C library that implements the OTR protocol
https://github.com/off-the-record/libotr

This is a library and toolkit which implements Off-the-Record (OTR) Messaging.

OTR allows you to have private conversations over IM 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.

For more information on Off-the-Record Messaging, see
https://otr.cypherpunks.ca/