DomainKeys: Proving and Protecting Email Sender Identity

DomainKeys: Proving and Protecting Email Sender Identity

Email spoofing - the forging of another person's or company's email address to get users to trust and open a message - is one of the biggest challenges facing both the Internet community and anti-spam technologists today. Without sender authentication, verification, and traceability, email providers can never know for certain if a message is legitimate or forged and will therefore have to continually make educated guesses on behalf of their users on what to deliver, what to block, and what to quarantine, in the pursuit of the best possible user experience.

DomainKeys is a technology proposal that can bring black and white back to this decision process by giving email providers a mechanism for verifying both the domain of each email sender and the integrity of the messages sent (i.e,. that they were not altered during transit). And, once the domain can be verified, it can be compared to the domain used by the sender in the From: field of the message to detect forgeries. If it's a forgery, then it's spam or fraud, and it can be dropped without impact to the user. If it's not a forgery, then the domain is known, and a persistent reputation profile can be established for that sending domain that can be tied into anti-spam policy systems, shared between service providers, and even exposed to the user.

For well-known companies that commonly send transactional email to consumers, such as banks, utilities, and ecommerce services, the benefits of verification are more profound, as it can help them protect their users from "phishing attacks" - the fraudulent solicitation for account information, such as credit card numbers and passwords, by impersonating the domain and email content of a company to which users have entrusted the storage of these data. For these companies, protecting their users from fraud emails translates directly into user protection, user satisfaction, reduced customer care costs, and brand protection.

For consumers, such as Yahoo! Mail users or a grandparent accessing email through a small mid-western ISP, industry support for sender authentication technologies will mean that they can start trusting email again, and it can resume its role as one of the most powerful communication tools of our times.

Standardization and License Terms

DKIM is the result of the ongoing commitment from numerous industry players to develop an open-standard e-mail authentication specification, and industry collaboration has played a critical role in the process. Industry leaders who played a valuable role in furthering the development of the DKIM specification include, Alt-N Technologies, AOL, Brandenburg Internetworking, Cisco, EarthLink, IBM, Microsoft, PGP Corporation, Sendmail, StrongMail Systems, Tumbleweed, VeriSign and Yahoo!. The participation of these companies has been instrumental in creating this single, signature-based e-mail authentication proposal. The authoring companies will continue to work with these organizations and the IETF on the standardization of the DomainKeys Identified Mail (DKIM) specification so that industry-wide agreement on the best method for validating the identification of email senders can be reached. DomainKeys Identified Mail has begun advancing through the IETF Internet standards process to be ultimately approved as an IETF Internet Standard.

For historical reference, Yahoo! has submitted the DomainKeys framework as an Internet-Draft entitled "draft-delany-domainkeys-base-03.txt. Yahoo!'s DomainKeys Intellectual Property may be licensed under either of the following terms:

- Yahoo! DomainKeys Patent License Agreement
- GNU General Public License version 2.0 (and no other version)

Yahoo!'s DomainKeys Intellectual Property includes the following patent(s) and patent application(s).

A: U.S. Patent Number 6,986,049, issued January 10, 2006
B: U.S. Patent Application Serial Number 10/805,181, filed March 19, 2004
C: PCT Application PCT/US2004/007883, filed March 15, 2004
D: PCT Application PCT/US2005/008656, filed March 15, 2005

In accordance with RFC2026, Yahoo! has also submitted the above license statement to the IETF as an IPR Disclosure. Have license feedback?

Reference Implementation

In addition to the Internet-Draft, Yahoo! has developed a reference implementation for DomainKeys that can be plugged into Message Transfer Agents (MTAs), such as qmail. A version of this software has been released and is available at Sendmail has developed a DomainKey implementation for their popular MTA (both the commercial and freeware versions). In fact, Sendmail, Inc. has released an open source implementation of the Yahoo! DomainKeys specification for testing on the Internet and is actively seeking participants and feedback for this Pilot Program.
How DomainKeys Works

How DomainKeys Works

How it Works - Sending ServersThere are two steps to signing an email with DomainKeys:
Set up: The domain owner (typically the team running the email systems within a company or service provider) generates a public/private key pair to use for signing all outgoing messages (multiple key pairs are allowed). The public key is published in DNS, and the private key is made available to their DomainKey-enabled outbound email servers. This is step "A" in the diagram to the right.

