This is the OTR plugin for Pidgin.

Off-the-Record (OTR) Messaging allows you to have private
conversations over instant messaging by providing:

Encryption
    No one else can read your instant messages.
Authentication
    You are assured the correspondent is who you think it is.
Deniability
    The messages you send do not have digital signatures that are
    checkable by a third party. Anyone can forge messages after a
    conversation to make them look like they came from you. However,
    during a conversation, your correspondent is assured the messages
    he sees are authentic and unmodified.
Perfect forward secrecy
    If you lose control of your private keys, no previous conversation
    is compromised.

Homepage:
http://www.cypherpunks.ca/otr/
