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I've expended 0 effort to find the answer to this myself, but I wonder if this is based on the OTR protocol, and if not, why not.

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The TextSecure protocol is now named the "Signal Protocol"; it's developed by Open Whisper Systems. It is the protocol used by the Signal app on Android and iPhone, and as of this week, also used by WhatsApp.

Here is an older post where the authors of the protocol explain why not OTR: https://whispersystems.org/blog/advanced-ratcheting/

The main takeaway: text messaging, unlike traditional instant messaging, is primarily asynchronous with long-lived sessions, where traditional instant messaging is primarily synchronous with short-lived sessions.

The protocol used to be called Axolotl, if you want to search for older discussions and research on it.

The crypto primitives they use are called Axolotl as a group. Axolotl is to signal what RSA is to TLS.

The Axolotl construction was invented for Signal Protocol, which was itself originally called "Axolotl".

Do you happen to know whether this is the same protocol used in SMSSecure? I know it is a fork of TextSecure but am not clear on whether TextSecure changed their protocol after the fork in the process of becoming Signal.

It's the same. TextSecure called it "Axolotl", but the "Signal protocol" appears to just be a branding change.

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