There is a crypto challenge that explains many of the flaws in crypto done not exactly right, by giving real examples / puzzles on how to break the crypto. See

There is a crypto challenge that explains many of the flaws in crypto done not exactly right, by giving real examples / puzzles on how to break the crypto. See http://www.matasano.com/articles/crypto-challenges/ http://www.matasano.com/articles/crypto-challenges/