DomainKeys vs. DKIM: What’s the Difference?
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Reading Time: 6 min
Key Takeaways
- Yahoo introduced DomainKeys in 2004 to verify the sender’s domain identity and reduce forged emails.
- DKIM (DomainKeys Identified Mail) was standardized by the IETF in RFC 4871 (2007), merging Yahoo’s DomainKeys and Cisco’s Identified Internet Mail into a single open standard.
- DKIM allows senders to sign a canonicalized hash of selected headers and body content, ensuring message integrity even when non-critical modifications (like whitespace changes) occur.
- Selectors in DKIM simplify key rotation and allow multiple keys for different services.
- DKIM validates that an email was authorized by the domain listed in the signature and that its content hasn’t been altered in transit.
- DKIM contributes to DMARC’s alignment checks, helping domain owners enforce anti-spoofing policies.
DomainKeys and DKIM are two related but distinct email authentication technologies designed to protect against spoofing, phishing, and spam. DomainKeys, introduced by Yahoo in 2004, was the first step toward validating sender authenticity using cryptographic signatures. However, it had limited flexibility and adoption. DKIM evolved from DomainKeys and Cisco’s Identified Internet Mail to become a standardized and widely adopted solution. Today, DKIM serves as a cornerstone of secure email communication, ensuring that messages remain authentic and unaltered during transmission while supporting DMARC for policy enforcement.
What Was DomainKeys (DK)?
DomainKeys was an early email authentication protocol that was developed and released by Yahoo in 2004. It had a very simple, straightforward, and admirable goal: to verify the sender’s domain identity and reduce the increasing number of spam, phishing, and forged email.
Here is how DomainKeys worked. It used a simple signing scheme that applied cryptographic signatures over the entire message. Even though this was a step forward, it still lacked flexibility in header selection and was easily broken by forwarding or mailing list modifications. Because of these limitations, it was eventually replaced by DKIM and deprecated.
Drawbacks of DomainKeys
Here are a few reasons why DomainKeys had to eventually be replaced:
- It was proprietary: It was a proprietary Yahoo standard and never achieved widespread IETF standardization.
- It had no selector mechanism: It lacked “selectors,” which means a domain could only use a single public key. This made key rotation and managing different email-sending services extremely difficult.
- The signatures were quite brittle: The signing method was very rigid. Signatures almost always broke if the email was forwarded or modified slightly by a mailing list.
- It had limited flexibility: It provided very few options for signing algorithms and canonicalization (i.e., how the email is processed before signing).
Introduction to DKIM (DomainKeys Identified Mail)
DKIM, or DomainKeys Identified Mail, is an email authentication protocol that enables senders to stop email content from being manipulated during delivery. DKIM originated from a 2004–2005 collaboration between Yahoo and Cisco and became an IETF standard in 2007 (RFC 4871).
It works by adding a digital signature to the message header. As soon as the receiver gets the DKIM-signed email, they check the signature to ensure it’s valid. If it is valid, they know the message has been transmitted intact and has not been manipulated by hackers.
Here is an example of a DKIM-Signature:
DKIM-Signature: v=1; a=rsa-sha256; d=example.com; s=selector1; h=from:subject:date; bh=abc123…; b=xyz456…
How DKIM Works
DKIM relies on a pair of cryptographic keys: a private key and a public key. These keys are essential for ensuring that an email is authentic and hasn’t been altered.
When an email is sent, the sender’s mail server uses the private key to create a digital signature. This signature is added to the email in the DKIM header, a special part of the email that contains the signature itself along with information about the signing domain, the algorithm used, and which headers were included in the signature.
When the email reaches the recipient, their mail server retrieves the public key from the sender’s DNS. The server then uses this public key to check the DKIM header and verify that the signature matches the message. If it does, the email is confirmed as authentic and unaltered.
In short: the DKIM header carries the signature, the private key generates it, and the public key verifies it, protecting both email integrity and authenticity.
Key innovation: DKIM’s key innovation over DomainKeys was introducing standardized, flexible cryptographic signatures with selectors and robust canonicalization for reliable, domain-based authentication.
DomainKeys Vs DKIM: The Key Differences
There is a difference of only two letters in the acronym, but it makes a huge difference. DKIM came to fix the limitations of DomainKeys.
1. Standardization and Adoption
- DomainKeys: It was proprietary and driven by Yahoo, and its adoption was quite limited; today, it is completely obsolete.
- DKIM: This is an open IETF standard. This neutrality and technical superiority became the main reasons why every major email provider, including Google and Microsoft, adopted it.
2. Signature Flexibility
- DomainKeys: It signed specific headers and the entire message body, which often caused signature breakage if messages were modified in transit.
- DKIM: DKIM’s body hash and canonicalization make it more tolerant to minor formatting changes, though forwarding or mailing list modifications can still break signatures. It allows the sender to choose exactly which headers to sign (h= tag). What’s more, it signs a hash of the message body (bh= tag), which is more resilient to minor modifications like forwarding. It also uses canonicalization (c= tag) to define how message changes like whitespace are treated.
