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 2021, and breaks them down based on the length of the key, or the number of bits used in critical elements of the cryptographic operations.

CHART DKIM key lengths

The graph below shows the number of the two most popular RSA key sizes observed in use since 2013. Note that the overall number of DKIM keys observed trippled 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.

CHART Most popular DKIM key sizes 2013 to 2021

We hope to take a closer look at how and why these numbers changed so dramatically during 2021 in the future.