Opportunistic encryption (OE) refers to any system that, when connecting to another system, attempts to encrypt communications channels, otherwise falling back to unencrypted communications. This method requires no pre-arrangement between the two systems.
Opportunistic encryption can be used to combat passive wiretapping.[1] (an active wiretapper, on the other hand, can disrupt encryption negotiation to either force an unencrypted channel or perform a man-in-the-middle attack on the encrypted link.) It does not provide a strong level of security as authentication may be difficult to establish and secure communications are not mandatory. However, it does make the encryption of most Internet traffic easy to implement, which removes a significant impediment to the mass adoption of Internet traffic security.
Opportunistic encryption on the Internet is described in RFC 4322 "Opportunistic Encryption using the Internet Key Exchange (IKE)", RFC 7435 "Opportunistic Security: Some Protection Most of the Time", and in RFC 8164 "Opportunistic Security for HTTP/2".