matasano crypto class notes/assignments

matasano.hs

import Data.Char
import Data.Word
import Data.Bits
import Data.List
import Data.Maybe
import qualified Data.ByteString as B

import Codec.Crypto.AES

repeatedKeyXor = do
  bs <- B.readFile "./test.file"
  let key = [80, 25, 64, 3]
  let encrypted = B.pack $ zipWith xor (cycle key) $ B.unpack bs
  putStrLn $ show $ encrypted

aesBlockSize = 16 -- bytes
pad size char bs = B.append bs $ strToBs $ replicate (mod (size - mod (B.length bs) size) size) char
strToBs str = B.pack $ map ((.&. 0xFF) . fromIntegral . ord) str

ecbCrypt key str =
  let bs = strToBs str in
  let key' = pad aesBlockSize '\0' $ strToBs key in
  let iv = B.pack $ replicate aesBlockSize 0 in
  crypt' ECB key' iv Encrypt $ pad aesBlockSize '\0' bs

cbcCrypt key str =
  let bs = strToBs str in
  let key' = pad aesBlockSize '\0' $ strToBs key in
  let iv = B.pack $ replicate aesBlockSize 0 in -- XXX: unsafe
  crypt' CBC key' iv Encrypt $ pad aesBlockSize '\0' bs

ctrCrypt key str =
  let bs = strToBs str in
  let key' = pad aesBlockSize '\0' $ strToBs key in
  let iv = B.pack $ replicate aesBlockSize 0 in -- XXX: unsafe
  crypt' CTR key' iv Encrypt bs

suffix = ">>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>"
knownNumberOfBlocks = (length suffix `div` aesBlockSize) + 1
offsets = take (knownNumberOfBlocks * aesBlockSize) $ (cycle [(aesBlockSize - 1), (aesBlockSize - 2) .. 0])
server plaintext = ecbCrypt "their secret key" (plaintext ++ suffix)

findTheCharacter followingCharacters (offset, targetBlock) =
  let knownCiphertext = B.take (aesBlockSize * targetBlock) $ server $ replicate offset 'a' in
  let possibilities = [(ch, B.take (aesBlockSize * targetBlock) $ server $ replicate offset 'a' ++ followingCharacters ++ [ch]) | ch <- ['\0'..'\255']] in
  let matchingPossibility = find (\pair -> (snd pair) == knownCiphertext) possibilities in
  let nextChar = fst $ fromMaybe ('\0', B.empty) matchingPossibility in
  followingCharacters ++ [nextChar]

assignment = foldl findTheCharacter "" $ zip offsets (concat $ [replicate aesBlockSize x | x <- [1..]])