DomainKeys Identified Message (DKIM) is one of the email authentication standards that underpins DMARC. Since it can still be used to authenticate a message after it has been forwarded, when the message body isn’t altered, it is recommended that senders implement DKIM.
The figures below are derived from analysis of data generously provided by DomainTools.
RSA Key Size
The following graph takes the RSA keys observed in use for DKIM during 2025, and breaks them down based on the length of the key, or the number of bits used in the cryptographic operations.
The graph below shows the number of the two most popular RSA key sizes observed between 2015 and 2025. Note that the overall number of DKIM keys observed tripled from 2020 to 2021; the number of 1,024 bit keys roughly doubled, while the number of 2,048 bit keys increased by 4.5 times.
