What happens if I use a DKIM key length shorter than recommended?

Using a DKIM key length shorter than the recommended 2048 bits, such as 1024-bit, reduces the cryptographic strength of your email authentication. While it may still work, it makes your DKIM signature more vulnerable to brute-force attacks over time. This can negatively impact your email deliverability, as receiving mail servers might view your emails with weaker authentication as less trustworthy, potentially routing them to spam or rejecting them.

Are there any disadvantages to using longer DKIM keys, like 4096-bit?

Yes, while longer keys offer more security, they can introduce practical challenges. The most common issue is exceeding the character limit for DNS TXT records, which can make publishing the public key difficult or require splitting the record into multiple strings, as discussed in our article about DKIM key issues with DNS provider limits . Additionally, longer keys might slightly increase processing time for signing and verification, though this is usually negligible for modern systems.

How do I change my DKIM key length if I'm currently using 1024-bit?

Changing your DKIM key length typically involves generating a new pair of 2048-bit keys (public and private). Then, you'll need to update your DNS TXT record with the new public key for your domain. After updating, ensure your email sending system is configured to use the new private key for signing. It's crucial to verify the new setup to avoid any email delivery disruptions. Tools like Suped can help monitor this transition and report on your DKIM authentication status.

Does DKIM key length affect DMARC alignment?

DKIM key length itself doesn't directly affect DMARC alignment. DMARC alignment checks if the organizational domain in the DKIM signature (d=domain) matches the organizational domain in the "From" header of the email. However, a weak DKIM key (like 1024-bit) can lead to potential compromise or spoofing, which would then cause DKIM authentication failures and, consequently, DMARC failures. So, indirectly, using the recommended 2048-bit key length contributes to stronger DKIM authentication, which is vital for achieving DMARC alignment and successful email delivery.

Should I also consider the DKIM selector name in relation to key length?

While the DKIM selector name is separate from the key length, it is an important part of your DKIM record configuration. The selector name helps email receivers find the correct public key in your DNS records. It is crucial to have unique and descriptive selector names, especially if you manage multiple sending services or rotate keys frequently. Understanding how DKIM selector names are interpreted and how to choose them is as important as selecting the right key length for a robust DKIM setup.

What is the maximum recommended key length for DKIM? - DKIM - Email authentication - Knowledge base

Digital key securing an email envelope representing DKIM security

When it comes to securing your email communications with DKIM (DomainKeys Identified Mail), one of the most common questions that comes up is about key length. It's a critical decision that balances cryptographic strength with practical implementation challenges, directly impacting your email's authenticity and deliverability.

For a long time, 1024-bit RSA keys were the standard, but as computing power advanced, so did the need for stronger encryption. Today, the landscape has shifted, and recommendations have evolved to ensure robust protection against potential attacks. I'll walk you through the current best practices and why they matter.

Understanding DKIM key lengths

DKIM relies on a pair of cryptographic keys: a private key, which signs your outgoing emails, and a public key, which is published in your domain's DNS records. The length of these keys, measured in bits, determines the complexity of the encryption and, consequently, its resistance to brute-force attacks.

Generally, a longer key means stronger encryption. This is why the industry has been moving towards longer key lengths over time. The primary algorithms used for DKIM key generation are RSA and, less commonly, ECDSA, as detailed in our guide on the recommended DKIM key algorithm.

1024-bit DKIM keys

Lower security: While still functional, 1024-bit keys offer less cryptographic strength and are more susceptible to future attacks. Mailjet provides a good overview of 1024-bit versus 2048-bit keys.
Faster processing: Shorter keys generally require less computational power to sign and verify, though this difference is often negligible for modern systems.
Wider compatibility: Historically, some older DNS providers or email systems had limitations, but this is less of an issue today.

2048-bit DKIM keys

Enhanced security: These keys offer a significantly higher level of cryptographic security, making them much harder to crack. This is the recommended standard for most organizations today, as discussed in Twilio's insights on 2048-bit keys.
Future-proof: With the increasing sophistication of cyber threats, 2048-bit keys offer better longevity and align with evolving security standards.
Industry preference: Many major email providers and security experts now recommend or even require 2048-bit keys for optimal authentication and deliverability.

To fully understand the trade-offs, you can explore the pros and cons of 1024-bit versus 2048-bit DKIM keys in our detailed guide.

