ciphers.hs

module ROT13 ( rot13 ) where
 
import Data.Char   
import Data.List
 
rot13 :: String -> String
rot13 = map (rotate 13) 
 
rotate :: Int -> Char -> Char
rotate n c
    | isUpper c = rotFrom 65
    | isLower c = rotFrom 97
    | otherwise = c
    where
      rotFrom m = chr $ mod ((ord c) - (m - n)) 26 + m
 
--
-- Try that multiplication idea!
--
 
cipher53 :: Int -> String -> String
cipher53 n = map (c53 n)
 
decipher53 :: Int -> String -> String
decipher53 n = map (c53 n')
    where
      n' = case find (\l -> mod (n*l) 53 == 1) [1..52] of
             Nothing -> 0
             Just x -> x
 
 
c53 :: Int -> Char -> Char
c53 n c = ftoc (mod (n*(ctof c)) 53) 
 
 
ctof :: Char -> Int
ctof c
    | isUpper c = (ord c) - 64
    | isLower c = (ord c) - 70
    | c == ' ' = 0
    | otherwise = 0
 
ftoc :: Int -> Char
ftoc n
    | n > 0 && n <= 26 = chr (n + 64)
    | n > 26 && n <= 52 = chr (n + 70)
    | n == 0 = ' '
    | otherwise = ' '
 
-- rubbish
phi :: Int -> Int
phi n = length . filter (coprime n) $ [1..n-1]
    where
      coprime a b = gcd a b == 1