CryptoHack – A fun, free platform for learning cryptography
cryptohack.org
What Jamie wrote
This looks like a really great site to learn about a variety of cryptography approaches from basic to fairly advanced. I created a profile so I can try it out. This could be a fun one to do with your kids as well. I thought it was funny that they gave you a simple cypher to solve just to register (hint: ROT-13).