Why 2048-bit is the modern standard

Based on current cryptographic best practices and the recommendations of major email service providers, the maximum recommended key length for DKIM is 2048 bits for RSA keys. While 1024-bit keys are still technically functional, they are widely considered less secure and are increasingly being phased out or flagged by receiving mail servers.

For specific use cases or very high-security requirements, some organizations might even consider 3072-bit or 4096-bit keys. However, these longer keys come with their own set of challenges, particularly regarding DNS TXT record length limitations and potential processing overhead. My advice would be to carefully weigh these factors, as discussed in our exploration of whether people are using 4096-bit DKIM keys.

DNS TXT record limitations

One practical challenge with longer DKIM keys is that the public key must be published in a DNS TXT record. DNS providers often have character limits for these records, typically around 255 characters per string. For a 2048-bit key, the raw public key can be significantly longer, requiring the TXT record value to be split into multiple strings. If not configured correctly, this can lead to DKIM key issues when DNS provider limits TXT record length, causing authentication failures. Always confirm your DNS provider's capabilities before generating your keys.

Secure vault door with 2048-bit key symbol, symbolizing strong encryption

The recommendation for 2048-bit keys is not arbitrary. It's a balance between current security needs and the practicality of implementation. As cryptographic algorithms and computing power advance, what is considered secure today may not be tomorrow. Therefore, choosing a stronger key length now can help ensure your email authentication remains effective for years to come, reducing the need for frequent updates.

Addressing practical challenges and legacy systems

Despite the strong recommendation for 2048-bit DKIM keys, some organizations still find themselves using 1024-bit keys. This is often due to legacy systems, DNS provider limitations, or a lack of awareness regarding the security implications. Microsoft, for instance, has historically defaulted to 1024-bit keys, though many admins are now manually upgrading them.

Example DKIM TXT record (Public Key)DNS

selector1._domainkey IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDyXp/J...[rest of 2048-bit key]...tL/XQIDAQAB;"

If you are currently using 1024-bit keys, I highly recommend planning an upgrade to 2048-bit. While they might still work for now, they present a weaker link in your email security chain. Upgrading might require generating new keys, updating your DNS records, and then verifying the changes, but it's a worthwhile investment in your domain's security and reputation. You can learn how to generate an a=rsa-sha256 key for DKIM in our guide.

It is also crucial to identify your current DKIM key length. We have a dedicated article on how to quickly identify your DKIM key length, which can help you assess your current setup. This proactive step ensures you are aware of your security posture and can make informed decisions about necessary upgrades.

Implementing 2048-bit keys and key management

Beyond choosing the correct key length, effective key management is paramount. This includes regular key rotation to minimize the impact of a potential key compromise. I generally advise rotating your DKIM keys at least once a year, or more frequently if your security policy dictates. Our resource on why DKIM key rotation is recommended explains this further.

AI-powered recommendations: Suped provides actionable advice to fix issues and strengthen your DKIM policy, taking the guesswork out of email authentication.
Real-time alerts: Stay informed about any DKIM authentication failures or policy breaches as they happen.
Unified platform: Monitor DKIM, SPF, and DMARC together with blocklist and deliverability insights in one place.
SPF flattening: Simplify your SPF record to avoid common DNS lookup limit issues, enhancing overall email security.

Tools like Suped offer comprehensive DMARC monitoring and reporting, helping you to oversee your DKIM implementation and other authentication protocols. This unified approach simplifies the management of your email security, providing insights into your email's authentication status and pinpointing any issues that might arise. This is especially useful for managing DKIM temporary error rates with Microsoft, for example.

Ensuring your DKIM keys are of the recommended length and properly managed is a foundational step in maintaining excellent email deliverability and protecting your brand's reputation. It's an ongoing process, but with the right tools and knowledge, it's entirely manageable.

Securing your email's future

The maximum recommended key length for DKIM is 2048 bits for RSA. This length provides a strong balance of security and compatibility for the vast majority of email senders. While 1024-bit keys may still function, they fall short of modern security standards and are advised against for new implementations or when upgrading existing systems.

Prioritizing 2048-bit keys, ensuring proper DNS configuration, and regularly rotating your keys are key practices that will bolster your email authentication, prevent phishing and spoofing, and ultimately improve your overall email deliverability. Staying informed about evolving security requirements, such as whether 2048-bit DKIM keys will become new email authentication requirements, is also crucial for future-proofing your email strategy.