The Matasano crypto challenges ( http://www.matasano.com/articles/crypto-challenges/ http://www.matasano.com/articles/crypto-challenges/ ) are a great way to do

The Matasano crypto challenges ( http://www.matasano.com/articles/crypto-challenges/ http://www.matasano.com/articles/crypto-challenges/ ) are a great way to do this. Not by implementing crypto, but by breaking crypto in ways that will scare you away from ever implementing crypto.