Cryptopals Set 4

As advertised, Set 4 was easier to work through than Set 3.

https://github.com/tkuriyama/cryptopals/tree/master/set4

For better or for worse, I couldn't actually find implementations of SHA-1 and MD4 in F#, so I ended up implementing them using a combination of the standard descriptions and C# / Python implementations. (The MD4 implementation is still incorrect somewhere, so I'll probably need to fix it before attempting Wang's attack later on in the challenges.) As with the Mersenne Twister, there is something very reassuring about being forced to implement each digest iteration in a pure fashion without mutations.

For the SHA1-HMAC timing leak attacks, I fell back to Python (trying Python 3!) since I haven't yet been able to get Paket / Nuget to work on my OS X machine. The F# tooling via Visual Studio for Mac seems like it is coming along, but I'm still trying to work in emacs... I'd like to play around with Suave and some of F#'s HTTP message handling libraries at some point. Meanwhile, setting up a virtualenv for Python 3 and running a tiny server via Flask was incredibly easy.