app.cs

using System;
using System.Text;
using System.Web.Script.Serialization;
using System.Security.Cryptography;

/*
 *  The App class is a simple example using pre-defined constants to be able to test the
 *  FidoAuthenticator class. These constants are examples of what would be generated in the
 *  browser when using the Web Authentication APIs.
 */
class App {
	const string challenge = "Y2xpbWIgYSBtb3VudGFpbg";
	const string pk = "{ 	\"kty\" : \"RSA\", 	\"alg\" : \"RS256\", 	\"ext\" : false, 	\"n\" : \"k-d_ZbVlAu-EBhRNlevnd0cBqJEPlhBefOMMLgHeGTS28Uev6_xHVCZ774I2XdtVF-M5En0aTdekAX82K2SVnO9TEZ4GdJFR1kW1jppLtrlUyjQqggq60OwkUxHM14XDQBhvgeW3fjpETraB-R_scyGeK7lNWMF8jW-NjvU_nljTyuHoDVnYJxPuCunlQ7uzg80iURp0jFKhw7FedPkzyQllG0HRRfzwQG1WOyECxkdPmMk7iPdF-B-Z78S9Fd8Dx2R8OZHEpFMdQ3Z3bMLG8prSbXcmBXlBtmwSLPzEEb3FuPdJQtXzVyg2i3jAR25zWA_XemXHBv7XE5Mf3JYldQ\", 	\"e\" : \"AQAB\" }";
	const string d = "ew0KCSJjaGFsbGVuZ2UiIDogIlkyeHBiV0lnWVNCdGIzVnVkR0ZwYmciLA0KCSJ1c2VyUHJvbXB0IiA6ICJIZWxsbyEiDQp9AA";
	const string s = "M-GT64y3FXoFQI8fRPq8ogckxuVYqv65R2eJEXGpbmVtm3Zn9Oa6ik4nClFMsN4h42e9bSBslMTEKW-J1oAoxF8n4JkDH82b9j4bFhhSRMHCbmE-uZm1RX8zVrGIgoWnXDy2nGQSu5xN-BhGubru1x0sXo9ZAdXKc-5hkp6SfIdXAY15o9flsag_H_CpIJ1_L1-vO5K8xhya_iOezflNlqa8-D1lI-xMJ7dOqyPwqg33ryW4l6iTtexuiYhZaGOOyJ5ZxzchjKrw9zMgQOsjbsrM7Q6bu7K7YvOoULxM5WJFdCLj0OBZznrskEHlLrSe0TSr_WrY1SkLhRaUCetKkg";
	const string a = "AQAAAAA";

	static void Main() {
		var v = FidoAuthenticator.validateSignature(pk,d,a,s,challenge) ? "verified" : "unverified";
		Console.WriteLine(v);
	}
}

/*
 *  The FidoAuthenticator class contains the logic for validating the signature returned from getAssertion in the
 *  browser. This code is currently specific to the early implementation in Microsoft Edge and will need to change
 *  when the final standard is adopted. This code would run on the server in order to validate that the user
 *  really is able to validate using the credentials previously created with the makeCredential API.
 *
 *  The public key in pk would have been stored on the server using the results of makeCredential. The challenge
 *  would have been created on ther server and sent to the client for use in the getAssertion call. The other
 *  parameters are returned by getAssertion and transmitted from the browser to the server for validation. 
 */
class FidoAuthenticator {
	public static bool validateSignature(string pk, string clientData, string authnrData, string signature, string challenge)
	{
		try {
			var c = rfc4648_base64_url_decode(clientData);
			var a = rfc4648_base64_url_decode(authnrData);
			var s = rfc4648_base64_url_decode(signature);

			// Make sure the challenge in the client data matches the expected challenge
			var cc = Encoding.ASCII.GetString(c);
			cc = cc.Replace("\0","").Trim();
			var json = new JavaScriptSerializer();
			var j = (System.Collections.Generic.Dictionary<string,object>)json.DeserializeObject(cc);
			if((string)j["challenge"] != challenge) return false;

			// Hash data with sha-256
			var hash = new SHA256Managed();
			var h = hash.ComputeHash(c);

			// Create data buffer to verify signature over
			var b = new byte[a.Length + h.Length];
			a.CopyTo(b,0);
			h.CopyTo(b,a.Length);

			// Load public key
			j = (System.Collections.Generic.Dictionary<string,object>)json.DeserializeObject(pk);
			var keyinfo = new RSAParameters();
			keyinfo.Modulus = rfc4648_base64_url_decode((string)j["n"]);
			keyinfo.Exponent = rfc4648_base64_url_decode((string)j["e"]);
			var rsa = new RSACng();
			rsa.ImportParameters(keyinfo);

			// Verify signature is correct for authnrData + hash
			return rsa.VerifyData(b,s,HashAlgorithmName.SHA256,RSASignaturePadding.Pkcs1);
		} catch(Exception) {
			return false;
		}
	}

	private static byte[] rfc4648_base64_url_decode(string url) {
		url = url.Replace('-','+');
		url = url.Replace('_','/');

		switch(url.Length % 4) { // Pad with trailing '='s
			case 0:
				// No pad chars in this case
				break;
			case 2:
				// Two pad chars
				url += "==";
				break;
			case 3:
				// One pad char
				url += "=";
				break;
			default:
				throw new Exception("Invalid string.");
		}
		return Convert.FromBase64String(url);
	}
}