[ root-me ]Shift cipher

In a shift-ciphered text the key consists of only one character: it is added (modulo 256) for each of the character of the original message to obtain the ciphertext.

Since the key space is only of 256 elements is not a problem do a bruteforce and visually inspect each transposition to find a message with sense.

The following python code solves the challenge:

1. with open('ch7.bin') as f:
2. msg = f.read()
3. for x in range(256):
4. print ''.join([chr((ord(y) + x) % 256) for y in msg])

shift cipher，经典的密码类型，透过字母的位移来达成简易的加密，最有名的例子就是Caesar cipher凯萨加密法。

题目提供了一个binary档案，里面有一串密文：

L|ky+*^*zo*kvsno|*kom*vo*zk}}*cyvksr

稍微观察一下，发现「*」一直出现。其实还满好猜测的，如果是一个句子，会一直出现的不外乎就是「空格」，查一下ASCII表格，空格是32，而星号则是42，推测位移量应该是10，写个简单的程式转换一下字串：

1 2 3 4 5

<?php $file = fopen( "ch7.bin" , "r" ); $code = str_split(fgets($file)); for ($i = 0 ; $i < count($code) ; $i++) echo chr(ord($code[$i]) - 10 );

输出结果是一串法文，不过没关系，我们只要看懂pass就是知道后面那个字串应该就是密码了：

Bravo! Tu peux valider avec le pass Yolaihu