Algebra - 8. Hausaufgabe

decrypt.hs

import Data.Char

alphabet = ['A'..'Z']++['ß','Ä','Ö','Ü']

charToInteger :: Char -> Integer
charToInteger char = maybe 0 id isValue
    where charMap = zip alphabet [0..29]
          isValue = lookup (toUpper char) charMap

integerToChar :: Integer -> Char
integerToChar int
    | 0 > int || int > 29 = ' '
    | otherwise = alphabet !! (fromInteger int)

decrypt :: Integer -> Integer -> [Char] -> [Char]
decrypt d n crypt = map integerToChar plain
    where nums = map charToInteger crypt
          plain = map (\m -> ((m `mod` n)^d) `mod` n) nums

main = print $ (decrypt 13 31 $ decrypt 7 31 "ÄMDARQYC") == "ÄMDARQYC"