[TLS] Version pinning and indicating the signing algorithm in draft-ietf-tls-subcerts-02

[Watson Ladd <[e-mail]>]

Dear TLS WG,

While we were implementing delegated credentials we came up with the
following two problems. I thank David Benjamin for communicating the
first to me.

The first is that a delegated credential is pinned to a particular
version of TLS. Unfortunately this means that the rollout of a new
version of TLS (which libraries consider that they can do on their
own) makes an existing set of delegated credentials unusable, and the
signer of delegated credentials needs to accept a new version of TLS
as valid before the extension can be used with a new version. This is
similar to the issues prompting
draft-wood-tls-external-psk-importer-00 where PSKs depend on
negotiated parameters, and thus are difficult to use in practice.

The second is that the DelegatedCredential indicates the algorithm
that was used to sign it. Someone who is a bit careless when
implementing might take that literally with the result that they
verify a DelegatedCredential with RSA algorithm and the certificate is
really an ECDSA certificate. I'll submit a PR with text for security
considerations/rewording the verification steps.

Sincerely,
Watson