Friday, September 22, 2017

Cryptopals Set 2 -- F#

After a hiatus, Set 2 completed in F#: https://github.com/tkuriyama/cryptopals

After struggling with type mismatches using sequences vs lists vs arrays for a while, I settled on using byte arrays as inputs and outputs wherever possible. While it seems more functional to recurse over lists, many of the problems are much simpler when it is possible to index directly into arrays. Also, the .NET AES function assumes an array.

The F# solutions have fewer lines than the Python ones, but verbosity is comparable. In part this is due to the fact that using immutable objects is just more verbose for some operations (e.g. creating a new array with one byte difference in the middle). It will be interesting to see how the comparison develops with some of the more complex problems. (Of course, the real significance does not lie in word or line count, but correctness and conciseness of solutions...)

Notes

  • Challenge 14 Byte-at-a-time ECB decryption -- I realized that my previous Python implementation did not interpret the problem correctly (it assumed that the target bytes could be fed into the oracle one block at a time, whereas the oracle already ships with all target bytes, so to speak). I also interpreted the oracle function slightly different this time, insofar as having an unknown, random prefix that does not change once the oracle function is initialized (as opposed to returning a different prefix each time the oracle function is called). 

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