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OTR – Off-the-Record Messaging

OTR, Off-the-Record, is a secure encryption algorythm.

PGP has the problem, that after getting the private fingerprint/key you can, for all old files/conversations you have, tell it was that person/that key encrypting them.

OTR however automatically generates and uses new keys to ensure it’s secure, even in the future.

You can’t tell it was a specific person encrypting it as anyone could fake it.

A Plugin for Pidgin is available, as well as for many other Messengers.

Also see wikipedia article.