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-- Each character on a computer is assigned a unique code and the preferred | ||
-- standard is ASCII (American Standard Code for Information Interchange). For | ||
-- example, uppercase A = 65, asterisk (*) = 42, and lowercase k = 107. | ||
-- | ||
-- A modern encryption method is to take a text file, convert the bytes to | ||
-- ASCII, then XOR each byte with a given value, taken from a secret key. The | ||
-- advantage with the XOR function is that using the same encryption key on the | ||
-- cipher text, restores the plain text; for example, 65 XOR 42 = 107, then 107 | ||
-- XOR 42 = 65. | ||
-- | ||
-- For unbreakable encryption, the key is the same length as the plain text | ||
-- message, and the key is made up of random bytes. The user would keep the | ||
-- encrypted message and the encryption key in different locations, and without | ||
-- both "halves", it is impossible to decrypt the message. | ||
-- | ||
-- Unfortunately, this method is impractical for most users, so the modified | ||
-- method is to use a password as a key. If the password is shorter than the | ||
-- message, which is likely, the key is repeated cyclically throughout the | ||
-- message. The balance for this method is using a sufficiently long password | ||
-- key for security, but short enough to be memorable. | ||
-- | ||
-- Your task has been made easy, as the encryption key consists of three lower | ||
-- case characters. Using prob59.txt, a file containing the encrypted ASCII | ||
-- codes, and the knowledge that the plain text must contain common English | ||
-- words, decrypt the message and find the sum of the ASCII values in the | ||
-- original text. | ||
-- | ||
|
||
import Data.Bits (xor) | ||
import Data.Char (isSpace,isAlphaNum) | ||
import Data.Ord (comparing) | ||
import Control.Monad (replicateM) | ||
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getCiphertext :: IO String | ||
getCiphertext = do | ||
source <- fmap (takeWhile (not.isSpace)) $ readFile "prob59.txt" | ||
let bytes :: [Int] | ||
bytes = read $ "[" ++ source ++ "]" | ||
return $ map toEnum bytes | ||
|
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-- all possible passwords | ||
passwords :: [String] | ||
passwords = replicateM 3 ['a'..'z'] | ||
|
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-- decrypt password ciphertext | ||
decrypt :: String -> String -> String | ||
decrypt ciphertext pass = toString decrypted | ||
where decrypted :: [Int] | ||
decrypted = zipWith xor (fromString ciphertext) (cycle $ fromString pass) | ||
toString :: [Int] -> String | ||
toString = map toEnum | ||
fromString :: String -> [Int] | ||
fromString = map fromEnum | ||
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isGood :: String -> Bool | ||
isGood = and . map (flip any tests . flip ($)) | ||
where tests = [isAlphaNum, isSpace, isGoodPunct] | ||
isGoodPunct = flip elem ",.()'\"?!@#$%&*:;" | ||
|
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main :: IO () | ||
main = do | ||
cipher <- getCiphertext | ||
let decryptions = map (decrypt cipher) passwords | ||
validDecryptions = filter isGood decryptions | ||
-- we've experimentally confirmed that validDecryptions is 1 entry | ||
let best = head validDecryptions | ||
print best | ||
print $ sum $ map fromEnum best |
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