Verifying JWT Signature Using a Public Key Endpoint

Question

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

The endpoint returns an array of public JSON keys. Here is one example from an 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 implemented the SigningKeyResolver interface, and I need to return an RSAPublicKey instance for verification. I had a problem with correctly creating this key from JSON.

Should I start with a module and an 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)); 

Should I start with x5c, generate an X509Certificate object and extract a PublicKey from it?

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

Both approaches were fruitless.

If I generated RSAPublicKey modulo and exponent, can I print the encoded Base64Binary key according to the x5c property? Maybe this is not the way I should argue.

I might misunderstand how to use this.

As always, any documentation is appreciated.

Answer 1

x5c contains a certification chain. The first certificate in the chain must match the key value represented by other values ​​in the JWK, in this case n and e , so the public key extracted from x5c[0] and the one created using n and e must be exactly the same

JWK values ​​are encoded in base64url, not base64. The 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()));