As far as I know, mysql's password() function is non-reversible (but it is repeatable, of course).

reversible are: encode (by decode), des_encrypt/des_decrypt (for mysql with SSL support), aes_encrypt/aes_decrypt.

Make sure to check which mysql version you have, for these functions have changed between versions, or are only available in newer ones.

Searching with google for these words will find you many explanations.