Reverse: Level1 Cracked by Eswink插图
Reverse 1

Information

​ Flag:SYC{ppxdtedZrekKSpAMeqzNoewKQjEFTOMaFNCQiLeMFGg}

Necessary Tools

  1. IDA 7.5 Linux Cracked by ericyudatou
  2. Pwntools
  3. Pycharm && Python3

Debugging Environment

​ Linux kali 5.10.0-kali9-amd64

Decompile

检测程序架构信息

pwn checksec level1
Reverse: Level1 Cracked by Eswink插图1

目标程序为64位程序,选择使用IDA x64反编译该程序,并且跟随来到level1()函数入口,F5转换为伪代码,并对变量进行重命名,便于分析。

分析主要代码

v7 = __readfsqword(0x28u);

__readfsqword用于本地反调试,远程调试即可解决此问题

 for ( i = 0; i <= 47; ++i )
  {
    for ( j = 0; j <= 53; ++j )
    {
      v1 ^= j ^ v2 ^ 0x5E;
      s[i] ^= s[(i + 1) % 48];
      s[i] ^= v1;
      ++v2;
    }
  }
  for ( k = 0; k <= 47; ++k )
  {SYC{ppxdtedZrekKSpAMeqzNoewKQjEFTOMaFNCQiLeMFGg}
    if ( s[k] != compare_data[k] )
    {
      puts("Ohhhh, TRY HARD");
      return __readfsqword(0x28u) ^ v7;
    }
  }

循环中的异或操作,可由密文再次异或得到明文,并且位置不受影响,遵循交换律,如:

a ^ b = b ^ a

异或后,再与.data中的compare_data内存比较,如果相同则提示:You Win

IDA中,提取compare_data中的内容,并且转换为Python List,便于接下来的还原。

data = [13, 5, 137, 179, 22, 20, 106, 116, 250, 233, 94, 98, 164, 177, 41, 11, 173, 140, 43, 37, 99, 117, 200, 254, 65, 73, 173, 147, 39, 30, 167, 166, 202, 211, 71, 105, 224, 234, 17, 1, 167, 128, 31, 53, 80, 83, 229, 253]

Reverse Script

Script Language

​ Python3

data = [13, 5, 137, 179, 22, 20, 106, 116, 250, 233, 94, 98, 164, 177, 41, 11, 173, 140, 43, 37, 99, 117, 200, 254, 65,
        73, 173, 147, 39, 30, 167, 166, 202, 211, 71, 105, 224, 234, 17, 1, 167, 128, 31, 53, 80, 83, 229, 253]
v1 = -85
v2 = 0
s = data
# s=[95, 94, 201, 204]
for i in range(48):
    for j in range(54):
        v1 ^= j ^ v2 ^ 0x5e
        s[i] = s[i] ^ s[(i + 1) % 4]
        s[i] ^= v1
        v2 = v2 + 1
print(s)

s = ""
d = ""
#对data[i]进行转换,如果大于127,则%128再转换得到对应的ascii,即可得到对应的明文字符
for i in range(len(data)):
    d = data[i]
    if d > 127:
        d = d % 128
    s = s + "".join(chr(d))
print(len(s))
print(s)

运行该脚本后,即可得到对应的Flag:

SYC{ppxdtedZrekKSpAMeqzNoewKQjEFTOMaFNCQiLeMFGg}

Result verification

将得到的Flag的值输入,最后得到正确结果

Reverse: Level1 Cracked by Eswink插图2

Reflection & Proposal

  • 当ASCII大于127并且需要将其转换为可视字符,需要%128再进行转换
  • 异或运算,明文秘钥和密文相互转换,知道其二可求其一