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MimeKit

IDkimPublicKeyLocator Interface

An interface for a service which locates and retrieves DKIM public keys (probably via DNS).

Namespace:  MimeKit.Cryptography
Assembly:  MimeKit (in MimeKit.dll) Version: 2.14.0
Syntax
C#
public interface IDkimPublicKeyLocator

The IDkimPublicKeyLocator type exposes the following members.

Methods
  NameDescription
Public methodCode exampleLocatePublicKey
Locate and retrieve the public key for the given domain and selector.
Public methodCode exampleLocatePublicKeyAsync
Asynchronously locate and retrieve the public key for the given domain and selector.
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Remarks

An interface for a service which locates and retrieves DKIM public keys (probably via DNS).

Since MimeKit itself does not implement DNS, it is up to the client to implement public key lookups via DNS.

Examples
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.");
        }
    }
}
Examples
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 ArcVerifierExample
{
    // 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 ArcVerifier (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 result = verifier.Verify (message);

                switch (result.Chain) {
                case ArcSignatureValidationResult.None:
                    Console.WriteLine ("No ARC signatures to verify.");
                    break;
                case ArcSignatureValidationResult.Pass:
                    Console.ForegroundColor = ConsoleColor.Green;
                    Console.WriteLine ("PASS");
                    Console.ResetColor ();
                    break;
                case ArcSignatureValidationResult.Fail:
                    Console.ForegroundColor = ConsoleColor.Red;
                    Console.WriteLine ("FAIL");
                    Console.ResetColor ();
                    break;
                }
            }
        }

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