C# Validate JWT token with values from an authorize end point (crv, x, y)

Question

I have a web page being displayed on a vendors site in an iframe. They call my page and pass a JWT in the query string. I have to parse the jwt.

eyJraWQiOiIyMDIwLTA5LTAyVDE3OjM2OjE3LjU3MC5lYyIsInR5cCI6IkpXVCIsImFsZyI6IkVTMjU2In0.eyJzdWIiOiJ1cm46Y2VybmVyOmlkZW50aXR5LWZlZGVyYXRpb246cmVhbG06SFdPb0lsUlgyWWRGZjkyNGJBZTZSR0l5WmtuajZrTjctY2g6cHJpbmNpcGFsOnRhNDh6OWdkNTVkNndyNW0iLCJhdWQiOiJodHRwczpcL1wvdXJsMjU4dmowai5leGVjdXRlLWFwaS51cy1lYXN0LTIuYW1hem9uYXdzLmNvbSIsImlzcyI6Imh0dHBzOlwvXC9kZXYuYmF5Y2FyZS5wYXRpZW50cG9ydGFsLnVzLTEuaGVhbHRoZWludGVudC5jb20iLCJleHAiOjE1OTkxNTQ1MTYsImlhdCI6MTU5OTE1MzkxNiwic2lkIjoiZGUwNmJhNmUtYjQyYy00ZmY5LWI4MmQtYmM4NjY0ODJmODU4In0.6Ru5Lyd1Zq016uv84pP-GjSuz6koVNipa_cd939eF21-5N2_A0Nj3I6AkDhuHrE870WzyTiCmZfkIjMOFZkRCA

I am suppose to verify the signature by pulling values from the https://authorizion.x.com/jwk. I get values that look like this:

{
   "keys":[
      {
         "kty":"EC",
         "crv":"P-256",
         "kid":"2020-09-04T18:16:04.934.ec",
         "x":"82WEbXbnfGC1kmMfjJch6gFJRp7hEp08gzZQdBLLFIk",
         "y":"ytkPwl4IjLw8M94DzgTmdAbxjq0AmmYu9mMmxpU3eBI"
      },
      {
         "kty":"EC",
         "crv":"P-256",
         "kid":"2020-09-02T17:36:17.570.ec",
         "x":"uAfEPKELRuUVMtB0DCB5oyYWnfiV8-9zHYntvI0lsRE",
         "y":"32J6nVgeb9RLdWK21QNDHhWdOsZJbxvyEq2n0IOvLtQ"
      },
      {
         "kty":"EC",
         "crv":"P-256",
         "kid":"2020-08-31T17:36:17.359.ec",
         "x":"HsxFY2vihycZgYnkSTLDHJ0Cagr2nUcZTbf2yQKPS6A",
         "y":"4kLClPGM0TG_gCUlBKkYdXrlLFVasPxQ2UOvwSBKyt0"
      }
   ]
}

I pick the key based on the kid value in the header of the JWT. How do I validate the signature based on the crv, x and y values? It seems like I need either a public or private certificate but I don't have that.

What more can I add. It is using ECDSA p-256 encryption. There is a Json Web Token above and at jwt.io I get the following values,

Header

{
  "kid": "2020-09-02T17:36:17.570.ec",
  "typ": "JWT",
  "alg": "ES256"
}

Payload

{
  "sub": "urn:cerner:identity-federation:realm:HWOoIlRX2YdFf924bAe6RGIyZknj6kN7-ch:principal:ta48z9gd55d6wr5m",
  "aud": "https://url258vj0j.execute-api.us-east-2.amazonaws.com",
  "iss": "https://dev.baycare.patientportal.us-1.healtheintent.com",
  "exp": 1599154516,
  "iat": 1599153916,
  "sid": "de06ba6e-b42c-4ff9-b82d-bc866482f858"
}

Signature

6Ru5Lyd1Zq016uv84pP-GjSuz6koVNipa_cd939eF21-5N2_A0Nj3I6AkDhuHrE870WzyTiCmZfkIjMOFZkRCA

I included the Java Web Key Set matching the JWT.

Answer 1

Here's a very short solution based on Jose.

Basically you need to read the JWKS from the given URL, choose the right JWK with the matching key ID (kid), create a ECJWKey based on the given, Base64 encoded, x and y parameters and then validate the JWT with that key:

The following source code is only a short illustration of the process and the key is hard coded and not read from the JWKS endpoint.

using Jose;
using System;
using System.Security.Cryptography;

namespace JWKValiadation
{
    public class ECJWKey
    {
        public string kty { get; set; }
        public string crv { get; set; }
        public string kid { get; set; }
        public string x { get; set; }
        public string y { get; set; }
    }

    class Program
    {
        static void Main(string[] args)
        {
            ECJWKey ecjwkkey = new ECJWKey
            {
                kty = "EC",
                crv = "P-256",
                kid = "2020-09-02T17:36:17.570.ec",
                x = "uAfEPKELRuUVMtB0DCB5oyYWnfiV8-9zHYntvI0lsRE",
                y = "32J6nVgeb9RLdWK21QNDHhWdOsZJbxvyEq2n0IOvLtQ"
            };

            string tokenEC = "eyJraWQiOiIyMDIwLTA5LTAyVDE3OjM2OjE3LjU3MC5lYyIsInR5cCI6IkpXVCIsImFsZyI6IkVTMjU2In0.eyJzdWIiOiJ1cm46Y2VybmVyOmlkZW50aXR5LWZlZGVyYXRpb246cmVhbG06SFdPb0lsUlgyWWRGZjkyNGJBZTZSR0l5WmtuajZrTjctY2g6cHJpbmNpcGFsOnRhNDh6OWdkNTVkNndyNW0iLCJhdWQiOiJodHRwczpcL1wvdXJsMjU4dmowai5leGVjdXRlLWFwaS51cy1lYXN0LTIuYW1hem9uYXdzLmNvbSIsImlzcyI6Imh0dHBzOlwvXC9kZXYuYmF5Y2FyZS5wYXRpZW50cG9ydGFsLnVzLTEuaGVhbHRoZWludGVudC5jb20iLCJleHAiOjE1OTkxNTQ1MTYsImlhdCI6MTU5OTE1MzkxNiwic2lkIjoiZGUwNmJhNmUtYjQyYy00ZmY5LWI4MmQtYmM4NjY0ODJmODU4In0.6Ru5Lyd1Zq016uv84pP-GjSuz6koVNipa_cd939eF21-5N2_A0Nj3I6AkDhuHrE870WzyTiCmZfkIjMOFZkRCA";

            // first read the header to get the kid
            var headers = Jose.JWT.Headers(tokenEC);
            if(headers.TryGetValue("kid", out var keyId))
            {
                // in a real application you would need the kid 
                // to select the right key from the JKWS
                Console.WriteLine(keyId);
            }

            // create the key based on the parameters from the JWK
            ECDsa eckey = ECDsa.Create(new ECParameters
            {
                Curve = ECCurve.NamedCurves.nistP256,
                Q = new ECPoint
                {
                    X = Base64Url.Decode(ecjwkkey.x),
                    Y = Base64Url.Decode(ecjwkkey.y)
                }
            });
            
            // verify and decode the token
            string payload = Jose.JWT.Decode(tokenEC, eckey);
            Console.WriteLine(payload);
        }
    }
}