MyOwnCBC[AFCTF2018]

分组密码的CBC与EBC模式

学习出处:

https://www.cnblogs.com/eleven-elv/p/7289579.html

https://www.jianshu.com/p/79a225c2650e

题目

#!/usr/bin/python2.7
# -*- coding: utf-8 -*-

from Crypto.Cipher import AES
from Crypto.Random import random
from Crypto.Util.number import long_to_bytes

def MyOwnCBC(key, plain):
	if len(key)!=32:
		return "error!"
	cipher_txt = b""
	cipher_arr = []
	cipher = AES.new(key, AES.MODE_ECB, "")
	plain = [plain[i:i+32] for i in range(0, len(plain), 32)]
	print plain
	cipher_arr.append(cipher.encrypt(plain[0]))
	cipher_txt += cipher_arr[0]
	for i in range(1, len(plain)):
		cipher = AES.new(cipher_arr[i-1], AES.MODE_ECB, "")
		cipher_arr.append(cipher.encrypt(plain[i]))
		cipher_txt += cipher_arr[i]
	return cipher_txt
	
key = random.getrandbits(256)
key = long_to_bytes(key)

s = ""
with open("flag.txt","r") as f:
	s = f.read()
	f.close()

with open("flag_cipher","wb") as f:
	f.write(MyOwnCBC(key, s))
	f.close()

分析

不同于一般的CBC,该题不是将加密后的一小段密文作为iv，而是直接作为下一次加密的key。

由于给出密文，将其分段后，就可以从最后一段逐段推出前一段明文，但由于不知道初始key,所以无法推出第一段明文（flag放到了文末）

知识点

ECB模式

电码本模式

这种模式是将整个明文分成若干段相同的小段，然后对每一小段进行加密。

CBC模式

密码分组链接模式

这种模式是先将明文切分成若干小段，然后每一小段与初始块或者上一段的密文段进行异或运算后，再与密钥进行加密。

搬运自:https://www.jianshu.com/p/79a225c2650e

解题

from Crypto.Cipher import AES
from Crypto.Util.number import long_to_bytes
cipher_text=open('https://y4ny4nblog.oss-cn-beijing.aliyuncs.com/img/flag_cipher','rb').read()
cipher=[]
plain=[]
mode=AES.MODE_ECB
for i in range(0,len(cipher_text),32):
    cipher.append(cipher_text[i:i+32])
for i in range(len(cipher)-1):
    cryptor=AES.new(cipher[(-1)*i-2],mode)
    plain.append(cryptor.decrypt(cipher[(-1)*i-1]))
flag=[i.decode('utf-8') for i in  plain]
print(''.join(flag[::-1]))