ssl:ssl_peer_certificate/2

ssl_peer_certificate(+Stream, -Certificate) is semidet

True if the peer certificate is provided (this is always the case for a client connection) and Certificate unifies with the peer certificate. The example below uses this to obtain the Common Name of the peer after establishing an https client connection:

  http_open(HTTPS_url, In, []),
  ssl_peer_certificate(In, Cert),
  memberchk(subject(Subject), Cert),
  memberchk('CN' = CommonName), Subject)