PBKDF2 is a secure password hashing algorithm that uses the techniques
of "key strengthening" to make the complexity of a brute-force
attack arbitrarily high. PBKDF2 uses any other cryptographic hash
or cipher (by convention, usually HMAC-SHA1, but Crypt::PBKDF2 is
fully pluggable), and allows for an arbitrary number of iterations
of the hashing function, and a nearly unlimited output hash size
(up to 2**32 - 1 times the size of the output of the backend hash).
The hash is salted, as any password hash should be, and the salt
may also be of arbitrary size.
