DkimVerifier Class

public class DkimVerifier : DkimVerifierBase

using System;
using System.IO;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Collections.Generic;

using Heijden.DNS;

using Org.BouncyCastle.Crypto;

using MimeKit;
using MimeKit.Cryptography;

namespace DkimVerifierExample
{
    // Note: By using the DkimPublicKeyLocatorBase, we avoid having to parse the DNS TXT records
    // in order to get the public key ourselves.
    class ExamplePublicKeyLocator : DkimPublicKeyLocatorBase
    {
        readonly Dictionary<string, AsymmetricKeyParameter> cache;
        readonly Resolver resolver;

        public ExamplePublicKeyLocator ()
        {
            cache = new Dictionary<string, AsymmetricKeyParameter> ();

            resolver = new Resolver ("8.8.8.8") {
                TransportType = TransportType.Udp,
                UseCache = true,
                Retries = 3
            };
        }

        AsymmetricKeyParameter DnsLookup (string domain, string selector, CancellationToken cancellationToken)
        {
            var query = selector + "._domainkey." + domain;
            AsymmetricKeyParameter pubkey;

            // checked if we've already fetched this key
            if (cache.TryGetValue (query, out pubkey))
                return pubkey;

            // make a DNS query
            var response = resolver.Query (query, QType.TXT);
            var builder = new StringBuilder ();

            // combine the TXT records into 1 string buffer
            foreach (var record in response.RecordsTXT) {
                foreach (var text in record.TXT)
                    builder.Append (text);
            }

            var txt = builder.ToString ();

            // DkimPublicKeyLocatorBase provides us with this helpful method.
            pubkey = GetPublicKey (txt);

            cache.Add (query, pubkey);

            return pubkey;
        }

        public AsymmetricKeyParameter LocatePublicKey (string methods, string domain, string selector, CancellationToken cancellationToken = default (CancellationToken))
        {
            var methodList = methods.Split (new char[] { ':' }, StringSplitOptions.RemoveEmptyEntries);
            for (int i = 0; i < methodList.Length; i++) {
                if (methodList[i] == "dns/txt")
                    return DnsLookup (domain, selector, cancellationToken);
            }

            throw new NotSupportedException (string.Format ("{0} does not include any suported lookup methods.", methods));
        }

        public Task<AsymmetricKeyParameter> LocatePublicKeyAsync (string methods, string domain, string selector, CancellationToken cancellationToken = default (CancellationToken))
        {
            return Task.Run (() => {
                return LocatePublicKey (methods, domain, selector, cancellationToken);
            }, cancellationToken);
        }
    }

    class Program
    {
        public static void Main (string[] args)
        {
            if (args.Length == 0) {
                Help ();
                return;
            }

            for (int i = 0; i < args.Length; i++) {
                if (args[i] == "--help") {
                    Help ();
                    return;
                }
            }

            var locator = new ExamplePublicKeyLocator ();
            var verifier = new DkimVerifier (locator);

            for (int i = 0; i < args.Length; i++) {
                if (!File.Exists (args[i])) {
                    Console.Error.WriteLine ("{0}: No such file.", args[i]);
                    continue;
                }

                Console.Write ("{0} -> ", args[i]);

                var message = MimeMessage.Load (args[i]);
                var index = message.Headers.IndexOf (HeaderId.DkimSignature);

                if (index == -1) {
                    Console.WriteLine ("NO SIGNATURE");
                    continue;
                }

                var dkim = message.Headers[index];

                if (verifier.Verify (message, dkim)) {
                    // the DKIM-Signature header is valid!
                    Console.ForegroundColor = ConsoleColor.Green;
                    Console.WriteLine ("VALID");
                    Console.ResetColor ();
                } else {
                    // the DKIM-Signature is invalid!
                    Console.ForegroundColor = ConsoleColor.Red;
                    Console.WriteLine ("INVALID");
                    Console.ResetColor ();
                }
            }
        }

        static void Help ()
        {
            Console.WriteLine ("Usage is: DkimVerifier [options] [messages]");
            Console.WriteLine ();
            Console.WriteLine ("Options:");
            Console.WriteLine ("  --help               This help menu.");
        }
    }
}