Opportunistic encryption - Definition from the Hotspot Shield Glossary
This type of data encryption is not permanent or fixed - it does not provide full security benefits. In essence, Opportunistic Encryption applies to any system or device that attempts to encrypt data when connecting to another device or system - If it cannot encrypt the data it will revert to unencrypted form.
This form of encryption is defined in RFC 7435. As Opportunistic Encryption is straightforward to implement it is widely used on many platforms including UNIX and Linux (FreeS/WAN), Windows OS, Email traffic, VoIP and Websites. A simple way to describe Opportunistic Encryption can be described as “some protection most of the time”.