Why is 2048-bit key length recommended over 1024-bit?

2048-bit keys offer significantly stronger cryptographic security compared to 1024-bit keys. Modern computing power makes 1024-bit keys more vulnerable to brute-force attacks, while 2048-bit keys provide a much higher level of protection against spoofing and tampering. Many major mailbox providers now require or strongly recommend 2048-bit keys.

What is RSA/SHA-256 and why is it the standard?

RSA is an asymmetric encryption algorithm used for generating the public and private key pair for DKIM. SHA-256 is a hashing algorithm that creates a unique fingerprint of your email content. This combination is the standard because it offers a robust, widely supported, and trusted method for verifying email authenticity, balancing strong security with broad compatibility across email systems.

How often should I rotate my DKIM keys?

It's a best practice to rotate your DKIM keys periodically to enhance security. A common recommendation is to rotate them every 6 to 12 months. Regular rotation helps minimize the risk if a key were to be compromised, limiting the window of opportunity for attackers. Remember to update your DNS records after generating new keys.

Are there any alternatives to RSA/SHA-256?

While RSA/SHA-256 is the current standard, other algorithms like Ed25519 are emerging. Ed25519 offers strong security with smaller key sizes. However, it is not yet as widely adopted or supported by all mailbox providers as RSA/SHA-256, so for maximum compatibility and deliverability, RSA/SHA-256 remains the recommended choice for most organizations.

Can 4096-bit DKIM keys be used?

Yes, 4096-bit DKIM keys can be used, and they offer even greater cryptographic strength. However, the security benefits typically diminish beyond 2048-bit, while the increased computational overhead for signing and verifying can sometimes lead to performance issues or compatibility problems with some older email systems. For most use cases, 2048-bit provides an optimal balance of security and practicality.

What is the recommended DKIM key algorithm? - DKIM - Email authentication - Knowledge base

Digital signature sealing an email envelope, symbolizing DKIM authentication

When you're dealing with email deliverability, setting up DKIM correctly is crucial. It’s not just about having a DKIM record, but ensuring that record uses the strongest, most widely accepted cryptographic standards. The choices you make regarding your DKIM key algorithm and length directly impact how mailbox providers like Gmail and Outlook view your emails.

I often see confusion around the different options available for DKIM keys. It's easy to get bogged down in the technical jargon, but my goal here is to simplify things and give you clear, actionable recommendations. We want to ensure your emails are both secure and reliably delivered to the inbox.

The foundation of robust email authentication lies in selecting the right cryptographic components. I'll walk you through the current best practices and explain why these particular choices are paramount for your email ecosystem.

The standard: RSA/SHA-256 with 2048-bit keys

The current consensus among email security experts and mailbox providers strongly favors RSA/SHA-256 as the recommended DKIM signing algorithm. This combination provides a high level of security that is widely supported and trusted across the email landscape. RSA refers to the asymmetric encryption algorithm used for the public and private key pair, while SHA-256 is the hashing algorithm that creates a unique digital fingerprint of your email content.

Regarding key length, 2048-bit keys have become the industry standard. While 1024-bit keys were once common, they are now considered less secure against modern cryptographic attacks. Most major email providers now either require or strongly recommend 2048-bit keys for stronger authentication. You can read more about recommended DKIM signing from sources like Wander.Science.

The shift to 2048-bit keys reflects the increasing sophistication of cyber threats. A longer key simply means more possible combinations, making it exponentially harder for malicious actors to crack your DKIM signature and forge emails from your domain. This enhanced security is vital for protecting your brand reputation and preventing phishing attacks.

I always advise clients to move to 2048-bit keys if they haven't already. It's a fundamental step in ensuring your emails pass authentication checks and reach their intended recipients. You can dive deeper into the pros and cons of 1024-bit versus 2048-bit DKIM keys to understand the full implications.

Deciphering DKIM key algorithms and lengths

In your DKIM record, the algorithm used for signing is indicated by the a tag. For example, a=rsa-sha256 specifies that the RSA algorithm with SHA-256 hashing is being used. This is a critical part of your DKIM DNS record and needs to be configured accurately.

Example DKIM DNS RecordDNS

selector1._domainkey IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDz9N...; a=rsa-sha256; s=email; t=s;"

While RSA/SHA-256 is the dominant algorithm, emerging options like Ed25519 are gaining traction. Ed25519 offers comparable security with smaller key sizes and potentially faster processing. However, its adoption is not as widespread as RSA, and many mailbox providers may not yet fully support it. For broad compatibility and maximum deliverability, sticking with RSA/SHA-256 is the safer bet for now.

1024-bit keys

Security level: Adequate for some older systems, but generally considered less secure today.
Performance impact: Faster processing due to shorter key, but this benefit is often negligible.
Current status: Many providers, including Microsoft, are deprecating or penalizing these keys.

2048-bit keys

Security level: Highly robust against current cryptographic attacks, providing strong protection.
Performance impact: Slightly slower processing, but modern systems handle this without noticeable impact.
Current status: Recommended minimum standard by most major mailbox providers and security experts.

Ultimately, the choice of key length comes down to balancing security needs with compatibility. For now, 2048-bit with RSA/SHA-256 offers the best of both worlds, providing strong security without sacrificing broad acceptance. Make sure you check the recommended DKIM key size regularly for updates to these guidelines.

Key rotation and advanced considerations

Beyond selecting the right algorithm and length, actively managing your DKIM keys is paramount. This includes regular key rotation, which helps mitigate the risk of a compromised key. Even a 2048-bit key isn't invincible if it's never changed. Rotating keys adds another layer of security, making it harder for attackers to exploit an old, potentially leaked key.

Symbolic image of rotating gears, representing regular DKIM key rotation for security

While 2048-bit is the recommended minimum, some organizations consider 3072-bit or even 4096-bit keys for extremely high-security environments. However, the cryptographic strength gains often diminish beyond 2048-bit, while the computational overhead increases. This can sometimes lead to performance issues or compatibility problems with older systems. For most businesses, 2048-bit remains the sweet spot. If you're wondering about the use of 4096-bit DKIM keys, remember the diminishing returns.

Key management best practices

Regular rotation: Schedule regular DKIM key rotations, typically every 6-12 months.
Secure storage: Ensure your private DKIM keys are stored securely and are only accessible by authorized personnel.
Monitoring: Actively monitor your DKIM authentication results using a DMARC reporting tool to detect any issues quickly.

The rotation process involves generating a new key pair and updating your DNS records. This is a critical process that should be handled with care to avoid any service interruptions. Understanding why DKIM key rotation is recommended is the first step toward a more secure email infrastructure.

Tools and methods are available to generate DKIM public and private keys, including using OpenSSL or features provided by your email service provider. It's important to follow the correct procedures to create a a=rsa-sha256 key to meet the recommended standards.

Maintaining strong email authentication

Implementing the recommended DKIM key algorithm and length is a non-negotiable step for any organization serious about email security and deliverability. RSA/SHA-256 with a 2048-bit key provides the best balance of cryptographic strength, compatibility, and industry acceptance.

Beyond initial setup, continuous monitoring is essential. Tools like Suped offer comprehensive DMARC reporting and monitoring, giving you visibility into your DKIM authentication results and helping you identify any issues quickly. Our AI-powered recommendations provide actionable insights to keep your email infrastructure robust and secure.

By adhering to these standards and actively managing your DKIM keys, you'll significantly reduce the risk of email spoofing and enhance your email’s chances of landing in the inbox, not the spam folder. Prioritizing strong email authentication is an investment in your brand's trust and overall digital security.