US Patent 10298404 Certificate echoing for session security

A client establishes a cryptographically protected communications session with a server. To detect a man-in-the-middle, the client echoes information about a certificate purportedly received from the server. The information echoed by the client is digitally signed so as to be verifiable by the server without any cryptographic key used in the cryptographically protected communications session or its establishment, thereby rendering the echoed information unmodifiable by a man-in-the-middle without invalidating the signature. The server can therefore verify both the echoed information and the digital signature to determine whether it has established a cryptographically protected communications session with the client or with a man-in-the-middle purporting to be the client.