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.