Return a true value if the encrypted value of 'plaintext' is
def
encrypt(plaintext):
Return the encrypted value of plaintext.
def
validate(plaintext, encrypted):
Return a true value if the encrypted value of 'plaintext' is
equivalent to the value of 'encrypted'. In general, if this
method returns true, it can also be assumed that the value of
self.encrypt(plaintext) will compare equal to 'encrypted'.