Problem 59

Problem 59

XOR decryption

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 cipher.txt (right click and ‘Save Link/Target As…’), 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.


计算机上的每个字符都被指定了一个独特的代码,其中被广泛使用的一种是ASCII码(美国信息交换标准代码)。例如,大写字母A = 65,星号(*) = 42,小写字母k = 107。

一种现代加密方法是将一个文本文档中的符号先转化为ASCII码,然后将每个字节异或一个根据密钥确定的值。使用异或进行加密的好处在于,只需对密文使用相同的密钥再加密一次就能得到明文,例如,65 XOR 42 = 107,而107 XOR 42 = 65。