FireShell CTF 2020 Re Simple Encryption

1.代码分析

__int64 __fastcall sub_401CA0(signed int a1, __int64 *a2)
{
  __int64 v2; // rsi
  __int64 v3; // rcx
  __int64 v4; // rdi
  __int64 v5; // rdx
  __int64 result; // rax
  __int64 v7; // rcx
  unsigned __int64 v8; // rt1
  __int16 v9; // [rsp+16h] [rbp-1Ah]
  __int64 v10; // [rsp+18h] [rbp-18h]
  __int64 v11; // [rsp+20h] [rbp-10h]
  unsigned __int64 v12; // [rsp+28h] [rbp-8h]

  v12 = __readfsqword(0x28u);
  sub_401C2D();
  if ( a1 <= 2 )
  {
    v2 = *a2;
    sub_410890((unsigned __int64)"Use: %s <input_file> <output_file>

");
    sub_40FD60(0xFFFFFFFFLL, v2);
  }
  v10 = sub_41FC70(a2[1], "r");                 // 读取文件
  v11 = sub_41FC70(a2[2], "w+");                // 写入文件
  while ( (unsigned int)sub_410A20(v10, (__int64)"%c", &v9) != -1 )// 将读取到的信息，一字节一字节存入v9
  {
    v9 = 2 * (unsigned __int8)v9;
    LOBYTE(v9) = HIBYTE(v9) | v9;
    LOBYTE(v9) = ~(_BYTE)v9;
    sub_421B30((unsigned __int8)v9, v11, v11, v3);// 将变换后的信息存入文件
  }
  sub_41F7C0(v10, "%c");
  v4 = v11;
  sub_41F7C0(v11, "%c");
  result = 0LL;
  v8 = __readfsqword(0x28u);
  v7 = v8 ^ v12;
  if ( v8 != v12 )
    sub_45AF10(v4, "%c", v5, v7);
  return result;
}

实际的信息变换就是

v9 = 2 * (unsigned __int8)v9;
LOBYTE(v9) = HIBYTE(v9) | v9;
LOBYTE(v9) = ~(_BYTE)v9;

因为每次读入一字节，变换后存入文件。因此我们只需要每次遍历0x00~0xFF，能够满足变换后与输出文件相同，则该数为原数。

2.脚本解密

# -*- coding:utf-8 -*-

with open('C://Users//10245//Desktop//flag.enc','rb') as f:
    datas = f.read()
    for data in datas:
        for i in range(0,256):
            j = i * 2
            lbyte = ((j & 0xFF00) >> 8) | j
            lbyte = (~lbyte)&0xFF
            hbyte = (~((j & 0xFF00) >> 8))&0xFF
            num = ((hbyte&0xF000)>>4)*pow(16,3) + (hbyte&0xF00)*pow(16,2) + ((lbyte&0xF0)>>4)*16 + (lbyte&0xF)
            if num == data:
                print (chr(i),end="")

# -*- coding:utf-8 -*-

with open('C://Users//10245//Desktop//flag.enc','rb') as f:
    datas = f.read()
    for data in datas:
        for i in range(0,256):
            j = ~((i >> 7) | (2 * i))&0xff
            if j == data:
                print (chr(i),end="")

Congratulations!

I hope you liked this small challenge.

The flag you are looking for is F#{S1mpl3_encr1pt10n_f0und_0n_g1thub!}

3. get flag！

 F#{S1mpl3_encr1pt10n_f0und_0n_g1thub!}