Scrypto is a project to port one of many existing Javascript encryption suites to Greasemonkey. Currently, the target is Juergen Gaertner's self-decrypting archives.

The initial release version of the user script is complete, but I need to clear up some ambiguities in the license for the original code. To date, I have been unable to contact the author. meh

Another possible candidate is John Walker's Javascrypt, although it appears old enough to be broken in Firefox 1.5, at least.