A Nguyen A Nguyen - 1 month ago 21
C# Question

c# How to verify signature JWT?

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

enter image description here


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();

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

                RSAPKCS1SignatureDeformatter rsaDeformatter = new RSAPKCS1SignatureDeformatter(rsa);
                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.