c# How to verify signature JWT?

Question

I have a token, a file containing public key and I want to verify the signature. I tried to verify signature based on this.

However, decodedCrypto and decodedSignature don't match.

Here is my code:

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();
    }

I'm sure that the signature of token is valid. I try to verify on https://jwt.io/ and it showed that Signature verified. So the problem is the algorithm to encode, decode.

Is there anyone can solve this problem? The algorithm is RS256

Answer 1

I finally got a solution from my colleague.

For those who have the same problem, try my code:

public static string Decode(string token, string key, bool verify = true)
{
    string[] parts = token.Split('.');
    string header = parts[0];
    string payload = parts[1];
    byte[] crypto = Base64UrlDecode(parts[2]);

    string headerJson = Encoding.UTF8.GetString(Base64UrlDecode(header));
    JObject headerData = JObject.Parse(headerJson);

    string payloadJson = Encoding.UTF8.GetString(Base64UrlDecode(payload));
    JObject payloadData = JObject.Parse(payloadJson);

    if (verify)
    {
        var keyBytes = Convert.FromBase64String(key); // your key here

        AsymmetricKeyParameter asymmetricKeyParameter = PublicKeyFactory.CreateKey(keyBytes);
        RsaKeyParameters rsaKeyParameters = (RsaKeyParameters)asymmetricKeyParameter;
        RSAParameters rsaParameters = new RSAParameters();
        rsaParameters.Modulus = rsaKeyParameters.Modulus.ToByteArrayUnsigned();
        rsaParameters.Exponent = rsaKeyParameters.Exponent.ToByteArrayUnsigned();
        RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
        rsa.ImportParameters(rsaParameters);

        SHA256 sha256 = SHA256.Create();
        byte[] hash = sha256.ComputeHash(Encoding.UTF8.GetBytes(parts[0] + '.' + parts[1]));

        RSAPKCS1SignatureDeformatter rsaDeformatter = new RSAPKCS1SignatureDeformatter(rsa);
        rsaDeformatter.SetHashAlgorithm("SHA256");
        if (!rsaDeformatter.VerifySignature(hash, FromBase64Url(parts[2])))
            throw new ApplicationException(string.Format("Invalid signature"));
    }

    return payloadData.ToString();
}

It works for me. The algorithm is RS256.