3. Key Management and Selectors
- DomainKeys: Lacked the selector mechanism, forcing all messages from a domain to use a single public key, which complicated key rotation and management.
- DKIM: Introduced the concept of “selectors.” A selector is a name that points to a specific public key in your DNS. This allows you to:
- Use different keys for different services, be it for Google Workspace, for your marketing platform, etc.
- Rotate your keys for security without disrupting mail flow.
If you need help creating your DKIM record, you can use a free DKIM record generator to ensure the syntax is correct.
4. Security and Integrity
- DomainKeys: It was focused on sender authenticity.
- DKIM: The body hash check (bh=) confirms that the email’s content has not been altered since it was signed.
DomainKeys Vs DKIM: Comparison Table
| Feature | DomainKeys (DK) | DomainKeys Identified Mail (DKIM) |
|---|---|---|
| Developed By | Yahoo (proprietary) in 2004 | Joint effort by Yahoo & Cisco, later standardized by the IETF. Introduced in 2007 (RFC 4871), updated in RFC 6376 (2011) |
| Purpose | Authenticate the sender’s domain to reduce forged emails and spam | Authenticate sender’s domain and ensure message integrity |
| Header Field Name | DomainKey-Signature: | DKIM-Signature: |
| Selector Mechanism | Not supported | Supported (selector._domainkey.example.com) |
| Key Management | Single key for the entire domain | Multiple keys via selectors; easy key rotation |
| Signing Scope | Entire message body and some headers | Specific headers chosen by sender (h= tag) and hashed message body (bh= tag) |
| Canonicalization Options | Basic or none | Simple and relaxed canonicalization options for flexibility |
| Body Hashing | Entire body signed directly | Uses body hash (bh=) for better resilience to minor changes |
| Verification Domain | Based on “From” or “Sender” domain | Based on d= tag in signature |
| Integration with DMARC | Not supported | Fully supported and used for alignment checks |
| Adoption & Status | Limited adoption; obsolete | Widely adopted and actively used |
| Main Limitation | Rigid signing process, no selectors, signature breakage | Doesn’t protect visible “From” header (needs DMARC) |
Why DKIM Replaced DomainKeys
DKIM replaced DomainKeys because it solved the protocol’s main technical limitations – lack of standardization, no selector mechanism for key rotation, and fragile signatures that broke with minor message changes. DKIM introduced flexible header signing, stronger cryptography, and IETF standardization, making it more reliable, scalable, and widely adopted for email authentication.
The Business Benefits of Implementing DKIM
DKIM comes with numerous benefits:
Spoofing and Phishing Prevention
While DKIM alone cannot stop spoofing and phishing attacks, it works with DMARC to enhance domain security and keep out fakes.
Brand Protection
When a scammer uses your domain to send malicious emails, your brand’s reputation suffers. Recipients associate your name with spam and fraud, which leads to a loss of trust. DKIM preserves your brand’s integrity in the inbox.
Email Deliverability
Major inbox providers like Google and Microsoft expect and require valid authentication. Emails that pass DKIM are seen as more trustworthy.
Message Integrity
The DKIM signature guarantees that the email’s content hasn’t been tampered with after it was sent. This helps ensure the message your recipient reads is the exact message you sent.
Why DKIM Alone Isn’t Enough
DKIM is a powerful tool, but it has one major limitation when used alone: it doesn’t stop header “From” spoofing.
DKIM checks that the email was signed by a domain listed in the d= tag of its signature. However, it doesn’t require that the domain match the visible “From” address the user sees. A phisher could send an email “from” the CEO’s behalf but sign it with their own malicious domain. The email would pass a DKIM check, but it’s still a spoofing attack.
This is where DMARC comes in.
DMARC bridges the gap between authentication and identity. It ensures that the domain authenticated by DKIM or SPF matches the domain visible to the recipient in the “From” header.
Summing Up
DomainKeys paved the way, but DKIM remains the standard for trusted email authentication today. Pairing it with DMARC ensures protection against spoofing and phishing.
If you configure something the wrong way, even legitimate emails can get blocked. To avoid such issues and other challenges with DKIM, PowerDMARC’s Hosted DKIM service can help. It is a cloud service that takes care of every aspect of the DKIM management process. From a centralized dashboard, you can add, edit, and manage multiple selectors and keys for all your domains, without having to deal with DNS-level changes.
Start a free trial today to boost deliverability, improve authentication, and safeguard your domain against spoofing!
Frequently Asked Questions
- Is DomainKeys still used today?
Not anymore. It was deprecated and replaced by the modern DKIM protocol.
- How can PowerDMARC help with DKIM management?
PowerDMARC offers a hosted DKIM solution that enables automatic DKIM deployment, selector, and key management.
Domain & Email Security Expert at PowerDMARC
Ahona is the Marketing Manager at PowerDMARC, with 5+ years of experience in writing about cybersecurity topics, specializing in domain and email security. Ahona holds a post-graduation degree in Journalism and Communications, solidifying her career in the security sector since 2019.
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