US Patent 10055591 Secure protocol attack mitigation

A handshake for establishing a secure connection between a client computer system and a service includes a CAPTCHA element. When the client computer system initiates the secure connection to the service, the service responds by generating a key seed and providing the key seed to the client computer system in the form of the CAPTCHA element. The CAPTCHA element is solvable by a human user at the client computer system to obtain a solution. The solution to the CAPTCHA is used to recover the key seed. The client and the server use the key seed to generate an encryption key which is used to encrypt communications between the client computer system and the service.