Forked from https://github.com/emerose/pbkdf2-ruby by Sam Quigley. This implementation conforms to RFC 2898, and has been tested using the test vectors in Appendix B of RFC 3962. Note, however, that while those specifications use HMAC-SHA-1, this implementation defaults to HMAC-SHA-256. (SHA-256 provides a longer bit length. In addition, NIST has stated that SHA-1 should be phased out due to concerns over recent cryptanalytic attacks.)