[chipsa]

Uh?

They normally store the User Principal Name from the cert, and then use the public/private key as part of the connection. Specifically, the connection is negotiated after the client sends the public client certificate, and uses it as part of the key exchange.

It doesn't necessarily need to store the public key, but it does need to store which certificate goes with which account. And the certificate is validated by checking that it's been issued by a CA the server trusts.

[q3k]

The server doesn't need to store the certificate, or even a mapping from certificate to identity. Just retrieving information encoded in the DN or SANs of a certificate presented by a client is enough to tie the connection originator/client to an identity. I mean, it's a design decision, whether you want to have a layer of indirection there - but keeping it without one allows TLS client certificates to be fully stateless, and be used across multiple backends that do not share any session/mapping store between them.

In addition, if I'm being picky, TLS 1.3 changes how client certificates are used, and they are now not part of the initial handshake.