Implement server acceptable certificate auths

[reynir]

The code for accepted certificate authorities in a client certificate
request was already implemented, but handshake_server.ml had hardcoded
an empty list of CAs. This commit adds a configuration option for this.

Note that this requires a change to ocaml-x509: mirleft/ocaml-x509#87

[reynir]

I should mention that I haven't tested this thoroughly. I have tested with openssl s_client. As mentioned, the TLS-part seem to have been implemented and it was just a matter of passing CAs and extracting the issuer DN (hence the ocaml-x509 change). It seems to work - I have tested with one and two certificates in config.accepted_cas.

The TODO in lib/handshake_server.ml mentions extracting "DN from authenticator", but since authenticator is a function it wouldn't be possible so I opted for adding Yet Another Configuration Option accepted_cas. It's not the best user experience since you have to repeat the CAs, once in the authenticator and once in the server config. Furthermore, the list of CAs will be static - there might be some use case for a dynamic list of CAs(?)

[hannesm]

thanks. indeed, I'd prefer to have the authenticator and accepted_cas unified. given that authenticator might as well be a set of fingerprints, this is certainly not easily doable atm, but we'll for sure find a way. I'm currently travelling..

[reynir]

Done. Have a nice nap+trip! :-)

Edit: oops, this was supposed to go to mirleft/ocaml-x509#87

[cfcs]

Did this ever get merged?

[reynir]

@cfcs No, it's not been merged.

[hannesm]

thanks, superseeded by #374 (please let me know if you disagree with the changes in 374, I amended your commit, i.e. you're still the author :)

[reynir]

Thanks!

The logic seems to be the same modulo naming and DN-unwrapping, so no complaints from me.

I look forward to play with client certificates again!