What's in a DKIM Record?

I had recently opened a test Aster Mail email account, which allows for one free own domain to be used. For my test using a spare own domain, I was setting up the usual DNS records (MX, SPF, DMARC), but when it came to the DKIM record I was slightly confused - the length of the record was much shorter than I had been accustomed to using.

The current industry standard is RSA, where a 2048-bit key results in a very long string of nearly 400 characters, and this was the format that I was used to.

This key begins with: 

v=DKIM1; k=rsa;

In contrast, Aster Mail were using the more modern Ed25519 format, which produces a much shorter key of exactly 44 characters, and this was a new one to me - I'd never seen it before.

This key begins with:

v=DKIM1; k=ed25519;

Both of the above keys would be TXT records. Because these keys are static, they're not 'rotatable' - you need to have a new key generated, and publish it alongside the old one to avoid delivery gaps, and eventually remove the outdated record once the new one has propagated.

There are other email providers (for example, mailbox.org) that provide the DKIM key in a CNAME format - which allows them to rotate the keys automatically on a regular basis.