シーザー暗号 (Caesar cipher)

シフト暗号

プログラム

頻度分析を行って暗号文から平文を復元する例

-- If you want to use it type something like:
-- let tmp = encode 3 "Zombies ate my brains"
-- crack tmp

import Data.Char

helper = zip ['a'..'z'] [0..25]

let2num::Char -> Int
let2num c = head [ b | (a,b) <- helper, a==c]
num2let d = head [ a | (a,b) <- helper, b==(d `mod` length helper)]

shift num c = if isLower c == False
              then c
              else num2let( (let2num c) + num )

encode::Int->String->String
encode num = map (shift num)
decode num = map (shift ((length helper) - num))

lowers s = fromIntegral (length (filter isLower s ) )
count c s = fromIntegral (length (filter(==c) s) )
percent a b = (a/b)*100

freq::String->[Float]
freq s = [percent (count a s) (lowers s) | a<-['a'..'z']]

rotate::Int->String->String
rotate d s = (drop d s) ++ (take d s)

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

position x xs = head [ b | (a,b) <- zip xs [0..(length xs -1)], a==x ]

freqArray s = [chisqr (freq (decode a s)) table | a <- [0..25]]

crack s = decode (position (minimum (freqArray s)) (freqArray s)) s

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]

参考リソース

• Programming in Haskell (2nd Edition)