crack.hs

import Data.Char

positions :: (Eq a) => a -> [a] -> [Int]
positions t xs = [n | (x,n) <- zip xs [0..(length xs)-1], x == t]

count :: Char -> String -> Int
count c (x:xs) | c == x = 1 + count c xs
               | otherwise = count c xs
count _ [] = 0

lowers :: String -> Int
lowers xs = sum [1 | x <- xs, isLower x]

let2int :: Char -> Int
let2int c = ord c - ord 'a'

int2let :: Int -> Char
int2let n = chr (ord 'a' + n)

shift :: Int -> Char -> Char
shift n c | isLower c = int2let((let2int c + n) `mod` 26)
          | otherwise = c

encode :: Int -> String -> String
encode n xs = [shift n x | x <- xs]

table :: [Float]
table = [8.2, 1.5, 2.8, 4.3, 12.7, 2.2, 2.0, 6.1, 7.0, 0.2, 0.8, 4.0, 2.4, 6.7, 7.5, 1.9, 0.1, 6.0, 6.3, 9.1, 2.8, 1.0, 2.4, 0.2, 2.0, 0.1]

percent :: Int -> Int -> Float
percent n m = (fromIntegral n / fromIntegral m) * 100

freqs :: String -> [Float]
freqs xs = [percent (count x xs) n | x <- ['a'..'z']]
           where n = lowers xs

chisqr :: [Float] -> [Float] -> Float
chisqr os es = sum [((o - e)^2) / e | (o,e) <- zip os es]

rotate :: Int -> [a] -> [a]
rotate n xs = drop n xs ++ take n xs

crack :: String -> String
crack xs = encode (-factor) xs
           where
             factor = head (positions (minimum chitab) chitab)
             chitab = [chisqr (rotate n table') table | n <- [0..25]]
             table' = freqs xs

-- crack "kdvnhoo lv ixq" 

count :: Char -> String -> Int
count c cs = length $ filter (== c) cs