逆向攻防世界CTF系列55-Replace

32位,upx壳

int __cdecl main(int argc, const char **argv, const char **envp){
  char Buffer[40];
  memset(Buffer, 0, sizeof(Buffer));
  printf("Welcome The System\nPlease Input Key:");
  gets_s(Buffer, 0x28u);
  if ( strlen(Buffer) - 35 <= 2 ){
    if ( sub_401090(Buffer) == 1 ) printf("Well Done!\n");
    else printf("Your Wrong!\n");
  }
  return 0;
}

跟进sub401090:

int __fastcall sub_401090(int a1, int a2){
  if ( a2 != 35 ) return -1;
  v4 = 0;
  while ( 1 ) {
    v5 = a1[v4];
    v6 = (v5 >> 4) % 16;          //取高四位
    v7 = ((16 * v5) >> 4) % 16;  //取低四位
    v8 = byte_402150[2 * v4];   //32h
    if ( v8 < 48 || v8 > 57 ) v9 = v8 - 87;
    else v9 = v8 - 48;
    v10 = byte_402151[2 * v4];//61h
    v11 = 16 * v9;
    if ( v10 < 48 || v10 > 57 ) v12 = v10 - 87;
    else v12 = v10 - 48;
    if ( (unsigned __int8)byte_4021A0[16 * v6 + v7] != ((v11 + v12) ^ 0x19)) break;
    if ( ++v4 >= 35 ) return 1;
  }
  return -1;
}

a2可以猜到是长度，看汇编也行，必须是35，a1是输入，v5每次取出一个

16 * v6 + v7这个其实就是v5

if ( v10 < 48 || v10 > 57 ) v12 = v10 - 87;
else v12 = v10 - 48;

有点像将字母和数字映射到0开始，比如一个字符a，就将a的低4位和高四位分别映射在加起来。

怎么推得flag没考虑清楚，下了几个断点调试一下，我猜测50，51，和后面那串字符都能被2*v4访问到

我猜前面是flag，试了输入flaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa，验证了我的猜测

一开始是32h，后面是34h，66h….并且5d2151开始得是61h,39h,36h…..

enc =[
  0x32, 0x61, 0x34, 0x39, 0x66, 0x36, 0x39, 0x63, 0x33, 0x38,
  0x33, 0x39, 0x35, 0x63, 0x64, 0x65, 0x39, 0x36, 0x64, 0x36,
  0x64, 0x65, 0x39, 0x36, 0x64, 0x36, 0x66, 0x34, 0x65, 0x30,
  0x32, 0x35, 0x34, 0x38, 0x34, 0x39, 0x35, 0x34, 0x64, 0x36,
  0x31, 0x39, 0x35, 0x34, 0x34, 0x38, 0x64, 0x65, 0x66, 0x36,
  0x65, 0x32, 0x64, 0x61, 0x64, 0x36, 0x37, 0x37, 0x38, 0x36,
  0x65, 0x32, 0x31, 0x64, 0x35, 0x61, 0x64, 0x61, 0x65, 0x36
]

