Crypto Challenge Set 2
This is the first of several sets on block cipher cryptography. This is bread-and-butter crypto, the kind you'll see implemented in most web software that does crypto.
This set is relatively easy. People that clear set 1 tend to clear set 2 somewhat quickly.
Three of the challenges in this set are extremely valuable in breaking real-world crypto; one allows you to decrypt messages encrypted in the default mode of AES, and the other two allow you to rewrite messages encrypted in the most popular modes of AES.