c# Base64 Encoding Decoding wrong result

Question

I need to create a hash-signature in c#.

The pseudo-code example that i need to implement in my c# code:

Signatur(Request) = new String(encodeBase64URLCompatible(HMAC-SHA-256(getBytes(Z, "UTF-8"), decodeBase64URLCompatible(getBytes(S, "UTF-8")))), "UTF-8")

Z: apiSecret S: stringToSign

The coding for expectedSignatur and apiSecret is Base64 URL Encoding [RFC 4648 Section 5]

My problem is that I always get the wrong result.

public static string Base64Decode(string base64EncodedData)
{
    var base64EncodedBytes = Convert.FromBase64String(base64EncodedData);
    return Encoding.UTF8.GetString(base64EncodedBytes);
}

public static string Base64Encode(string plainText)
{
    var plainTextBytes = Encoding.UTF8.GetBytes(plainText);
    return Convert.ToBase64String(plainTextBytes);
}

private static byte[] HmacSha256(string data, string key)
{
     using (var hmac = new HMACSHA256(Encoding.UTF8.GetBytes(key)))
     {
       return hmac.ComputeHash(Encoding.UTF8.GetBytes(data));
     }
}

static void Main(string[] args)
{
     var apiSecret = "JrXRHCnUegQJAYSJ5J6OvEuOUOpy2q2-MHPoH_IECRY=";
     var stringToSign = "f3fea5f3-60af-496f-ac3e-dbb10924e87a:20160201094942:e81d298b-60dd-4f46-9ec9-1dbc72f5b5df:Qg5f0Q3ly1Cwh5M9zcw57jwHI_HPoKbjdHLurXGpPg0yazdC6OWPpwnYi22bnB6S";
     var expectedSignatur = "ps9MooGiTeTXIkPkUWbHG4rlF3wuTJuZ9qcMe-Y41xE=";

     apiSecret = apiSecret.Replace('-', '+').Replace('_', '/').PadRight(apiSecret.Length + (4 - apiSecret.Length % 4) % 4, '=');

     var secretBase64Decoded = Base64Decode(apiSecret);

     var hmac = Convert.ToBase64String(HmacSha256(secretBase64Decoded, stringToSign));

     var signatur = hmac.Replace('+', '-').Replace('/', '_');


     Console.WriteLine($"signatur: {signatur}");
     Console.WriteLine($"expected: {expectedSignatur}");

     Console.WriteLine(signatur.Equals(expectedSignatur));
     Console.ReadLine();
 }

Answer 1

You're assuming that your key was originally text encoded with UTF-8 - but it looks like it wasn't. You should keep logically binary data as binary data - you don't need your Base64Encode and Base64Decode methods at all. Instead, your HmacSha256 method should take a byte[] as a key, and you can just use Convert.FromBase64String to get at those bytes from the base64-encoded secret:

using System;
using System.Text;
using System.Security.Cryptography;

class Test
{    
    private static byte[] HmacSha256(byte[] key, string data)
    {
        using (var hmac = new HMACSHA256(key))
        {
            return hmac.ComputeHash(Encoding.UTF8.GetBytes(data));
        }
    }

    static void Main(string[] args)
    {
        var apiSecret = "JrXRHCnUegQJAYSJ5J6OvEuOUOpy2q2-MHPoH_IECRY=";
        var stringToSign = "f3fea5f3-60af-496f-ac3e-dbb10924e87a:20160201094942:e81d298b-60dd-4f46-9ec9-1dbc72f5b5df:Qg5f0Q3ly1Cwh5M9zcw57jwHI_HPoKbjdHLurXGpPg0yazdC6OWPpwnYi22bnB6S";
        var expectedSignatur = "ps9MooGiTeTXIkPkUWbHG4rlF3wuTJuZ9qcMe-Y41xE=";

        apiSecret = apiSecret.Replace('-', '+').Replace('_', '/').PadRight(apiSecret.Length + (4 - apiSecret.Length % 4) % 4, '=');

        var secretBase64Decoded = Convert.FromBase64String(apiSecret);

        var hmac = Convert.ToBase64String(HmacSha256(secretBase64Decoded, stringToSign));

        var signatur = hmac.Replace('+', '-').Replace('/', '_');
        Console.WriteLine($"signatur: {signatur}");
        Console.WriteLine($"expected: {expectedSignatur}");            
        Console.WriteLine(signatur.Equals(expectedSignatur));
    }
}

Personally I'd change your HmacSha256 method to:

private static byte[] ComputeHmacSha256Hash(byte[] key, byte[] data)
{
    using (var hmac = new HMACSHA256(key))
    {
        return hmac.ComputeHash(data);
    }
}

so that it's more general purpose, maybe adding another method to compute the hash after encoding as UTF-8 for convenience. That way you can sign any data, not just strings.