Signing: When each email is sent by an authorized end-user within the domain, the DomainKey-enabled email system automatically uses the stored private key to generate a digital signature of the message. This signature is then pre-pended as a header to the email, and the email is sent on to the target recipient's mail server. This is step "B" in the diagram to the right.
How it Works - Receiving ServersThere are three steps to verifying a signed email:
Preparing: The DomainKeys-enabled receiving email system extracts the signature and claimed From: domain from the email headers and fetches the public key from DNS for the claimed From: domain. This is step "C" in the diagram to the right.

Verifying: The public key from DNS is then used by the receiving mail system to verify that the signature was generated by the matching private key. This proves that the email was truly sent by, and with the permission of, the claimed sending From: domain and that its headers and content weren't altered during transfer.

Delivering: The receiving email system applies local policies based on the results of the signature test. If the domain is verified and other anti-spam tests don't catch it, the email can be delivered to the user's inbox. If the signature fails to verify, or there isn't one, the email can be dropped, flagged, or quarantined. This is step "D" in the diagram on the right. In general, Yahoo! expects that DomainKeys will be verified by the receiving email servers. However, end-user mail clients could also be modified to verify signatures and take action on the results.

How will this help stop spam?

Several ways. First, it can allow receiving companies to drop or quarantine unsigned email that comes from domains that are known to always sign their emails with DomainKeys, thus impacting spam and phishing attacks. Second, the ability to verify sender domain will allow email service providers to begin to build reputation databases that can be shared with the community and also applied to spam policy. For example, one ISP could share their "spam vs. legit email ratio" for the domain with other ISPs that may not yet have built up information about the credibility and "spamminess" of email coming from Last, by eliminating forged From: addresses, we can bring server-level traceability back to email (not user-level - we believe that should be a policy of the provider and the choice of the user). Spammers don't want to be traced, so they will be forced to only spam companies that aren't using verification solutions.

How will this help stop fraud/phishing attacks?

Companies that are susceptible to phishing attacks can sign all of their outgoing emails with DomainKeys and then tell the world this policy so that email service providers can watch and drop any messages that claim to come from their domain that are unsigned. For example, if the company signs all of its outgoing email with DomainKeys, Yahoo! can add a filter to its SpamGuard system that drops any unsigned or improperly signed messages claiming to come from the domain, thus protecting tens of millions of's customers or prospective customers from these phishing attacks.

Won't spammers just sign their messages with DomainKeys?

Hopefully! If they do, they'll make it easier for the Internet community to isolate and drop/quarantine their messages using the methods described above in "How will this help stop spam?" Eliminating the uncertainty of "did this email really come from the domain" will facilitate a whole range of anti-spam solutions.

What does DomainKeys verify?

DomainKeys examines the From: and Sender: headers' domain to protect the user and deliver the best possible user experience. Desktop mail clients like Microsoft Outlook show these headers in their user interfaces. If the user establishes their trust based on the these domains, then so should any system built to verify whether that trust is warranted.

Why sign the entire message?

DomainKeys signs the entire message to allow the receiving server to also verify that the message wasn't tampered with or altered in transit. By signing the headers and the body, DomainKeys makes it impossible to reuse parts of a message from a trusted source to fool users into believing the email is from that source.

Does DomainKeys encrypt each message?

DomainKeys does not encrypt the actual message - it only pre-pends a "digital signature" as a header.

What public/private key technology is used for DomainKeys?

DomainKeys currently uses an RSA public/private key method. The key length is decided by the domain owner.

Who issues the public/private key pairs required by DomainKeys?

The domain owner, or an agent or service provider acting on their behalf, should generate the key pairs that are used for their DomainKeys-enabled mail system.

Does DomainKeys require signing of the public key by a Certificate Authority (CA)?

DomainKeys does not require a CA. Much like a trusted Notary Public, Certificate Authorities are used in public/private key systems to sign, or "endorse," public keys so that the external users of public keys can know that the public keys they receive are truly owned by the people who sent them. Since DomainKeys leverages DNS as the public key distribution system, and since only a domain owner can publish to their DNS, external users of DomainKeys know that the public key they pull is truly for that domain. The CA is not needed to verify the owner of the public key - the presence in that domain's DNS is the verification. However, it is possible that Certificate Authorities may become a valuable addition to the DomainKeys solution to add an even greater level of security and trust.

