Quick and dirty JWT Token decoder class to use in Blazor based projects, since trying to use the official MS Jwt Token libs at present causes things to blow up. :-)

JwtDecoder.cs

using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.Text;

namespace IntranetV6.Client.Classes
{
  public static class StringExtensions
  {
    public static int GetNextHighestMultiple(this int source, int multipicand)
    {
      int result = source;
      while ((result % multipicand) != 0)
      {
        result++;
      }
      return result;
    }
  }

  // Shamelessly ripped from the MS implementation :-)
  public struct JwtRegisteredClaimNames
  {
    //
    // Summary:
    //     http://tools.ietf.org/html/rfc7519#section-4
    public const string Actort = "actort";
    //
    // Summary:
    //     http://tools.ietf.org/html/rfc7519#section-4
    public const string Typ = "typ";
    //
    // Summary:
    //     http://tools.ietf.org/html/rfc7519#section-4
    public const string Sub = "sub";
    //
    // Summary:
    //     http://openid.net/specs/openid-connect-frontchannel-1_0.html#OPLogout
    public const string Sid = "sid";
    //
    // Summary:
    //     http://tools.ietf.org/html/rfc7519#section-4
    public const string Prn = "prn";
    //
    // Summary:
    //     http://tools.ietf.org/html/rfc7519#section-4
    public const string Nbf = "nbf";
    //
    // Summary:
    //     http://tools.ietf.org/html/rfc7519#section-4
    public const string Nonce = "nonce";
    //
    // Summary:
    //     http://tools.ietf.org/html/rfc7519#section-4
    public const string NameId = "nameid";
    //
    // Summary:
    //     http://tools.ietf.org/html/rfc7519#section-4
    public const string Jti = "jti";
    //
    // Summary:
    //     http://tools.ietf.org/html/rfc7519#section-4
    public const string Iss = "iss";
    //
    // Summary:
    //     http://tools.ietf.org/html/rfc7519#section-4
    public const string Iat = "iat";
    //
    // Summary:
    //     http://tools.ietf.org/html/rfc7519#section-4
    public const string GivenName = "given_name";
    //
    // Summary:
    //     http://tools.ietf.org/html/rfc7519#section-4
    public const string FamilyName = "family_name";
    //
    // Summary:
    //     http://tools.ietf.org/html/rfc7519#section-4
    public const string Gender = "gender";
    //
    // Summary:
    //     http://tools.ietf.org/html/rfc7519#section-4
    public const string Exp = "exp";
    //
    // Summary:
    //     http://tools.ietf.org/html/rfc7519#section-4
    public const string Email = "email";
    //
    // Summary:
    //     http://openid.net/specs/openid-connect-core-1_0.html#CodeIDToken
    public const string AtHash = "at_hash";
    //
    // Summary:
    //     http://tools.ietf.org/html/rfc7519#section-4
    public const string CHash = "c_hash";
    //
    // Summary:
    //     http://tools.ietf.org/html/rfc7519#section-4
    public const string Birthdate = "birthdate";
    //
    // Summary:
    //     http://openid.net/specs/openid-connect-core-1_0.html#IDToken
    public const string Azp = "azp";
    //
    // Summary:
    //     http://openid.net/specs/openid-connect-core-1_0.html#IDToken
    public const string AuthTime = "auth_time";
    //
    // Summary:
    //     http://tools.ietf.org/html/rfc7519#section-4
    public const string Aud = "aud";
    //
    // Summary:
    //     http://openid.net/specs/openid-connect-core-1_0.html#IDToken
    public const string Amr = "amr";
    //
    // Summary:
    //     http://openid.net/specs/openid-connect-core-1_0.html#IDToken
    public const string Acr = "acr";
    //
    // Summary:
    //     http://tools.ietf.org/html/rfc7519#section-4
    public const string UniqueName = "unique_name";
    //
    // Summary:
    //     http://tools.ietf.org/html/rfc7519#section-4
    public const string Website = "website";

    //
    // Summary:
    //     Microsoft have to be different, if you want to use roles for auth in the API, you have to use this
    //     rather than the ietf standard "Role"..... (Added by shawty, as not in official JWT constants list)
    public const string Role = "http://schemas.microsoft.com/ws/2008/06/identity/claims/role";

  }

  public class JwtDecoder
  {
    private Dictionary<string, object> _header;
    private Dictionary<string, object> _payload;
    private string _token = String.Empty;

    public JwtDecoder(string token, bool autoDecode = false)
    {
      _token = token;
      if (autoDecode) Decode();
    }

    public void Decode()
    {
      string[] parts = _token.Split('.');
      if (parts.Length != 3) throw new Exception("Malformed JWT token");

      string b64Header = parts[0];
      string b64Payload = parts[1];

      // B64 strings must be a length that is a multiple of 4 to decode them, so ensure this before we try to use them
      if (b64Header.Length % 4 != 0)
      {
        var lengthToBe = b64Header.Length.GetNextHighestMultiple(4);
        b64Header = b64Header.PadRight(lengthToBe, '=');
      }

      if (b64Payload.Length % 4 != 0)
      {
        var lengthToBe = b64Payload.Length.GetNextHighestMultiple(4);
        b64Payload = b64Payload.PadRight(lengthToBe, '=');
      }

      string headerJson = Encoding.UTF8.GetString(Convert.FromBase64String(b64Header));
      string payloadJson = Encoding.UTF8.GetString(Convert.FromBase64String(b64Payload));

      _header = JsonConvert.DeserializeObject<Dictionary<string, object>>(headerJson);
      _payload = JsonConvert.DeserializeObject<Dictionary<string, object>>(payloadJson);

    }

    public object GetKey(string keyName)
    {
      if (!_payload.ContainsKey(keyName)) throw new Exception($"JWT Token does not contain key {keyName}");
      return _payload[keyName];
    }

  }

}