BCrypt implements OpenBSD-style Blowfish password hashing using
the scheme described in "A Future-Adaptable Password Scheme" by
Niels Provos and David Mazieres.
Perform the "enhanced key schedule" step described by
Provos and Mazieres in "A Future-Adaptable Password Scheme"
http://www.openbsd.org/papers/bcrypt-paper.ps