Manually validating a JWT token in C#

Check out ValidateJwt() in this sample:

https://github.com/IdentityServer/IdentityServer4/blob/master/samples/Clients/old/MvcManual/Controllers/HomeController.cs

The bit you're missing is loading the public key from the discovery document.

Try changing the length of your private key. Your private key is too small to be encoded I suppose.

For manual verification, you could just use

static byte[] FromBase64Url(string base64Url)
        {
            string padded = base64Url.Length % 4 == 0
                ? base64Url : base64Url + "====".Substring(base64Url.Length % 4);
            string base64 = padded.Replace("_", "/")
                                  .Replace("-", "+");
            return Convert.FromBase64String(base64);
        }

This also answers @henk-holterman 's question

Encoding.UTF8.GetBytes( can't be the right way to do this.

Although realistically a better way to do this is via the OIDC discovery-endpoint Auth0 has a good article about this using standard NuGet packages. Basically, you load everything needed from the discovery end-point.

IConfigurationManager<OpenIdConnectConfiguration> configurationManager = new ConfigurationManager<OpenIdConnectConfiguration>($"{auth0Domain}.well-known/openid-configuration", new OpenIdConnectConfigurationRetriever());
OpenIdConnectConfiguration openIdConfig = await configurationManager.GetConfigurationAsync(CancellationToken.None);
TokenValidationParameters validationParameters =
    new TokenValidationParameters
    {
        ValidIssuer = auth0Domain,
        ValidAudiences = new[] { auth0Audience },
        IssuerSigningKeys = openIdConfig.SigningKeys
    };
SecurityToken validatedToken;
JwtSecurityTokenHandler handler = new JwtSecurityTokenHandler();
var user = handler.ValidateToken("eyJhbGciOi.....", validationParameters, out validatedToken);

You can read more about it here Or their GitHub sample page about this here

In my case, I did not have a discovery endpoint. Just a JWKS endpoint. So I opted to do this.

using Microsoft.IdentityModel.Tokens;
using Newtonsoft.Json;
using RestSharp;
using System;
using System.Collections.Generic;
using System.IdentityModel.Tokens.Jwt;
using System.Linq;
using System.Threading.Tasks;

public class ExpectedJwksResponse
    {
        [JsonProperty(PropertyName = "keys")]
        public List<JsonWebKey> Keys { get; set; }
    }

private static async Task<List<SecurityKey>> GetSecurityKeysAsync()
        {
            // Feel free to use HttpClient or whatever you want to call the endpoint.
            var client = new RestClient("<https://sample-jwks-endpoint.url>");
            var request = new RestRequest(Method.GET);
            var result = await client.ExecuteTaskAsync<ExpectedJwksResponse>(request);
            if (result.StatusCode != System.Net.HttpStatusCode.OK)
            {
                throw new Exception("Wasnt 200 status code");
            }

            if (result.Data == null || result.Data.Keys == null || result.Data.Keys.Count == 0 )
            {
                throw new Exception("Couldnt parse any keys");
            }

            var keys = new List<SecurityKey>();
            foreach ( var key in result.Data.Keys )
            {
                keys.Add(key);
            }
            return keys;
        }

private async Task<bool> ValidateToken(token){
    TokenValidationParameters validationParameters = new TokenValidationParameters
            {
                RequireExpirationTime = true,
                RequireSignedTokens = true,
                ValidateLifetime = true,
                ValidIssuer = "https://sample-issuer.com",
                ValidAudiences = new[] { "https://sample-audience/resource" },
                IssuerSigningKeys = await GetSecurityKeysAsync()
            };
    var user = null as System.Security.Claims.ClaimsPrincipal;
    SecurityToken validatedToken;
    try
    {
        user = handler.ValidateToken(token, validationParameters, out validatedToken);
    }
    catch ( Exception e )
    {
        Console.Write($"ErrorMessage: {e.Message}");
        return false;
    }
    var readToken = handler.ReadJwtToken(token);
    var claims = readToken.Claims;
    return true;
}