Off-The-Record Messaging Protocol is a custom tailored encryption scheme for instant messaging based on public key encryption. In addition to the standard benefits of Public Key encryption, such as proving your identity, and encrypting your communication, OTR also offers plausible deniability.

The best part of OTR Messaging Protocol is that its not just a white paper: You can download plugins for most open source IM clients (Pidgin, Kopete, Miranda, Trillian, and others. Adium has support built in), run a local proxy to encrypt any other AIM client!

There is a technical white paper on the OTF Messaging Protocol available at http://www.cypherpunks.ca/otr/Protocol-v2-3.1.0.html, and a discussion on why one would use OTR for instant messaging rather than PHP at www.cypherpunks.ca/otr/otr-wpes.pdf