Based on the beta version of the Chrome Extension, they use
https://github.com/wwwtyro/cryptico to create a RSA keypair based on a user-provided keyphrase. The private part is stored in the LocalStorage of the extension and the public part seems to be submitted to the server. Actual message encryption then happens using AES, but I'm not sure about the details. They delegate all actual crypto to the cryptico lib, so the description on GitHub is probably accurate for BlockPRISM's encryption too.