http://ssd.iykpqm7jiradoeezzkhj7c4b33g4hbgfwelht2evxxeicbpjy44c7ead.onion/module/deep-dive-end-end-encryption-how-do-public-key-encryption-systems-work
Off the Record (OTR) OTR was once a popular add-on to early messaging chat clients, like Pidgin, and was used for communicating on services like Google Hangouts. For OTR to work, both parties must be online to set up a session, which limits its use. With OTR, every back-and-forth message is a “session.” This means that if a single key is leaked, only messages from that session are compromised, not anything before or after.