Before we investigate OTR, let's consider how we could encrypt our chat sessions. We could use GPG for chat. We'd have to know the public key of our correspondent, and each time we'd enter a message, it would encrypt and/or sign the message and send it along. Some chat networks don't have an equivalent e-mail address, so it could be awkward finding and verifying public keys. However, you can certainly imagine a chat system that worked this way; it's a slightly more synchronous version of GPG with e-mail.

Even if those technical problems are addressed there is a bigger issue lurking in PGP's design. Let's return to our friends Alice and Bob. Alice and Bob have been communicating with GPG for quite some time now. They use GPG flawlessly and religiously practice the best security hygiene. Until one day, when somebody gets a hold of Bob's private key. Now, there are several ways this could happen. Despite...