TCPCRYPT

Tcpcrypt is a TCP extension defined in RFC 8548 that encrypts connection payloads opportunistically, without certificates, configuration, or application changes. Because it does not authenticate the peer it stops passive eavesdropping but not an active machine-in-the-middle attacker, so it is a floor rather than a replacement for TLS. Adoption stayed limited, and TLS plus QUIC became the practical answer instead.