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.

See also: RFC2898, PKCS#5 version 2.0: http://tools.ietf.org/html/rfc2898