DKIM key length

What it means

RFC 8301 (2018) set the floor: signers must use RSA keys of at least 1024 bits and should use at least 2048, and verifiers must accept keys from 1024 to 4096 bits and must reject anything shorter than 1024. The same update made rsa-sha256 mandatory and retired rsa-sha1, which must no longer be used for signing or verifying.

Gmail requires a key of 1024 bits or longer for mail to personal Gmail accounts and recommends 2048 bits where the DNS host supports it; its setup guide keeps 1024 as the option for hosts that cannot store longer keys.

Key length is visible to anyone: look up the selector's record and measure the p= value. A 2048-bit key needs splitting across TXT strings, which some DNS hosts do not handle. Ed25519 keys, at 256 bits, are far shorter and fit in one string.