南京邮电大学CTF密码学部分Writeup

异性相吸

1.xor

2.hex2binary

3.len(bin(miwen))==len(bin(mingwen))

# -*- coding:utf-8 -*-
file_de = open('decode1.txt')
file_en = open('ene.txt')
de = file_de.read()
en = file_en.read()

s = ''
for i, j in zip(de, en):
    s += chr(ord(i) ^ ord(j))
print s
file_de.close()
file_en.close()

MD5

明文:TASC?O3RJMV?WDJKX?ZM

MD5 密文：e9032???da???08????911513?0???a2

查了下 Python MD5的库，发现是内置了一个模块的：

http://www.cnblogs.com/the4king/archive/2012/02/06/2340660.html

暴力猜明文缺失字符。

import hashlib
s1 = "TASC"
s2 = "O3RJMV"
s3 = "WDJKX"
s4 = "ZM"

dict = ['0','1','2','3','4','5','6','7','8','9','0','A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z']

for i in range(36):
    for j in range(36):
        for k in range(36):
            src = s1 + dict[i] + s2 + dict[j] + s3 + dict[k] + s4
            pre = hashlib.md5()
            pre.update(src)
            md5 = str(pre.hexdigest())
            if ('e9032' and 'da' and '911513') in md5 :
                print md5,'  ',src

easy&base64&n次base64&mixed_base64

这几道题都是一个类型的……

import base64
with open('string.txt') as f:
        s = f.read().replace('\n','')
while 1:
        try:
                s = base64.b16decode(s)
        except:
                try:
                     s = base64.b32decode(s)
                except:
                        s = base64.b64decode(s)
        if 'nctf' in s:
                print s
                break

KeyBoard

看键盘画图形，不过我没做对，想象力拙计……

骚年来一发吗

密文

iEJqak3pjIaZ0NzLiITLwWTqzqGAtW2oyOTq1A3pzqas

function encode($str){
    $_o = strrev($str);
    for($_0 = 0; $_0 < strlen($_o); $_0++){
        $_c = substr($_o, $_0, 1);
        $__ = ord($_c) + 1;
        $_c = chr($__);
        $_ = $_.$_c;
    }
    return str_rot13(strrev(base64_encode($_)));

没学过这种世界上最好的语言。去搜索了下 $ 貌似是“xx变量”的意思，暂且当成 CPP 来看了……然后直接搜一些函数语法：

substr() 返回字符串
strrev(string)     反转字符串
ord(string)   返回字符串中第一个字符的 ASCII 值
chr(ascii)  从指定 ASCII 值返回字符
(.)         PHP 并置运算符,将两个字符串连接起来

执行完函数后得到一个字符串 $_

然后将这个字符串 base64 再反转再 ROT13 。

ROT13 实际上是凯撒加密的变体。,不过神奇的是，PHP 竟然自带 rot13 的函数。

解密的时候逆过来操作就好了。

我感到了智商缺陷，最开始在逆向操作 return那一行时从内层到外层写的……

import codecs
import base64

with open('rot13decode.txt') as f:
    s = f.read().replace('\n', '')
flag = ""
s = codecs.encode( s,"rot13" )
s = s[::-1]
s = base64.b64decode(s)
for i in s:
    s = s + chr(ord(i) - 1)
s = s[::-1]
print s

Vigenere

It is said that Vigenere cipher does not achieve the perfect secrecy actually