[gnutls-devel] GnuTLS | With TLS 1.3 enabled, gnutls_handshake() succeeds in client when client fails to send required certificate (#615)

[Development of GNU's TLS library]

Aha. Unfortunately, there is not much we can do for that. The TLS1.3 handshake is:
```

       Client                                           Server

Key  ^ ClientHello
Exch | + key_share*
     | + signature_algorithms*
     | + psk_key_exchange_modes*
     v + pre_shared_key*       -------->
                                                  ServerHello  ^ Key
                                                 + key_share*  | Exch
                                            + pre_shared_key*  v
                                        {EncryptedExtensions}  ^  Server
                                        {CertificateRequest*}  v  Params
                                               {Certificate*}  ^
                                         {CertificateVerify*}  | Auth
                                                   {Finished}  v
                               <--------  [Application Data*]
     ^ {Certificate*}
Auth | {CertificateVerify*}
     v {Finished}              -------->
       [Application Data]      <------->  [Application Data]
```

meaning that the client has seen the client finished before he sends his certificate. That is, the server has no way to indicate rejection of the session due to certificate in the normal handshake, thus `gnutls_handshake()` for the client will not see that error.

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