Key Length (DKIM)

What is Key Length (DKIM)?

Key length in DKIM (DomainKeys Identified Mail) refers to the size of the cryptographic key used to sign and verify email messages. It determines the strength of the digital signature applied by the sending mail server and the difficulty for an attacker to forge it. In DKIM, longer key lengths provide stronger security, making it harder to break or spoof a signature through brute-force or cryptographic attacks.

Each DKIM key pair consists of a private key (used to sign messages) and a public key (published in the domain’s DNS TXT record). When a message is received, the mail server retrieves the public key from DNS, verifies the signature using that key, and confirms that the message was not altered in transit.

How Key Length Works in DKIM

The key length defines the size of the encryption key in bits. DKIM supports several key lengths depending on the algorithm used, but most domains use RSA with 1024-bit or 2048-bit keys. The key size directly affects both performance and security:

• 512-bit keys: Obsolete and insecure. Easily cracked using modern computing power.
• 1024-bit keys: Minimum acceptable for many mail systems but considered weak by today’s standards.
• 2048-bit keys: Recommended best practice for secure and compliant DKIM implementations.
• 4096-bit keys: Provides additional security but may increase DNS record size and lookup time.

Example of a DKIM public key published in DNS:

The p= tag contains the Base64-encoded public key, which mail receivers use to verify the cryptographic signature in each email’s DKIM-Signature header.

Security and Compliance Considerations

Key length has a direct impact on DKIM security. Shorter keys (1024-bit or less) are more vulnerable to cryptographic attacks, especially as computing power increases. Major mailbox providers and security frameworks, including DMARC, recommend using 2048-bit RSA keys or stronger to prevent forgery and protect against compromise.

Important best practices include:

• Use at least 2048-bit keys for new DKIM records.
• Regularly rotate keys to limit exposure in case of compromise.
• Monitor DNS size limits - 2048-bit keys can increase record length, so split long strings into multiple quoted segments if necessary.
• Avoid reusing the same selector across different services or mail streams.
• Periodically audit all DKIM selectors to ensure they meet modern cryptographic standards.

Many enterprise security standards (such as ISO 27001 and NIST SP 800-177) emphasize strong key management as part of secure email authentication policies. Using outdated or weak DKIM keys can lead to message rejection or failure to pass alignment checks during DMARC evaluation.

How Key Length Affects Deliverability

Mailbox providers like Google, Yahoo, and Microsoft actively check DKIM key strength. Messages signed with weak keys (especially 512-bit or 1024-bit RSA keys) may be flagged as insecure or fail authentication altogether. This can lower sender reputation and increase spam folder placement.

Upgrading to 2048-bit keys improves both security and trustworthiness. However, DNS configuration must be tested carefully, as longer records can exceed size limits or cause lookup failures if not properly formatted. Verifying the DKIM signature after publishing the new key ensures smooth delivery and accurate reporting in DMARC analytics.

Key Length and DMARCeye

DMARCeye continuously monitors DKIM configurations across all domains and subdomains to ensure cryptographic best practices. The platform analyzes each selector’s key length, flags outdated or insecure keys, and alerts administrators before they impact deliverability or authentication success.

By correlating key data with message authentication results, DMARCeye provides detailed insights into which selectors are in use, which need rotation, and where key strength may fall below compliance standards. This helps organizations maintain a secure, standards-aligned DKIM setup that supports reliable domain protection and strong DMARC enforcement.

Sign up for a free trial of DMARCeye today and secure your email domain.

To learn more about DMARC and DMARC-related terms, explore the DMARCeye Glossary.