DKIM Key Size: 1024 vs 2048 Bits — Which Should You Use?

When generating DKIM keys, you must choose a key size. The two common options are 1024 bits and 2048 bits. Here's what you need to know.

Quick Comparison

Security Considerations

1024-bit Keys

1024-bit RSA keys were standard for years but are now considered weak:

• Theoretically breakable with sufficient computational resources
• NIST deprecated 1024-bit RSA in 2013
• RFC 8301 (2018) recommends against 1024-bit DKIM keys
• Some security audits flag 1024-bit keys as vulnerabilities

A 1024-bit key hasn't been publicly broken for DKIM purposes, but the security margin is shrinking.

2048-bit Keys

2048-bit RSA provides substantially stronger security:

• 128-bit security level — considered secure through 2030+
• Industry standard for DKIM, TLS certificates, and more
• Required by some compliance frameworks
• Recommended by Google, Microsoft, and email security experts

The computational cost to break a 2048-bit key is approximately 2^32 times harder than 1024-bit.

DNS Record Size

The practical challenge with larger keys is DNS record length.

1024-bit DNS Record

selector._domainkey TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC..."

• Approximately 180 characters for the public key
• Fits easily in a single DNS TXT record
• No compatibility issues

2048-bit DNS Record

selector._domainkey TXT "v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA..."

• Approximately 400 characters for the public key
• May require splitting across multiple strings
• Some older DNS providers have issues

Splitting Long Records

DNS TXT records have a 255-character limit per string. For 2048-bit keys, the record is split:

selector._domainkey TXT ("v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8A"
                         "MIIBCgKCAQEA...rest of key...")

Most DNS providers handle this automatically. Receivers concatenate the strings when validating.

Provider Support

Providers Requiring 2048-bit

Some email services mandate 2048-bit keys:

• Google Workspace — 2048-bit default since 2016
• Microsoft 365 — 2048-bit recommended
• Proofpoint — 2048-bit required
• Many enterprise email gateways

DNS Providers With 2048-bit Issues

A few older or limited DNS providers struggle with long TXT records:

• Some budget hosting control panels
• Older on-premise DNS servers
• Certain registrar DNS services

If your DNS provider can't handle 2048-bit keys, consider:

1. Migrating DNS to a modern provider (Cloudflare, Route 53, etc.)
2. Using 1024-bit as a temporary measure while migrating
3. Contacting your provider about TXT record limits

Performance Impact

Key size affects signing and verification speed:

These differences are negligible for normal email volumes. Even high-volume senders (millions of emails/day) won't notice meaningful performance impact from 2048-bit keys.

When to Use 1024-bit

Despite the recommendation for 2048-bit, there are limited cases for 1024-bit:

1. DNS provider limitation — Provider truly cannot support long TXT records
2. Legacy system constraint — Older mail server software with hardcoded limits
3. Temporary migration — Short-term use while upgrading infrastructure

If using 1024-bit, plan to migrate to 2048-bit within 6-12 months.

When to Use 2048-bit

Use 2048-bit keys for:

• New deployments — Start with current standards
• Production email — Anything customer-facing
• Compliance requirements — PCI-DSS, SOC 2, etc.
• Long-term use — Keys you won't rotate frequently

Key Rotation Strategy

Key size choice affects rotation planning:

1024-bit Strategy

If forced to use 1024-bit:

• Rotate every 6 months
• Plan migration to 2048-bit
• Monitor security advisories

2048-bit Strategy

• Rotate annually or on security events
• Standard practice for most organizations
• No urgent migration pressure

Migration Path

Moving from 1024-bit to 2048-bit:

What About 4096-bit?

Some wonder about even larger keys:

• Not recommended for DKIM currently
• DNS record would be ~800 characters — compatibility issues
• Performance overhead increases significantly
• 2048-bit provides sufficient security for foreseeable future
• Consider algorithm change (Ed25519) before key size increase

Future: Ed25519

DKIM supports Ed25519 (elliptic curve) as an alternative to RSA:

Ed25519 offers equivalent security with much smaller keys. However, support isn't yet universal. For now, RSA 2048-bit remains the safe choice.

Recommendation

Use 2048-bit keys unless you have a specific, documented reason not to.

The security improvement over 1024-bit is substantial, performance impact is negligible, and DNS compatibility is nearly universal. Starting with 2048-bit avoids future migration work.

---

Generate secure 2048-bit DKIM keys in your browser.