Define precisely how to marshal SubjectPublicKeyInfo structure in tls.md

[kpp]

This is a clarification how to marshal SubjectPublicKeyInfo according to the go implementation (https://github.com/libp2p/go-libp2p-tls/blob/7530faa07acbfc0aa918c072c2cb35d3c8d5d859/crypto.go#L143 and https://pkg.go.dev/crypto/x509#MarshalPKIXPublicKey).

[mxinden]

This looks good to me. Thanks @kpp for extending the docs. This kind of contribution is very much appreciated!

Any additional comments @marten-seemann? If not, I will merge here.

[marten-seemann]

All that we do is marshal the public key: https://github.com/libp2p/go-libp2p-tls/blob/7530faa07acbfc0aa918c072c2cb35d3c8d5d859/crypto.go#L167-L174
I'm not sure if we need to say anything more than https://pkg.go.dev/crypto/x509#MarshalPKIXPublicKey says, as this seems to be the standard way of marshaling the key. I find spelling it out explicitly, as suggested in this PR, confusing, as it suggests that we're doing something non-standard here.

[kpp]

@marten-seemann Would it be better if I replace that statement with: "The public key is encoded as a SubjectPublicKeyInfo structure as described in RFC 5280, Section 4.1:"?

[marten-seemann]

Would it be better if I replace that statement with: "The public key is encoded as a SubjectPublicKeyInfo structure as described in RFC 5280, Section 4.1:"?

Yes, I think referencing the RFC makes a lot of sense here.

[kpp]

Done. Sorry for the delay.

[mxinden]

Friendly ping @marten-seemann.