Understanding Cryptography practically
If you're really interested in practical problems and want to know what people care about when actually deploying crypto in practice, then you can start by e.g. reading through the messages of the IETF Crypto Forum Research Group mailing list. The discussions cover mostly crypto standards, which means that it's as practical as it can get. Archived discussions can be found here:

http://dir.gmane.org/gmane.ietf.irtf.cfrg

You won't find any homework problems there, but you can find discussions that will lead up to future standards.