there are hundred of password crackers out there made by software houses for legitmate reasons, use one of those, but if you want to learn how to do it then this problby isnt the place as jwenting said. Its a touchy subject because as it could be used legally on your own files as you say, it could also be used to hack
I always thought it was fun to build a crack not because it goes around the law, but because it is something totally new to program.
Granted I have never distrubuted one, and have only managed to build one for a md5 brute force cracker, which given my computing power could never be used to generate anything more than 5 characters in a day or two (350 mhz). I've been to lazy/cheap to take my 2ghz laptop in for repair.
but the answer to bob's question, is it possible, absolutely! but no i can't tell you how to do it (hmm... this might be my next project)
That would be an extremely hard project. You'll be working with decrypting possibly 128 bit keys. Mathematically, it SHOULD take millions of years to crack that. So, you would need some very sophisticated methods to break passwords.
Actually it's an extremely easy project. Realisticaly, as you have mentioned it will take forever and have virtually no practical use because of the time! And like i said it takes a day or two with a 5 character password ... mind you I am limiting my character set to the most commonly used characters, ie ignoring special symbols. Adding just 1 more character to the 5 char would make it go through all the previous keys times that number of characters you have entered.
So supposed i used all numbers (10), lower (26), and upper (26) case letters my character set would be 62. So if a 5 character password took a day to crack a 6 character set would take 62 days. But remember this is completly dependent on the computers cpu speed.