Pbkdf2 (part of the pkcs#5 standard) is a secure password hashing algorithm that uses the techniques of "key strengthening" to make the complexity of a brute-force attack arbitrarily high. the crypt::pbkdf2 module supports sha-1, sha-2 and sha-3 as the underlying hash functions natively and can also use arbitrary digest-compatible classes. it 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.