DKIM: What is it and why is it important?

What is DKIM? #

DKIM (DomainKeys Identified Mail) is an email security standard that helps detect whether messages are altered in transit between sending and receiving mailservers. 

DKIM uses public-key cryptography to sign email with a responsible party's private key as it leaves a sending server; recipient servers then use a public key published to the DKIM's domain to verify the source of the message, and that the parts of the message included in the DKIM signature haven't changed since the message was signed.
Once the signature is verified with the public key by the recipient server, the message passes DKIM and is considered authentic.

What is a DKIM record?  #

A DKIM record is a specially formatted DNS TXT record; it stores the public key the receiving mail server will use to verify a message’s signature.

A DKIM record is formed by a name, version, key type, and the public key itself, and is often made available by the provider that is sending your email (for example, Postmark. That’s us 👋).

DKIM record visualization via Postmark

Why is DKIM important?

While DKIM isn't required, having emails that are signed with DKIM appear more legitimate to your recipients and are less likely to go to Junk or Spam folders. Spoofing email from trusted domains is a popular technique for malicious spam and phishing campaigns, and DKIM makes it harder to spoof email from domains that use it.

DKIM is compatible with existing email infrastructure and works with SPF and DMARC to create multiple layers of security for domains sending emails. Mail servers that don’t support DKIM signatures are still able to receive signed messages without any problems. It’s an optional security protocol, and DKIM is not a universally adopted standard.

Even though it’s not required, we recommend you add a DKIM record to your DNS whenever possible to authenticate mail from your domain. We use it to sign messages at Postmark, and ISPs like Yahoo, AOL, and Gmail use it to check incoming messages. We’ve done testing that proved messages are more likely to be delivered when they use these security protocols.

An additional benefit of DKIM is that ISPs use it to build a reputation on your domain over time. As you send email and improve your delivery practices (low spam and bounces, high engagement), you help your domain build a good sending reputation with ISPs, which improves deliverability.

While it’s important to understand what DKIM does, it’s also important to be clear about what it doesn’t solve. Using DKIM will make sure your message hasn’t been altered, but it doesn’t encrypt the contents of your message. Many ESPs use opportunistic TLS to encrypt messages as they move between sender and recipients, but it’s still possible to send unencrypted messages if an email server refuses a TLS connection. Once a message has been delivered, the DKIM signature will remain in the email headers but won’t encrypt the content of the message in any way.

Now that we’ve described what DKIM does, let’s move on to how it protects your domain’s email.

How do DKIM records work? #

DKIM uses two actions to verify your messages. The first action takes place on a server sending DKIM signed emails, while the second happens on a recipient server checking DKIM signatures on incoming messages. The entire process is made possible by a private/public key pair. Your private key is kept secret and safe, either on your own server or with your ESP, and the public key is added to the DNS records for your domain to broadcast it to the world to help verify your messages. Let’s dive a little deeper into how DKIM works on servers that are sending and receiving email.

Diagram of how DKIM protects messages.

Sending a signed DKIM message #

If you run your own mail server, you can generate this pair on your own. Any time you use a service like Google Apps, Campaign Monitor, Postmark, or other email providers that support DKIM they’ll normally generate the key for you.

To give you an idea of how DKIM works, let’s explain the process on Postmark. We keep your private key securely stored on our servers and sign each message as it is sent. When a message is sent we create a hash from the content of the message headers and then use your private key to sign the hash. This signature carries everything a recipient server needs to validate the message and looks like this:

DKIM-Signature: v=1; a=rsa-sha1; c=relaxed/relaxed; s=20130519032151pm;;
h=From:Date:Subject:MIME-Version:Content-Type:Content-Transfer-Encoding:To:Message-ID;; bh=vYFvy46eesUDGJ45hyBTH30JfN4=;

Here’s what each part of the header means:

  • DKIM-Signature: The header registered for DKIM signed messages.
  • v=1; The version of DKIM being used by the sending server.
  • a=rsa-sha1; The algorithm used to generate the hash for the private/public key. There are two officially supported signature algorithms for this hash, rsa-sha1 and rsa-sha256.
  • c=relaxed/relaxed; Sets the canonicalization posture for the sending domain. This regulates whitespace and text wrapping changes in a message. There are two canonicalized postures. `Simple` doesn’t allow any changes, and `relaxed` allows common changes to whitespace and header line-wrapping. Canonicalization in the header and body can be managed individually and uses a header/body format.
  • s=20130519032151pm; Used as a selector for the public DKIM key for verification. Domains can have multiple public DKIM keys, and the selector value makes sure recipient servers are using the correct public key.
  •; The email domain that signed the message. It’s important that your DKIM signature use your domain name here because this bolsters your domain’s reputation with ISPs as you send valid email, regardless of the Email Service Provider you use.
  • From:Date:Subject:MIME-Version:Content-Type:Content-Transfer-Encoding:To:Message-ID; The headers included with the message when it was cryptographically signed.
  •; The identity of the signer and is usually provided as an email address.
  • bh=vYFvy46eesUDGJ45hyBTH30JfN4=; The value of a body hash generated before the message headers are signed.
  • b=iHeFQ+7rCiSQs3DPjR2eUSZSv4i/Kp+sipURfVH7BGf+SxcwOkX7X8R1RVObMQsFcbIxnrq7Ba2QCf0YZlL9iqJf32V+baDI8IykuDztuoNUF2Kk0pawZkbSPNHYRtLxV2CTOtc+x4eIeSeYptaiu7g7GupekLZ2DE1ODHhuP4I=The cryptographic signature of all the preceding information from the DKIM-Signature field. This entry is treated as an empty string during the verification process.

