Reversible encryption is not usually used for authentication. Instead, a crypt or MD5 digest of the password is stored. On login, the offered password is digested and compared to the stored hash.

That improves password security by making it impossible for anyone (including root) to read a password from the host. Sites which observe this elementary precaution cannot mail you a password reminder, only a new password - which should be changed on first use.

After Compline,
Zaxo