Hmm, don't think this is allowed?
But it depends on the modem/wireless adapter your PC/Laptop/Mobile has...
For PC/Laptop I think you can use backtrack or similar, for android bcmon & Reaver? idk I've never tried since, as I said, depends on the modem.
btw, it could take up to a day to crack the pw, also depends on the encryption (WPA/WEP)