OpenDKIM has not been updated in the last 7 years and failed to adopt
RFC8463, which introduces Ed25519-SHA256 signatures.
It has thereby held back the DKIM ecosystem, which relies on the DNS
system to publish its public keys. The DNS system in turn does not handle
large record sizes well (see RFC8301), which is why Ed25519 public keys
would be preferable, but I'm not sure the ecosystem has caught up, so we
stay on the conservative side with RSA for now.
Closes: #210
I recently went through this, and the generated file looks a bit
different than was previously documented.
I opted to be explicit about `k=rsa` (even though [the default is
"rsa"](https://datatracker.ietf.org/doc/html/rfc6376#section-3.6.1)).
I also opted to be explicit about `s=email` ([the default is
"*"](https://datatracker.ietf.org/doc/html/rfc6376#section-3.6.1)).
Honestly not sure what the consequences of this are, I don't know if
DKIM is used for anything besides email.
Add a certificate scheme for using an existing ACME certificate without
setting up Nginx.
Also use names instead of magic numbers for certificate schemes.
