SecurityTokenSignatureKeyNotFoundException when validating JWT signature

Question

I'm trying to implement the OpenID Connect specification for my organisation. I'm using Microsoft's OWIN implementation of OpenID Connect in a test relying party application to verify my implementation of the protocol.

I've exposed the following metadata document:

{
  "issuer": "https://acs.contoso.com/",
  "authorization_endpoint": "http://localhost:53615/oauth2/auth",
  "token_endpoint": "http://localhost:53615/oauth2/token",
  "userinfo_endpoint": "http://localhost:53615/connect/userinfo",
  "jwks_uri": "http://localhost:53615/connect/keys",
  "ui_locales_supported": [
    "en-GB"
  ]
}

The signing key is exposed as this document:

{
  "keys": [
    {
      "n": "xpXxl3M-YkZlzQJdArO1TfOGT2no-UL4dbZ7WuSCNIsSfyGDaqUXjMMHNyq9yD3vp-NCyk8kmn7d5XqHufnceXJM8q4xTrhN3lvywdBSbR-dwXsA-B-MJVgfiK0d_z-mxP9ew2Hj9-KkWbWCzsswlWp3gZ4mB4RGutB1IRSzXVIbvZ-MtKUb6XUDU4LDb_c1xCEXWZxhR-o1a1dLfObH2hHJ-w5y6odGlKtOFx4i4h0u7-Oj5R6k5b2YXEHM0IuYeN0u0sQvrTecokntGzPrvhnKy69I7Z_az5rC5kgloh25D9lTbe4vcRU7FXlYCFYDZsT0_IkGIXRi7brOS4f1ow",
      "e": "AQAB",
      "kty": "RSA",
      "use": "sig",
      "alg": "RS256",
      "kid": "F8A59280B3D13777CC7541B3218480984F421450"
    }
  ]
}

The identity token is being generated using the JwtSecurityToken class and its associated handler, using the X509SigningCredentials class. This code is representative of how the token is constructed and returned to the calling system as a parameter of the response data.

var credentials = new X509SigningCredentials(cert); // My certificate.
var issuedTime = DateTime.UtcNow;
var expiresTime = issuedTime.AddMinutes(5);
var epoch = new DateTime(1970, 01, 01, 0, 0, 0);

var claims = new[]
{
    new Claim("sub", Guid.NewGuid().ToString()),
    new Claim("iat" Math.Floor((issuedTime - epoch).TotalSeconds).ToString()),
    new Claim("nonce", nonce), // Value from client
}

var token = new JwtSecurityToken(
    "https://acs.contoso.com",
    client_id, // Value from client
    claims,
    new Lifetime(issuedTime, expiresTime),
    credentials);

var handler = new JwtSecurityTokenHandler();
parameters.Add("id_token", handler.WriteToken(token)); // Outgoing parameters.

When I attempt to pass the signed token back to the relying party application, the OWIN middleware accepts the POST and attempts to verify the signature of the token. In doing so, the following exception is thrown:

SecurityTokenSignatureKeyNotFoundException: IDX10500: Signature validation failed. Unable to resolve SecurityKeyIdentifier: 'SecurityKeyIdentifier ( IsReadOnly = False, Count = 1, Clause[0] = X509ThumbprintKeyIdentifierClause(Hash = 0xF8A59280B3D13777CC7541B3218480984F421450) ) ', token: '{"typ":"JWT","alg":"RS256","x5t":"-KWSgLPRN3fMdUGzIYSAmE9CFFA"}.{"iss":"https://test.accesscontrol.net/","aud":"test","nbf":1404917162,"exp":1404917462,"sub":"60eb55ec-0699-4068-bfa6-41666fc2b2e9","iat":"1404917162"} RawData: eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiIsIng1dCI6Ii1LV1NnTFBSTjNmTWRVR3pJWVNBbUU5Q0ZGQSJ9.eyJpc3MiOiJodHRwczovL2Fjcy5zdXJlY2xvdWQuY29tLyIsImF1ZCI6InRlc3QiLCJuYmYiOjE0MDQ5MTcxNjIsImV4cCI6MTQwNDkxNzQ2Miwic3ViIjoiNjBlYjU1ZWMtMDY5OS00MDY4LWJmYTYtNDE2NjZmYzJiMmU5IiwiaWF0IjoiMTQwNDkxNzE2MiJ9.xkP0RwlX3CYfU0KhFsVvLJC94WK22DTqNTm71cfjiJ8VUHv3b2YhDqfq70N8mQEyiR8vTR6OQqnO6UqXqX4RXUs6ZkfK9Liv3n9NhCs97wJhP2jfefJYeScYtRmWcNNWSSL7vkm2JXQfwKOQTnOGp-ba04TtI6jVrjhOQXH43eCJ9vNuBUzdD-t8CAdmnbvH0nWpIB8kWbw5v8Sa0aQuxMjJYbLC_2Iw3X13dqnyVjp4fA7eSB8N7c1it0KEB-VKfUqiGD3VecyEZGGZbaGE8rvVet5QrY1lJ3V4yM8j6-xDc5Yndc4swOun0L3D6TYk-8gdVXUJDRjbv1ZuhZltsw'.

The component is still pre-release, so this may be a flaw in the implementation, however I want to assume it's my error until all possibilities have been ruled out.

Is there anything I'm doing which is obviously wrong, or is there something I should do to understand exactly why the signature is failing to be validated?

Answer 1

The problem is nestled in the exception message here:

Clause[0] = X509ThumbprintKeyIdentifierClause(Hash = 0xF8A59280B3D13777CC7541B3218480984F421450)

The token is signed with the default key identifier clause for an X.509 certificate: its thumbprint. The metadata is exposing just the RSA parameters and a name identifier. When the client retrieves the metadata, it sets up an RSA key using this information, not an X.509 thumbprint.

To correct this error, the signing credentials have to be changed to include the correct name identifier:

var credentials = new X509CertificateCredentials(
    cert,
    new SecurityKeyIdentifier(
        new NamedKeySecurityKeyIdentifierClause(
            "kid",
            "F8A59280B3D13777CC7541B3218480984F421450")));

This includes the expected identifier in the signature, and the signature is validated successfully.