Sunday July 17, 2011
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 in cryptography is that using the same encryption key on the cipher text restores the plain text; for example, 65 XOR 42 = 107, and 107 XOR 42 = 65. However, your task in this problem is to exploit this property.

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 job has been made easy, as the encryption key consists of precisely three lower case characters. Using cipher.txt below, a file containing the encrypted ASCII codes, and the knowledge that the plain text must contain common English words (including upper- and lower-case letters, numbers, and common punctuation), decrypt the message.

(This problem and its text come from Project Euler).
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