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Why would you want to, out of curiousity? I mean, if you lost your password, just request a new one to login, then change it back to something you know as soon as you get in...... If users are requesting that you do that for them, it very well might be a huge security risk if you do, because identiy theft happens all the time, if you know what I mean.
Basically thats the main reason passwords are encrypted and stored that way in the db and not in plain text, but to answer your question, I'm not real sure of a way to decode them. Im sure there probably is someone that knows, but finding one here in the main XOOPS site might be tough.... Just FYI....