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MimeKit

IDkimPublicKeyLocatorLocatePublicKey Method

Locate and retrieve the public key for the given domain and selector.

Namespace: MimeKit.Cryptography
Assembly: MimeKit (in MimeKit.dll) Version: 4.3.0
Syntax
C#
AsymmetricKeyParameter LocatePublicKey(
	string methods,
	string domain,
	string selector,
	CancellationToken cancellationToken = default
)

Parameters

methods  String
A colon-separated list of query methods used to retrieve the public key. The default is "dns/txt".
domain  String
The domain.
selector  String
The selector.
cancellationToken  CancellationToken  (Optional)
The cancellation token.

Return Value

AsymmetricKeyParameter
The public key.
Remarks

Locates and retrieves the public key for the given domain and selector.

Example
C#
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.");
        }
    }
}
See Also