Firm Grip Handshakes: A Tool for Bidirectional Vouching

Clients trust servers over the Internet due to their trust in digital signatures of certification authorities (CAs) which comprise the Internet’s trust infrastructure. Based on the recent DigiNotar attack and other attacks on CAs, we formulate here a very strong attack denoted “Certificate in The Middle” (CiTM) and propose a mitigation for this attack. The solution is embedded in a handshake protocol and makes it more robust: It adds to the usual aspect of “CA vouching” a client side vouching for the server “continuity of service,” thus, allowing clients and server to detect past and future breaches of the trust infrastructure. We had simplicity, flexibility, and scalability in mind, solving the problem within the context of the protocol (with the underlying goal of embedding the solution in the TLS layer) with minor field changes, minimal cryptographic additions, no interaction with other protocol layers, and no added trusted parties.