Resurrect SIG(0)

[jschlyter]

I'm considering resurrecting support for our old friend SIG(0) in order to support secure DDNS for https://datatracker.ietf.org/doc/draft-johani-dnsop-delegation-mgmt-via-ddns/.

Any prior work on SIG(0)? I've done some work in https://github.com/jschlyter/dnspython/tree/sig0 starting to add a RRSigBase as RRIG and SIG(0) basically share everything.

[rthalley]

I don't think dnspython has ever supported SIG(0), and I've been avoiding it for more than 20 years. That's not to say you shouldn't do it! I'd merge support for it. Starting from an RRSIGBase seems reasonable. I'm guessing we'll have to give some thought to the message API and keyring management, and that it would be similar to TSIG in many ways (e.g. the need to reserve space in the message. I expect Brian will also have good advice!