This is Implementation of PBKDF2 (Password-Based Key Derivation Function 2) in Rebol2 & Rebol3.
Based on this PHP code, which seemed far the cleanest. https://defuse.ca/php-pbkdf2.htm
Conversion to 4 byte endians uses function from: http://www.codeconscious.com/rebol/tips-and-techniques.html
Tests pass.