This is the base class for a system of objects that encapsulate
passphrases.  An object of this type is a passphrase recogniser: its
job is to recognise whether an offered passphrase is the right one.
For security, such passphrase recognisers usually do not themselves
know the passphrase they are looking for; they can merely recognise it
when they see it.  There are many schemes in use to achieve this
effect, and the intent of this class is to provide a consistent
interface to them all, hiding the details.

The CPAN package Authen::Passphrase contains implementations of
several specific passphrase schemes in addition to the base class.