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.
