Crypto Challenge Set 3
This is the next set of block cipher cryptography challenges (even the randomness stuff here plays into block cipher crypto).
This set is moderately difficult. It includes a famous attack against CBC mode, and a "cloning" attack on a popular RNG that can be annoying to get right.
We've also reached a point in the crypto challenges where all the challenges, with one possible exception, are valuable in breaking real-world crypto.