using System;

using System.Linq;

using System.Security.Cryptography;

using System.Text;

​

public class Program

{

    public static void Main(string[] args)

    {

        string publicKey = "MIGbMBAGByqGSM49AgEGBSuBBAAjA4GGAAQBgc4HZz+/fBbC7lmEww0AO3NK9wVZPDZ0VEnsaUFLEYpTzb90nITtJUcPUbvOsdZIZ1Q8fnbquAYgxXL5UgHMoywAib476MkyyYgPk0BXZq3mq4zImTRNuaU9slj9TVJ3ScT3L1bXwVuPJDzpr5GOFpaj+WwMAl8G7CqwoJOsW7Kddns=";

        string token = "eyJ0eXAiOiJKV1QiLCJhbGciOiJFUzUxMiJ9.eyJpc3MiOiJPbmxpbmUgSldUIEJ1aWxkZXIiLCJpYXQiOjE1NjUwOTk2ODgsImV4cCI6MTU5ODE4OTg4NSwiYXVkIjoiaHR0cDovL2xvY2FsaG9zdDozNTg4Iiwic3ViIjoiaHR0cDovL2xvY2FsaG9zdDo1MDM3NiIsImZpcnN0bmFtZSI6IkFydmluZCIsImxhc3RuYW1lIjoiS3VtYXIiLCJFbWFpbCI6ImFydmluZC5rdW1hckBzdHJlYW1hbWcuY29tIiwiSWQiOiIxMDEifQ.AVwAJeY44lKrnywnDs7CdUOu3gli2cGafSJ6iP3zT7lkZpd2QnL0k54aVmPVxAGuN5dDnzbYmMTdRl5u2QE92ccOAHrcf5yA2gsvhhAGuDAAeh6Io4VU7v5TOTvwWGRb-ubgdjUvagA_HSJOyeXvFR16_M_MzGfDnXfg02sj4y9VFjDr";

        VerifyES512Jwt(token, publicKey);

    }

​

    public static void VerifyES512Jwt(string token, string publicKey)

    {

        byte[] publicKeyBytes = Convert.FromBase64String(publicKey);

​

        string[] parts = token.Split('.');

        string header = parts[0];

        string payload = parts[1];

        string signature = parts[2];

​

        // Load key

        var keyType = new byte[] { 0x45, 0x43, 0x53, 0x35 };

        var keyLength = new byte[] { 0x42, 0x00, 0x00, 0x00 };

        var key = keyType.Concat(keyLength).Concat(publicKeyBytes.Skip(publicKeyBytes.Length - 132)).ToArray();

        CngKey cngKey = CngKey.Import(key, CngKeyBlobFormat.EccPublicBlob);

        ECDsaCng eCDsaCng = new ECDsaCng(cngKey);

​

        // Verify JWT

        byte[] headerPayloadBytes = Encoding.UTF8.GetBytes(header + "." + payload);

        byte[] signatureBytes = Base64UrlDecode(signature);

        bool verified = eCDsaCng.VerifyData(headerPayloadBytes, signatureBytes, HashAlgorithmName.SHA512);

        Console.WriteLine("Verification: " + verified);

    }

​

​

    // https://stackoverflow.com/a/26354677/9014097

    private static byte[] Base64UrlDecode(string base64urlString)

    {

        string base64String = base64urlString.Replace('_', '/').Replace('-', '+');

        switch (base64urlString.Length % 4)

        {

            case 2: base64String += "=="; break;

            case 3: base64String += "="; break;

        }

        byte[] bytes = Convert.FromBase64String(base64String);

​

        return bytes;

    }

}