There are multiple possible answers there. You are correct that 0 means none of the letters in the word you picked match the password. To change your signature go to the upper left drop-down arrow and select My Profile, then look to the upper right for a button that says Edit my Profile. Your signature will then be one of the options along the left column.
Also, terminal hacking is made much easier (at least for me) by use of the <>, (), [], and {} symbols. If they are facing together like-so, on the same line without a word in-between then you can select the left one to either reset your guesses to max (4 or 5 with bobble-head) or remove a dud password. You can easily just guess 3 times and if you don't get it then go through all the matching brackets you can find to reset your guesses and remove duds. I used to do that, now I just hunt down all the brackets first and then figure it out from there. (sometimes you won't find one that resets your guesses, so bringing it down to one can be risky)