DKIM RSA Key Size: 1024 vs 2048 Bits and the Future with Ed25519

Your DKIM setup works, signatures pass. But have you checked the size of your RSA key? A key that’s too short is a ticking time bomb: it could be cracked, allowing an attacker to sign emails on your behalf.

The History of DKIM Keys: From 512 to 2048 Bits

2012: The End of 512-Bit Keys

In 2012, researchers demonstrated that a 512-bit RSA key could be cracked in under 72 hours using cheap cloud computing power. The result: anyone could impersonate a domain using a 512-bit key and send perfectly DKIM-signed emails.

Google and Yahoo then began rejecting DKIM signatures based on 512 and 768-bit keys.

2013: The Standard Moves to 1024 Bits

RFC 6376 established 1024 bits as the minimum size for DKIM RSA keys. At the time, cracking such a key was unthinkable.

2020: 1024 Bits Starts Showing Its Age

The latest RSA Factoring Challenge record: an 829-bit key was cracked in 2020, requiring approximately 2,700 core-years (2,700 CPU cores at 2.1 GHz for a full year). 1024-bit keys aren’t practically breakable yet, but the deadline is approaching.

2026: The Recommendation Is 2048 Bits

Today, best practice is clear:

Key Size	Status in 2026
512 bits	Rejected by all providers
768 bits	Rejected by all providers
1024 bits	Functional but discouraged for sensitive sectors
2048 bits	Recommended, current standard
4096 bits	Possible but often too long for a single DNS TXT record

The DNS Problem with Large Keys

A DNS TXT record is limited to 255 characters per string. A 2048-bit RSA key encoded in Base64 is approximately 390 characters, requiring the value to be split across multiple strings in the TXT record.

Most DNS providers handle this automatically. But a 4096-bit key (approximately 740 characters) can cause compatibility issues with some DNS resolvers.

That’s why 2048 bits is the ideal compromise: robust security and universal DNS compatibility.

Ed25519: The Future of DKIM?

Ed25519 is a signature algorithm based on elliptic curves. Compared to RSA:

Criteria	RSA 2048	Ed25519
Public key size	~390 characters	~44 characters
Signature size	~340 characters	~88 characters
Security level	~112 bits	~128 bits
Performance	Slow	Very fast
Email support	Universal	Partial (growing)

Ed25519 produces much shorter keys, eliminating DNS issues. It’s also faster and offers equivalent or superior security.

The Problem: Support

In 2026, Ed25519 for DKIM isn’t yet supported by all mailbox providers. Google and a few major players accept it, but many others silently ignore Ed25519 signatures.

Recommendation: if you use Ed25519, also keep an RSA 2048-bit signature in parallel (dual signing). RFC 8463 defines the use of Ed25519 with DKIM.

How to Check Your Key Size

DKIM Checker: enter your domain and selector, the tool displays key size and algorithm
Free audit: send a test email, key size is analyzed automatically
Command line:

dig +short selector._domainkey.yourdomain.com TXT

Then decode the public key with OpenSSL:

echo "MIIBIjANBg..." | base64 -d | openssl rsa -pubin -inform DER -text -noout

The output will show RSA Public-Key: (2048 bit) or the corresponding size.

How to Migrate to 2048 Bits

Generate a new 2048-bit key pair
Publish the new public key on a new selector (e.g., s2._domainkey)
Configure your MTA/ESP to sign with the new selector
Wait 48 hours for DNS propagation
Verify with the DKIM Checker
Remove the old DNS record once migration is confirmed

Using a new selector allows a zero-downtime migration: old and new coexist during the transition.

Regular Key Rotation

Regardless of size, rotating DKIM keys is a best practice. If a private key is compromised, the exploitation window is limited to the period between rotations.

Recommended frequency: every 6 to 12 months.