Are people using 4096-bit DKIM keys, and what is the recommended DKIM key length?
Matthew Whittaker
Co-founder & CTO, Suped
Published 23 May 2025
Updated 19 Aug 2025
7 min read
When it comes to email authentication, DKIM, or DomainKeys Identified Mail, plays a crucial role in preventing spoofing and phishing. It allows a sender to cryptographically sign emails, giving the receiving server a way to verify that the email truly originated from the claimed domain and that its content hasn't been tampered with in transit. The strength of this cryptographic signature heavily relies on the length of the DKIM key.
For many years, the standard key length was 1024-bit. However, as computational power increased and security best practices evolved, the industry began a shift towards stronger encryption. This led to a widespread adoption of 2048-bit DKIM keys, which provide a significantly higher level of security against brute-force attacks.
The natural progression of this security mindset brings us to larger key sizes. I've often wondered if organizations are beginning to adopt 4096-bit DKIM keys and whether they are even necessary. Let's delve into the current state of DKIM key usage and what the recommended key length truly is for optimal email security and deliverability.
Historically, 1024-bit RSA keys were the norm for DKIM. They provided a decent level of security for their time. However, cryptographic recommendations are constantly evolving, and what was once considered secure may become vulnerable as technology advances. This is why many in the email community have advocated for and implemented 2048-bit keys as the new baseline.
The change was largely driven by the increasing feasibility of cracking shorter keys. For instance, a 512-bit RSA key can now be broken with relatively minimal computational cost. While 1024-bit keys are still in use, the general consensus is that they offer diminishing security. The Internet Engineering Task Force (IETF) has even published RFC 8301, which effectively deprecates 1024-bit keys for new implementations due to security concerns, recommending at least 2048-bit keys.
Most major email providers, including Google and Microsoft, now fully support and often recommend 2048-bit DKIM keys. This length provides a robust level of cryptographic security, balancing protection against attacks with reasonable processing overhead for both senders and receivers. If you're using 1024-bit keys, it's generally a good idea to consider transitioning to 2048-bit for enhanced security and compliance with evolving standards. We've discussed the pros and cons of 1024-bit versus 2048-bit DKIM keys in more detail elsewhere.
The shift to 2048-bit keys also aligns with broader recommendations for RSA key lengths in general, ensuring that email authentication mechanisms keep pace with advancements in cryptographic attacks. Staying current with these recommendations is a key part of maintaining strong email domain reputation.
Are 4096-bit DKIM keys being used?
While 2048-bit DKIM keys are the widely accepted standard, the question of whether people are using 4096-bit keys is a valid one. I have indeed observed instances of 4096-bit DKIM keys in the wild. Some organizations, particularly those with very high-security requirements or those looking to future-proof their email authentication, might experiment with or implement these larger key sizes.
The primary benefit of a 4096-bit key is enhanced cryptographic strength, making it exponentially harder for an attacker to break the signature through brute-force methods. However, this increased security comes with practical considerations. Larger keys result in longer DNS TXT records, which can sometimes exceed DNS provider limits for TXT record length, leading to potential authentication failures if not handled correctly.
Another consideration is the processing overhead. Both the sending and receiving mail servers need more computational resources to generate and verify signatures with larger keys. While modern servers can generally handle 2048-bit keys efficiently, 4096-bit keys introduce additional strain, which could theoretically impact performance for very high-volume senders or older receiving systems. It's worth noting that RFC 6376, the original DKIM specification, requires verifiers to be able to validate signatures with keys ranging from 512 to 2048 bits, but anything beyond that is often considered implementation-defined.
The reality is that while 4096-bit keys are technically permissible, they are not universally required or even consistently supported across all mail systems. The marginal security benefit over 2048-bit keys may not always outweigh the potential compatibility and performance challenges. We've also explored whether 2048-bit DKIM keys are well accepted by ISPs, which is an important consideration when debating larger key sizes.
Recommended DKIM key length and best practices
Based on current cryptographic standards and industry acceptance, the recommended DKIM key length remains 2048 bits. This size offers an excellent balance of strong security and broad compatibility, ensuring your emails are consistently authenticated without encountering unnecessary deliverability issues.
