hacker.org
https://www.hacker.org/challenge/about.php
The Challenge System - hacker
The hacker.org challenges are a series of puzzles, tricks, tests, and brainteasers designed to probe the depths your hacking skills. To master this series you will need to crack cryptography, write clever code, and dissect the impenetrable; and in the process you will enrich your understanding of the world of hacking.
