XCTF-base64stego base64隐写
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题目：
给了一个压缩包，打开后发现需要密码，Hex查看发现是伪加密，修改加密位；解压后得到一个文本文档，里面是一大串的base64编码。解密后是一篇文章，介绍了一下隐写，不过应该没有我们要的flag。下面把文档贴出来。
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原理：
base64在加密时以6位二进制数一组加密，如果明文字节数不为3的倍数，即位数不为8位的倍数时会在二进制数值末尾填充零，然后在密文末尾添加等号，表示添加的零值字节数；解密以8位二进制数一组，首先丢弃等号，多余的位（即加密时填充的零）也会被丢弃。既然多余的位不影响加解密，那就可以在其中做文章，这就是base64隐写的原理。要注意的是“=”位的二进制数不能被修改，因为这样会改变密文中“=”号的数量，也就破坏了源字符串。
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解密：
# -*- coding: cp936 -*-
b64chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
with open('1.txt', 'rb') as f:
bin_str = ''
for line in f.readlines():
stegb64 = ''.join(line.split())
rowb64 = ''.join(stegb64.decode('base64').encode('base64').split())
offset = abs(b64chars.index(stegb64.replace('=','')[-1])-b64chars.index(rowb64.replace('=','')[-1]))
equalnum = stegb64.count('=') #no equalnum no offset
if equalnum:
bin_str += bin(offset)[2:].zfill(equalnum * 2)
print ''.join([chr(int(bin_str[i:i + 8], 2)) for i in xrange(0, len(bin_str), 8)]) #8 位一组
Summary
学到了一种新姿势，隐写还真是无处不在，深入掌握原理才能掌握隐写。
