DomainKeys Identified Mail (DKIM)

DKIM (DomainKeys Identified Mail) is an E-Mail domain authentication method, designed to protect E-Mail sender domains (RFC5322.From) from forgery (spoofing). DKIM is defined in RFC 6376 with updates in RFC 8301 and RFC 8463. DKIM is a requirement of DMARC.

Functionality

Each message is digitally signed by the sending server when it's being sent. DKIM works with a public key and a private key for signing and a selector for identification.

Selector

The selector assures that multiple DKIM records can be set on a single sender domain. Selectors can be any phrase. Here's an example from RFC 6376:

   selectors might indicate the names of office locations (e.g.,
   "sanfrancisco", "coolumbeach", and "reykjavik"), the signing date
   (e.g., "january2005", "february2005", etc.), or even an individual
   user.

Public and private key

The public key is publicly accessible in a DNS TXT record. The selector is used to compile a subdomain for this DNS record. If, for example, the selector is "reykjavik" and the senderdomain is "email.example.com", the following subdomain should be created: reykjavik._domainkey.email.example.com. The content of the DNS TXT record may look like this:

v=DKIM1; t=s; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA0vuPa8g6qdfYLi9TWfbMzFoijdNfJC6/a0uGfIj6fOr+z1fJlOsM1DhKaEaSkNeI0ClKjLx9648CfMl02TxViTvG1Ne2sDsFvGc53NzEd65I2BsPuLpBsHo5zXbZ1ZvLhFm+iOjXlPnD1WlOeQuDhFdIdR+1lWt5aExNwBvIqBr+nYfJt094h9fUwXxMpJ+75GtBdAo3j2nOlWlZtCkWnDmCsXd0j6nNrHz0fO8VqCcJmQsP1ThUgBlO7T3L4PiVg1yHbDpKyTgVb6zHpYt/cXiKmIxVn6nQoDxL9ZfQ2EmVi7hUfMcSoFpWdIpYuOnMmPgPk47J+YZjv4N2X6UpSQIDAQAB

There are numerous switches that can be applied to a DKIM record. The ones we see here are:

Caption text

Switch	Example	Description	Required
v	v=DKIM1	Version	Required
t	t=s	Alignment / Testing	Recommended
k	k=rsa	Key type	Optional
p	p=LONG KEY	Public key	Required

The long key (p-switch) is the public key that matches the private key on the signing server. This key can be obtained from your [ISP]/[ESP] or your mail server administrator.

Once a message has been received, the DKIM signature can be found in the E-Mail header and it looks like this:

   DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=email.example.com;
	s=reykjavik; t=1117574938; i=@email.example.com;
	bh=MTIzNDU2Nzg5MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTI=;
	h=To:Message-ID:Date:Content-Type:Subject:From:List-Unsubscribe:
	 From:To:Cc:Subject;
	b=nXiJoG9QuMwPyLsCw0yCx2bCd92K89bGgOb/nUsFpUuHvRfM9M1QnQaPdTaJu7pBm
	 2Yl7xHdSqXj6cU2Y2MoDeFgBkFpSa14ZiByX7VwPq8eGiNzB2580l52LtBeVxKtWrH
	 By9oU96j4h7bMxRgYvTe/r7dWaHbGaIwMwNc4eXa=

The meaning of the individual switches we see in the example is as follows:

Caption text

Switch	Example	Description	Required
v	v=1	Version	Required
a	a=rsa-sha256	Key type / Signing algorithm	Required
c	c=relaxed/relaxed	Canonicalization algorithm(s) for header and body	Optional
d	d=email.example.com	Signing Domain Identifier (SDID)	Required
s	s=reykjavik	Selector	Required
t	t=1117574938	Timestamp	Recommended
i	i=@email.example.com	Sending domain (AUID)	Optional
bh	bh=MTIzNDU2Nzg5MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTI=	Body hash	Required
h	h=To:Message-ID:Date:Content-Type:Subject:From:List-Unsubscribe: From:To:Cc:Subject;	list of header fields that have been signed	Required
b	b=nXiJoG9QuMwPyLsCw0yCx2bCd92K89bGgOb/nUsFpUuHvRfM9M1QnQaPdTaJu7pBm 2Yl7xHdSqXj6cU2Y2MoDeFgBkFpSa14ZiByX7VwPq8eGiNzB2580l52LtBeVxKtWrH By9oU96j4h7bMxRgYvTe/r7dWaHbGaIwMwNc4eXa=	Signature of headers and body	Required

Alignment

Main article: Alignment

DKIM is aligned when the sender domain (also "RFC5322.From" or "Agent or User Identifier"), represented in the i-switch, matches the "Signing Domain Identifier", represented in the d-switch.

Double DKIM

Main article: Double DKIM

Useful links

• RFC 6376
• RFC 8301
• RFC 8463