How are DomainKeys revoked?

DomainKeys allows for multiple public keys to be published in DNS at the same time. This allows companies to use different key pairs for the various mail servers they run and also to easily revoke, replace, or expire keys at their convenience. Thus, the domain owner may revoke a public key and shift to signing with a new pair at any time.

Why not just use S/MIME?

S/MIME was developed for user-to-user message signing and encryption and by design should be independent of the sending and receiving servers. We believe that DomainKeys should be a natural server-to-server complement to S/MIME and not a replacement. Additionally, since S/MIME is used by many security-conscious industries, we need to ensure that the two technologies can work together without breaking each other. Finally, S/MIME is not yet supported by many of the email services, client software, and server software used across the Internet, and in Yahoo!'s opinion, that standardization effort would be much more difficult than the standardization of DomainKeys.

How does DomainKeys work with mailing lists?

Mailing lists that do not change the content or re-arrange or append headers will be DomainKey compatible with no changes required. Mailing lists that change the message and headers should re-sign the message with their own private key and claim authorship of the message.

Who implements DomainKeys?

DomainKeys will typically be implemented/enabled by the team within a company, ISP, or email service provider that deploys and runs the incoming and outgoing mail servers. Some companies may have service providers that handle their email. As MTA vendors add support for DomainKeys to their products, the implementation of DomainKeys will become simpler.

Which mail transfer agents (MTAs) support DomainKeys?

Sendmail has released a milter implementation for both the commercial and freeware versions of their MTA. A Qmail patch, an Exim version as well as a qpsmtpd plugin are also available. CERN, the creators of the WWW has released a C# library for use in MS Exchange 2003. Port 25's PowerMTA,'s acSMTP, ActivSoftware's XMServer, OmniTI's Ecelerity, StrongMail, Alt-N Technology's MDaemon MTA for Windows, Postfix, Communigate Pro, IronPort, and Merak Mail all have DomainKey versions of their software. Finally, Yahoo! has released an open source reference implementation for DomainKeys that can be plugged into other MTAs.

How do I deploy DomainKeys?

After installing a DomainKey aware MTA, there are several key distribution options from which to choose. Once chosen, the public key portion should be published to your domain's _domainkey subdomain's TXT record, and the private key inserted into your MTA. You can test your DNS record policy and selector, and there are several autoresponding email addresses to test your implementations.

I don't use my domain's SMTP server to send email. How do I use DomainKeys?

DomainKeys relies on the domain administrator to authorize the use of the domain in an email. If you can not use the domain's authorized SMTP server because of port 25 blocking, you have a number of options.

- You should encourage your domain to accept submission services on port 587. Your domain administrator should try to control authorization of the domain. Giving users a path to submit mail will help do this. Yahoo! Mail recently began offering a submission server on port 587.

- You may be able to convince the domain administrator to grant you a user specific key. With a DomainKey, it should be possible to sign your messages using your mail client or any submission server. In fact, you could ask your submission service if you could give them a private key to use to sign your domain's mail.

- You could consider using other headers to convey your identity. For instance, the Reply-to: header allows a recipient's mail client to choose an address to which replies should be sent. The Sender: header defines the address that injects the message into the SMTP stream. You might consider sending your message From: your domain, with the Sender: header set to the address of your submission service. Be aware however, that this strategy may be viewed suspiciously by anti-spam filters, as it may become a tactic for spammers and phishers.

- Finally, you could choose to send unauthenticated mail. While this will not be a good long term strategy, it will certainly take quite a while before the vast majority of Internet email is authenticated. If you choose this path, you should carefully monitor the amount of authenticated mail over time to ensure that this strategy does not impact the deliverability of your email.

How can I send you feedback?

Yahoo! welcomes your feedback on DomainKeys. You agree that Yahoo! shall own and have the right to use, without attribution or compensation to you, all feedback received by Yahoo!, in any form, to improve or modify DomainKeys or otherwise. Please use this email form to submit your comments. Note that due to the volume of emails we receive, it is unlikely that we'll be able to respond to your individual emails.