You should realize that the unix password is not enciphered as such, but is instead a one-way hash. The standard unix password, for instance, is a string of 8 binary zeros which are encrypted using the password given as a key, and a version of the DES algorithm modified to make it slightly more difficult to attack (including the addition of a "salt" value). The other algorithms out, such as 3DES or MD5, when used for passwords are probably used similarly (although you would have to research them further to be sure).

Likely, if you are wanting to decrypt rather than crack them, there are better approaches you could take; likewise, if you want to decrypt them as a way of testing their strength, there are also better approaches you could take.

Goodluck in your quest for knowledge in that realm.