Sign emails with DKIM
DKIM is short for DomainKeys Identified Mail.
Adding a signature looks like this:
// C#
var certificate = new X509Certificate2("Test.pfx");
var rsa = (RSACryptoServiceProvider)certificate.PrivateKey;
IMail email = Mail
.Text("text")
.From("alice@mail.com")
.To("bob@mail.com")
.Subject("subject")
.DKIMSign(rsa, "brisbane", "example.com")
.Create();
' VB.NET
Dim certificate = New X509Certificate2("Test.pfx")
Dim rsa = DirectCast(certificate.PrivateKey, RSACryptoServiceProvider)
Dim email As IMail = Mail _
.Text("text") _
.From("alice@mail.com") _
.[To]("bob@mail.com") _
.Subject("subject") _
.DKIMSign(rsa, "brisbane", "example.com") _
.Create()
So what you need is RSACryptoServiceProvider with your private key, and two strings: selector and domain.
Basically how this works is the recipient of the email queries the DNS server for TXT record for selector._domainkey.domain (in our sample it is: “brisbane._domainkey.example.com”) to get the public key and validate the signature.
You can use nslookup to get the public key for a domain:
Microsoft Windows [Version 6.1.7600]
Copyright (c) 2009 Microsoft Corporation. All rights reserved.
C:UsersPawel>nslookup
Default Server: UnKnown
Address: 192.168.0.1
> set type=TXT
> gamma._domainkey.gmail.com
Server: UnKnown
Address: 192.168.0.1
Non-authoritative answer:
gamma._domainkey.gmail.com text =
"k=rsa; t=y; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDIhyR3oItOy22ZOaBrI
Ve9m/iME3RqOJeasANSpg2YTHTYV+Xtp4xwf5gTjCmHQEMOs0qYu0FYiNQPQogJ2t0Mfx9zNu06rfRBD
jiIU9tpx2T+NGlWZ8qhbiLo5By8apJavLyqTLavyPSrvsx0B3YzC63T4Age2CDqZYA+OwSMWQIDAQAB"
gmail.com nameserver = ns1.google.com
gmail.com nameserver = ns4.google.com
gmail.com nameserver = ns3.google.com
gmail.com nameserver = ns2.google.com
ns1.google.com internet address = 216.239.32.10
ns2.google.com internet address = 216.239.34.10
ns3.google.com internet address = 216.239.36.10
ns4.google.com internet address = 216.239.38.10
>
But don’t worry as Mail.dll will make this DNS query for you.
Validating (including DNS query for public key) is simple:
// C#
IMail email = new MailBuilder()
.CreateFromEmlFile("signed_gamma.gmail.eml");
bool isValid = email.CheckDKIMSignature();
' VB.NET
Dim email As IMail = New MailBuilder() _
.CreateFromEmlFile("signed_gamma.gmail.eml")
Dim isValid As Boolean = email.CheckDKIMSignature()