = (X509Certificate) factory
          .generateCertificate(new ByteArrayInputStream(
           DatatypeConverter.parseBase64Binary(jsonKey.getX5c())));
publicKey = (RSAPublicKey)cert.getPublicKey();

Both approaches have proved fruitless.

If I generated the RSAPublicKey from the modulus and exponent should I be able to print the Base64Binary encoded key to match the x5c property? Maybe that's not how I should be validating.

I might be misunderstanding how to use this.

As always, any documentation is appreciated as well.

Answer 1

x5c contains the certification chain. The first certificate of the chain must match with the key value represented by the other values in the JWK, in this case n and e, therefore the public key extracted from x5c[0] and the one built with n and e must be exactly the same

JWK values are encoded in base64url, not in base64. Change

BigInteger modulus = new BigInteger(1, Base64.decodeBase64(jsonKey.getN()));
BigInteger exponent = new BigInteger(1, Base64.decodeBase64(jsonKey.getE()));

with

BigInteger modulus = new BigInteger(1, Base64.getUrlDecoder().decode(jsonKey.getN()));
BigInteger exponent = new BigInteger(1, Base64.getUrlDecoder().decode(jsonKey.getE()));