This code is not performing encryption, as the commentors already pointed out, but hashing. The central difference is that hashing, by design, is not reversible cheaply. This is important in password applications because you do explicitly not want to store the user's passwords in any accessible form, but only check against them.
So, in short words: You cannot "decrypt" your hash.
When you want to check a supplied password for correctness, you hash it again like in your code and compare the hash against the hash of the original password.