I'm trying to write a custom JWT token generator for C# .NET on Windows for use in Google Firebase. The source I lifted from a few different places here on StackOverflow (I'm trying to re-discover them so I can credit the original authors and ask a couple of additional questions too), so it's a bit of a hodge-podge together.
For the most part it seems to work, but it doesn't seem to generate a valid (firebase) token as expected. This could be entirely due to my lack of understanding of how it should work in the first place, but if I could get a second pair of world-class-awesome-engineers eyes from here to point out what I did wrong and make things work, it would be nice.
Here's the code:
// Token generator setup
using Newtonsoft.Json;
using System;
using System.IO;
using System.Web;
using System.Web.Configuration;
namespace myapp.Utils
{
public class GoogleJsonWebToken
{
public static string Encode(string uid)
{
var utc0 = new DateTime(1970, 1, 1, 0, 0, 0, 0, DateTimeKind.Utc);
var issueTime = DateTime.Now;
var iat = (int)issueTime.Subtract(utc0).TotalSeconds;
var exp = (int)issueTime.AddMinutes(60).Subtract(utc0).TotalSeconds;
var firebaseInfPath = HttpContext.Current.Server.MapPath(WebConfigurationManager.AppSettings["firebaseInf"]);
var firebaseInfJsonContent = File.ReadAllText(firebaseInfPath);
var firebaseInf = JsonConvert.DeserializeObject<dynamic>(firebaseInfJsonContent);
var privateKey = (string)firebaseInf.private_key;
var payload = new
{
iss = firebaseInf.client_email,
scope = firebaseInf.client_email,
aud = "https://identitytoolkit.googleapis.com/google.identity.identitytoolkit.v1.IdentityToolkit",
exp = exp,
uid = uid,
iat = iat,
claims = new { premium_account = true }
};
return myapp.Utils.JsonWebToken.Encode(payload, privateKey, JwtHashAlgorithm.RS256);
}
}
}
Here's the JWT hash algorithm code:
// JsonWebToken.cs
using Newtonsoft.Json;
using Newtonsoft.Json.Linq;
using System;
using System.Collections.Generic;
using System.Security.Cryptography;
using System.Text;
namespace myapp.Utils
{
public enum JwtHashAlgorithm
{
RS256,
HS384,
HS512
}
public class JsonWebToken
{
private static Dictionary<JwtHashAlgorithm, Func<byte[], byte[], byte[]>> HashAlgorithms;
static JsonWebToken()
{
HashAlgorithms = new Dictionary<JwtHashAlgorithm, Func<byte[], byte[], byte[]>>
{
{ JwtHashAlgorithm.RS256, (key, value) => { using (var sha = new HMACSHA256(key)) { return sha.ComputeHash(value); } } },
{ JwtHashAlgorithm.HS384, (key, value) => { using (var sha = new HMACSHA384(key)) { return sha.ComputeHash(value); } } },
{ JwtHashAlgorithm.HS512, (key, value) => { using (var sha = new HMACSHA512(key)) { return sha.ComputeHash(value); } } }
};
}
public static string Encode(object payload, string key, JwtHashAlgorithm algorithm)
{
var keyBytes = Encoding.UTF8.GetBytes(key);
return Encode(payload, keyBytes, algorithm);
}
public static string Encode(object payload, byte[] keyBytes, JwtHashAlgorithm algorithm)
{
var segments = new List<string>();
var header = new { alg = algorithm.ToString(), typ = "JWT" };
byte[] headerBytes = Encoding.UTF8.GetBytes(JsonConvert.SerializeObject(header, Formatting.None));
byte[] payloadBytes = Encoding.UTF8.GetBytes(JsonConvert.SerializeObject(payload, Formatting.None));
//byte[] payloadBytes = Encoding.UTF8.GetBytes(@"{"iss":"761326798069-r5mljlln1rd4lrbhg75efgigp36m78j5@developer.gserviceaccount.com","scope":"https://www.googleapis.com/auth/prediction","aud":"https://accounts.google.com/o/oauth2/token","exp":1328554385,"iat":1328550785}");
segments.Add(Base64UrlEncode(headerBytes));
segments.Add(Base64UrlEncode(payloadBytes));
var stringToSign = string.Join(".", segments.ToArray());
var bytesToSign = Encoding.UTF8.GetBytes(stringToSign);
byte[] signature = HashAlgorithms[algorithm](keyBytes, bytesToSign);
segments.Add(Base64UrlEncode(signature));
return string.Join(".", segments.ToArray());
}
public static string Decode(string token, string key)
{
return Decode(token, key, true);
}
public static string Decode(string token, string key, bool verify)
{
var parts = token.Split('.');
var header = parts[0];
var payload = parts[1];
byte[] crypto = Base64UrlDecode(parts[2]);
var headerJson = Encoding.UTF8.GetString(Base64UrlDecode(header));
var headerData = JObject.Parse(headerJson);
var payloadJson = Encoding.UTF8.GetString(Base64UrlDecode(payload));
var payloadData = JObject.Parse(payloadJson);
if (verify)
{
var bytesToSign = Encoding.UTF8.GetBytes(string.Concat(header, ".", payload));
var keyBytes = Encoding.UTF8.GetBytes(key);
var algorithm = (string)headerData["alg"];
var signature = HashAlgorithms[GetHashAlgorithm(algorithm)](keyBytes, bytesToSign);
var decodedCrypto = Convert.ToBase64String(crypto);
var decodedSignature = Convert.ToBase64String(signature);
if (decodedCrypto != decodedSignature)
{
throw new ApplicationException(string.Format("Invalid signature. Expected {0} got {1}", decodedCrypto, decodedSignature));
}
}
return payloadData.ToString();
}
private static JwtHashAlgorithm GetHashAlgorithm(string algorithm)
{
switch (algorithm)
{
case "RS256": return JwtHashAlgorithm.RS256;
case "HS384": return JwtHashAlgorithm.HS384;
case "HS512": return JwtHashAlgorithm.HS512;
default: throw new InvalidOperationException("Algorithm not supported.");
}
}
// from JWT spec
private static string Base64UrlEncode(byte[] input)
{
var output = Convert.ToBase64String(input);
output = output.Split('=')[0]; // Remove any trailing '='s
output = output.Replace('+', '-'); // 62nd char of encoding
output = output.Replace('/', '_'); // 63rd char of encoding
return output;
}
// from JWT spec
private static byte[] Base64UrlDecode(string input)
{
var output = input;
output = output.Replace('-', '+'); // 62nd char of encoding
output = output.Replace('_', '/'); // 63rd char of encoding
switch (output.Length % 4) // Pad with trailing '='s
{
case 0: break; // No pad chars in this case
case 2: output += "=="; break; // Two pad chars
case 3: output += "="; break; // One pad char
default: throw new System.Exception("Illegal base64url string!");
}
var converted = Convert.FromBase64String(output); // Standard base64 decoder
return converted;
}
}
}
