/// <summary>
/// based on  http://www.contentmaster.com/azure/creating-a-jwt-token-to-access-windows-azure-mobile-services
/// Reimplemented cryptographic part
/// </summary>
public class JsonWebToken
{
    /// <summary>
    /// Create a HMACSHA256 Signing HASH 
    /// </summary>
    /// <param name="signingKey"></param>
    /// <param name="bytesToSign"></param>
    /// <returns></returns>
    private static byte[] HMACSHA256(byte[] signingKey, byte[] bytesToSign)
    {
        var signingKeyBuffer = CryptographicBuffer.CreateFromByteArray(signingKey);
        var bytesToSignBuffer = CryptographicBuffer.CreateFromByteArray(bytesToSign);

        var hmacAlgorithm = MacAlgorithmProvider.OpenAlgorithm(MacAlgorithmNames.HmacSha256);
        var hash = hmacAlgorithm.CreateHash(signingKeyBuffer);
        hash.Append(bytesToSignBuffer);
        string base64Hash = CryptographicBuffer.EncodeToBase64String(hash.GetValueAndReset());
        return Convert.FromBase64String(base64Hash);
    }

    public static string Encode(object payload, string key)
    {
        return Encode(payload, Encoding.UTF8.GetBytes(key));
    }

    public static string Encode(object payload, byte[] keyBytes)
    {
        var segments = new List<string>();
        var header = new { alg = "HS256", typ = "JWT", kid = 0 };
        byte[] headerBytes = Encoding.UTF8.GetBytes(JsonConvert.SerializeObject(header, Formatting.None));
        byte[] payloadBytes = Encoding.UTF8.GetBytes(JsonConvert.SerializeObject(payload, Formatting.None));
        segments.Add(Base64UrlEncode(headerBytes)); segments.Add(Base64UrlEncode(payloadBytes));
        var stringToSign = string.Join(".", segments.ToArray());
        var bytesToSign = Encoding.UTF8.GetBytes(stringToSign);
        byte[] signature = HMACSHA256(keyBytes, bytesToSign);
        segments.Add(Base64UrlEncode(signature));
        return string.Join(".", segments.ToArray());
    }

    // 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;
    }

    internal static string TestJWT()
    {
        var privateKey = "secret";
        var issueTime = DateTime.Now;
        var utc0 = new DateTime(1970, 1, 1, 0, 0, 0, 0, DateTimeKind.Utc);
        var exp = (int)issueTime.AddMinutes(60).Subtract(utc0).TotalSeconds;
        var payload = new
        {
            exp = exp,
            ver = 1,
            aud = "[Your AUD]",
            uid = "[A unique identifier for the authenticated user]"
        };
        return JsonWebToken.Encode(payload, privateKey);
    }
}

Enjoy your day