Password encoding and validation using bcrypt.

Example usage:

>>> import Crypto.KDF.BCrypt (hashPassword, validatePassword)
>>> import qualified Data.ByteString.Char8 as B
>>> 
>>> let bcryptHash = B.pack "$2a$10$MJJifxfaqQmbx1Mhsq3oq.YmMmfNhkyW4s/MS3K5rIMVfB7w0Q/OW"
>>> let password = B.pack "password"
>>> validatePassword password bcryptHash
>>> True
>>> let otherPassword = B.pack "otherpassword"
>>> otherHash <- hashPassword 12 otherPassword :: IO B.ByteString
>>> validatePassword otherPassword otherHash
>>> True

See https://www.usenix.org/conference/1999-usenix-annual-technical-conference/future-adaptable-password-scheme for details of the original algorithm.

The functions hashPassword and validatePassword should be all that most users need.

Hashes are strings of the form $2a$10$MJJifxfaqQmbx1Mhsq3oq.YmMmfNhkyW4sMS3K5rIMVfB7w0QOW which encode a version number, an integer cost parameter and the concatenated salt and hash bytes (each separately Base64 encoded. Incrementing the cost parameter approximately doubles the time taken to calculate the hash.

The different version numbers evolved to account for bugs in the standard C implementations. They don't represent different versions of the algorithm itself and in most cases should produce identical results. The most up to date version is 2b and this implementation uses the 2b version prefix, but will also attempt to validate against hashes with versions 2a and 2y. Version 2 or 2x will be rejected. No attempt is made to differentiate between the different versions when validating a password, but in practice this shouldn't cause any problems if passwords are UTF-8 encoded (which they should be) and less than 256 characters long.

The cost parameter can be between 4 and 31 inclusive, but anything less than 10 is probably not strong enough. High values may be prohibitively slow depending on your hardware. Choose the highest value you can without having an unacceptable impact on your users. The cost parameter can also be varied depending on the account, since it is unique to an individual hash.