While 4096-bit keys might seem like a straightforward path to greater security, the real-world implications of their use, such as DNS record limitations and potential compatibility quirks with older or less robust mail systems, often outweigh the marginal security gains they offer over 2048-bit keys. For most senders, investing in robust DMARC monitoring and disciplined DKIM key rotation practices will yield far greater improvements in email security and deliverability.
Regularly rotating your DKIM keys, regardless of their length, is a critical security practice. This minimizes the window of opportunity for attackers should a key ever be compromised. I recommend reviewing your DKIM setup periodically, especially if you handle sensitive email communications. You can learn more about why DKIM key rotation is recommended and what key length is secure.
Ultimately, the choice of key length should be a pragmatic one, prioritizing a balance between security and the practical realities of email infrastructure. For nearly all senders, 2048-bit DKIM keys are the optimal choice, providing robust protection while ensuring maximum deliverability.
Optimal DKIM key length
Key length comparison
1024-bit key: While still functional, it is considered less secure and is being phased out by cryptographic standards. This length may not be sufficient for long-term security against evolving attack methods.
2048-bit key: This is the current industry standard and the recommended key length. It provides a strong level of security without introducing significant compatibility or performance issues across mail systems.
4096-bit key: Offers the highest cryptographic strength among these options. However, it can lead to larger DNS TXT records, which may exceed limits for some DNS providers, and could impose more processing overhead.
The choice of DKIM key length is often a balance between security, performance, and compatibility. Here's a quick overview of how different key lengths stack up:
The table above highlights the trade-offs involved with each key length. While 4096-bit offers superior theoretical security, the practical challenges it introduces, especially regarding DNS record length, often make it less ideal for mass email sending. Understanding DKIM selector names and recommended key size is also crucial for proper implementation.
Final thoughts on DKIM key length
My observation is that 4096-bit DKIM keys are being used by some, but they are far from being the norm. The industry has largely standardized on 2048-bit keys due to their strong security profile and excellent compatibility with existing email infrastructure. For most organizations, this is the optimal balance for ensuring email deliverability and protecting against spoofing.
Views from the trenches
Best practices
Regularly audit your DKIM records to ensure they are properly configured and use the recommended 2048-bit key length.
Implement a key rotation schedule to enhance security, even for robust 2048-bit keys.
Monitor DMARC reports to identify any authentication failures related to DKIM, including issues with key length or DNS record size.
Consider the capabilities of your DNS provider regarding TXT record length before implementing very large DKIM keys.
Common pitfalls
Using outdated 1024-bit DKIM keys, which are increasingly vulnerable to cryptographic attacks.
Attempting to deploy 4096-bit keys without first verifying compatibility with your DNS provider and mail systems.
Neglecting regular DKIM key rotation, leaving your domain exposed to potential compromise.
Failing to monitor DMARC reports, which can hide underlying DKIM authentication issues due to key length or other misconfigurations.
Expert tips
When moving to a larger key size, it's wise to test the change with a subset of your email traffic first.
Ensure your DNS records are clean and don't contain extraneous characters that might increase the TXT record length unnecessarily.
For very high-volume senders, consider the performance implications of larger key sizes on your signing infrastructure.
Prioritize a well-implemented 2048-bit key with regular rotation over a 4096-bit key with potential compatibility issues.
Expert view
Expert from Email Geeks says that they have seen 4096-bit DKIM keys in use.
2024-08-19 - Email Geeks
Marketer view
Marketer from Email Geeks noted the historical vulnerability of smaller keys, referencing an article about cracking a 512-bit RSA key for a low cost, which prompts broader questions about optimal key length and security.
Google and 
Microsoft, now fully support and often recommend 2048-bit DKIM keys. This length provides a robust level of cryptographic security, balancing protection against attacks with reasonable processing overhead for both senders and receivers. If you're using 1024-bit keys, it's generally a good idea to consider transitioning to 2048-bit for enhanced security and compliance with evolving standards. We've discussed the pros and cons of 1024-bit versus 2048-bit DKIM keys in more detail elsewhere.
