prepareStrings :: [Char] -> [Char] -> [(Char, Char)]
prepareStrings x y = zip  (filter isLetter x)  (filter isLetter (take 19  $ cycle y))

decodeLetter (key,secret)
   | isLower key = chr(ord 'a' + ((ord key + ord secret - ord 'a') - ord 'a') `mod` 26)
   | isUpper key = chr(ord 'A' + ((ord key + ord secret - ord 'A') - ord 'A') `mod` 26)

encodeLetter (key,secret)
   | isLower key = chr(ord 'a' + ((ord key - ord secret - ord 'a') - ord 'a') `mod` 26)
   | isUpper key = chr(ord 'A' + ((ord key - ord secret - ord 'A') - ord 'A') `mod` 26)
vigienere tekst key  =  map decodeLetter $ prepareStrings  tekst key

unvigienere tekst key = map encodeLetter $ prepareStrings tekst key