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Add prob59.hs and prob59.txt

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1 parent a484211 commit 0260be823cfe389cbabd2da67789b096cfe5a6d0 @kballard committed Mar 16, 2012
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  1. +68 −0 prob59.hs
  2. +1 −0 prob59.txt
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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)
+
+getCiphertext :: IO String
+getCiphertext = do
+ source <- fmap (takeWhile (not.isSpace)) $ readFile "prob59.txt"
+ let bytes :: [Int]
+ bytes = read $ "[" ++ source ++ "]"
+ return $ map toEnum bytes
+
+-- all possible passwords
+passwords :: [String]
+passwords = replicateM 3 ['a'..'z']
+
+-- 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
+
+isGood :: String -> Bool
+isGood = and . map (flip any tests . flip ($))
+ where tests = [isAlphaNum, isSpace, isGoodPunct]
+ isGoodPunct = flip elem ",.()'\"?!@#$%&*:;"
+
+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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@@ -0,0 +1 @@
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