Add API for verifying that a private key is valid for a certificate

[briansmith]

TLS servers, and TLS clients that do client authentication, need way to verify that a private key that they're about to use for authentication is the one that corresponds to the certificate they present to the peer. And/or they need an API that selects a certificate that matches a private key.

See *ring issue 419 for more context.

@ctz Do you have any suggestions for an API that you could use in Rustls to verify that the certificate and the private key being used are consistent?

[ctz]

I'd be happy with a function which computes a key hash from a webpki EndEntityCert or a ring RSAKeyPair. I understand the usual thing to do is hash a DER encoding of the SPKI. I guess that would mean ring needs to learn how to construct one of those from a RSAKeyPair.

[briansmith]

See https://twitter.com/avadacatavra/status/836732794445381632: "accidentally switched my certificate and key files in a call to an openssl library[.]" That would be a deadly mistake in a web server.

[briansmith]

See https://blog.hboeck.de/archives/888-How-I-tricked-Symantec-with-a-Fake-Private-Key.html.