This signature is computed and added to the outgoing email headers. The message is now ready for a recipient server to verify the message hasn’t been modified in transit.

Verifying a signed DKIM message #

Mail systems start DKIM verification by making sure the version number meets the DKIM specification, the identity of the sender’s domain matches the domain set in the signature, and the “h=“ tag contains the From header field.

Once the signature has been validated, the recipient server tries to retrieve the public key for the sending domain. The server uses the “d=“ tag to look up the DNS records for the sending domain, and the “s=“ tag to select the correct DKIM key. Here’s what the public key for Postmark looks like: 3599 IN TXT "k=rsa\; 

The public key decrypts the encrypted hash sent. The receiving mail server then computes its own hash. If the two matches, the message is let through.

How does DKIM prevent domain spoofing? #

DKIM alone does not prevent domain spoofing. It’s possible to sign a message using a DKIM key linked to a different domain than the one specified in the "From" header.

However, if you have a DMARC policy set for your domain, the receiving mail server will check that the DKIM key used to sign the message matches the From domain when determining DMARC compliance.

How does DKIM improve deliverability? #

ISPs like Gmail, Yahoo, and AOL use DKIM as a signal when determining whether a message is legitimate or not. Our testing has found that using email authentication methods like SPF and DKIM is critical to good deliverability.

What do SPF and DMARC have to do with DKIM? #

While DKIM ensures messages aren’t altered in transit between the sending and recipient servers, SPF validates that the sending server is authorized to send messages using a domain in the first place.

DMARC gives domain owners a mechanism for communicating how they’d like unauthenticated messages to be handled by receivers. It uses DKIM and SPF to determine if a message is legitimate and whether it should be delivered to the recipient or blocked.

How do I add a DKIM record to my domain? #

No matter which ESP or mail server you use, the general setup for DKIM is the same. You need a private key stored somewhere safe, and you need to share a public key in your domain’s DNS records. Similar to SPF, DKIM also uses DNS TXT records with a special format.

It’s considered best practice to periodically rotate your DKIM keys. The DKIM standard recommends rotating your keys every quarter, and it also recommends you revoke your old DKIM keys as part of this rotation. The best way to manage this is by adding your new keys, and a few days later removing your old keys DNS records for your domain. Postmark is one of the only ESPs that make it easy to manage this rotation because we keep your old private key active while your new public key propagates.

We have some helpful videos that go in-depth on how to add a DKIM and CNAME records to various DNS providers that can be viewed here.

The very best approach to securing your domain's email is to layer DKIM with SPF and DMARC. Don't miss our other guides on these protocols to learn more about how they work together to protect your domain.

How can I test if I set up DKIM correctly? #

Once you’ve set up DKIM for an email service, send a message to an email address you manage and examine the DKIM-Signature and Authentication-Results headers to ensure DKIM passed successfully.

How can I read message headers?

You can also use DMARC reports to check that the messages sent using your domain are correctly authenticated with DKIM and SPF. A DMARC monitoring service like DMARC Digests will process these reports for you and provide a useful summary of DKIM, SPF, and DMARC results for messages sent from all the email providers you use.

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Resources #

Implementation tools #

Email security guides from Postmark #


Do I need a certificate to run DKIM? #

No. DKIM gives domain owners a quick way to create, setup, and destroy keys.

Does DKIM give my messages end-to-end encryption? #

No. DKIM works as more of a handshake to verify a message hasn’t been tampered in transit to a recipient server.

Wasn’t DKIM compromised in 2012? #

Yes, and no. Zachary Harris found a vulnerability in short DKIM keys that allowed him to factor 512-bit keys in about 24 hours. The DKIM standard calls for a minimum key length of 1024-bits, so the shorter keys were compromised not the DKIM standard.

Postmark uses 1024-bit keys to sign all email for our customers.

How Can I Read the DKIM Header? #

Most email clients have a way of viewing the raw headers for a message.

In Gmail, select the Show original option from the context menu (⁝) in the top right corner of the message.

In the Mail app on Mac, select the message then go to ViewMessageRaw Source.

In Outlook, right-click the message and select View Source.

Once you have access to the raw message headers, look for the DKIM-Signature header to confirm which DKIM key was used to sign the message.

The Authentication-Results header shows the results of the DKIM and SPF checks carried out by the receiving mail server.

       dkim=pass header.s=google header.b="ga9/RuJg";
       spf=pass ( domain of designates as permitted sender);
       dmarc=pass (p=REJECT sp=REJECT dis=NONE)

Can I have multiple DKIM records? #

Yes. A domain can have multiple DKIM records in the DNS. Each DKIM key has a different selector which is added to a message's DKIM signature. This tells the receiving mail server which DKIM key should be used for validation.

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This post was originally published Sep 05, 2017

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