且长度是70

[
  0x63, 0x7C, 0x77, 0x7B, 0xF2, 0x6B, 0x6F, 0xC5, 0x30, 0x01, 
  0x67, 0x2B, 0xFE, 0xD7, 0xAB, 0x76, 0xCA, 0x82, 0xC9, 0x7D, 
  0xFA, 0x59, 0x47, 0xF0, 0xAD, 0xD4, 0xA2, 0xAF, 0x9C, 0xA4, 
  0x72, 0xC0, 0xB7, 0xFD, 0x93, 0x26, 0x36, 0x3F, 0xF7, 0xCC, 
  0x34, 0xA5, 0xE5, 0xF1, 0x71, 0xD8, 0x31, 0x15, 0x04, 0xC7, 
  0x23, 0xC3, 0x18, 0x96, 0x05, 0x9A, 0x07, 0x12, 0x80, 0xE2, 
  0xEB, 0x27, 0xB2, 0x75, 0x09, 0x83, 0x2C, 0x1A, 0x1B, 0x6E, 
  0x5A, 0xA0, 0x52, 0x3B, 0xD6, 0xB3, 0x29, 0xE3, 0x2F, 0x84, 
  0x53, 0xD1, 0x00, 0xED, 0x20, 0xFC, 0xB1, 0x5B, 0x6A, 0xCB, 
  0xBE, 0x39, 0x4A, 0x4C, 0x58, 0xCF, 0xD0, 0xEF, 0xAA, 0xFB, 
  0x43, 0x4D, 0x33, 0x85, 0x45, 0xF9, 0x02, 0x7F, 0x50, 0x3C, 
  0x9F, 0xA8, 0x51, 0xA3, 0x40, 0x8F, 0x92, 0x9D, 0x38, 0xF5, 
  0xBC, 0xB6, 0xDA, 0x21, 0x10, 0xFF, 0xF3, 0xD2, 0xCD, 0x0C, 
  0x13, 0xEC, 0x5F, 0x97, 0x44, 0x17, 0xC4, 0xA7, 0x7E, 0x3D, 
  0x64, 0x5D, 0x19, 0x73, 0x60, 0x81, 0x4F, 0xDC, 0x22, 0x2A, 
  0x90, 0x88, 0x46, 0xEE, 0xB8, 0x14, 0xDE, 0x5E, 0x0B, 0xDB, 
  0xE0, 0x32, 0x3A, 0x0A, 0x49, 0x06, 0x24, 0x5C, 0xC2, 0xD3, 
  0xAC, 0x62, 0x91, 0x95, 0xE4, 0x79, 0xE7, 0xC8, 0x37, 0x6D, 
  0x8D, 0xD5, 0x4E, 0xA9, 0x6C, 0x56, 0xF4, 0xEA, 0x65, 0x7A, 
  0xAE, 0x08, 0xBA, 0x78, 0x25, 0x2E, 0x1C, 0xA6, 0xB4, 0xC6, 
  0xE8, 0xDD, 0x74, 0x1F, 0x4B, 0xBD, 0x8B, 0x8A, 0x70, 0x3E, 
  0xB5, 0x66, 0x48, 0x03, 0xF6, 0x0E, 0x61, 0x35, 0x57, 0xB9, 
  0x86, 0xC1, 0x1D, 0x9E, 0xE1, 0xF8, 0x98, 0x11, 0x69, 0xD9, 
  0x8E, 0x94, 0x9B, 0x1E, 0x87, 0xE9, 0xCE, 0x55, 0x28, 0xDF, 
  0x8C, 0xA1, 0x89, 0x0D, 0xBF, 0xE6, 0x42, 0x68, 0x41, 0x99, 
  0x2D, 0x0F, 0xB0, 0x54, 0xBB, 0x16
]

```python

enc =[
  0x32, 0x61, 0x34, 0x39, 0x66, 0x36, 0x39, 0x63, 0x33, 0x38,
  0x33, 0x39, 0x35, 0x63, 0x64, 0x65, 0x39, 0x36, 0x64, 0x36,
  0x64, 0x65, 0x39, 0x36, 0x64, 0x36, 0x66, 0x34, 0x65, 0x30,
  0x32, 0x35, 0x34, 0x38, 0x34, 0x39, 0x35, 0x34, 0x64, 0x36,
  0x31, 0x39, 0x35, 0x34, 0x34, 0x38, 0x64, 0x65, 0x66, 0x36,
  0x65, 0x32, 0x64, 0x61, 0x64, 0x36, 0x37, 0x37, 0x38, 0x36,
  0x65, 0x32, 0x31, 0x64, 0x35, 0x61, 0x64, 0x61, 0x65, 0x36
]

key = [
  0x63, 0x7C, 0x77, 0x7B, 0xF2, 0x6B, 0x6F, 0xC5, 0x30, 0x01,
  0x67, 0x2B, 0xFE, 0xD7, 0xAB, 0x76, 0xCA, 0x82, 0xC9, 0x7D,
  0xFA, 0x59, 0x47, 0xF0, 0xAD, 0xD4, 0xA2, 0xAF, 0x9C, 0xA4,
  0x72, 0xC0, 0xB7, 0xFD, 0x93, 0x26, 0x36, 0x3F, 0xF7, 0xCC,
  0x34, 0xA5, 0xE5, 0xF1, 0x71, 0xD8, 0x31, 0x15, 0x04, 0xC7,
  0x23, 0xC3, 0x18, 0x96, 0x05, 0x9A, 0x07, 0x12, 0x80, 0xE2,
  0xEB, 0x27, 0xB2, 0x75, 0x09, 0x83, 0x2C, 0x1A, 0x1B, 0x6E,
  0x5A, 0xA0, 0x52, 0x3B, 0xD6, 0xB3, 0x29, 0xE3, 0x2F, 0x84,
  0x53, 0xD1, 0x00, 0xED, 0x20, 0xFC, 0xB1, 0x5B, 0x6A, 0xCB,
  0xBE, 0x39, 0x4A, 0x4C, 0x58, 0xCF, 0xD0, 0xEF, 0xAA, 0xFB,
  0x43, 0x4D, 0x33, 0x85, 0x45, 0xF9, 0x02, 0x7F, 0x50, 0x3C,
  0x9F, 0xA8, 0x51, 0xA3, 0x40, 0x8F, 0x92, 0x9D, 0x38, 0xF5,
  0xBC, 0xB6, 0xDA, 0x21, 0x10, 0xFF, 0xF3, 0xD2, 0xCD, 0x0C,
  0x13, 0xEC, 0x5F, 0x97, 0x44, 0x17, 0xC4, 0xA7, 0x7E, 0x3D,
  0x64, 0x5D, 0x19, 0x73, 0x60, 0x81, 0x4F, 0xDC, 0x22, 0x2A,
  0x90, 0x88, 0x46, 0xEE, 0xB8, 0x14, 0xDE, 0x5E, 0x0B, 0xDB,
  0xE0, 0x32, 0x3A, 0x0A, 0x49, 0x06, 0x24, 0x5C, 0xC2, 0xD3,
  0xAC, 0x62, 0x91, 0x95, 0xE4, 0x79, 0xE7, 0xC8, 0x37, 0x6D,
  0x8D, 0xD5, 0x4E, 0xA9, 0x6C, 0x56, 0xF4, 0xEA, 0x65, 0x7A,
  0xAE, 0x08, 0xBA, 0x78, 0x25, 0x2E, 0x1C, 0xA6, 0xB4, 0xC6,
  0xE8, 0xDD, 0x74, 0x1F, 0x4B, 0xBD, 0x8B, 0x8A, 0x70, 0x3E,
  0xB5, 0x66, 0x48, 0x03, 0xF6, 0x0E, 0x61, 0x35, 0x57, 0xB9,
  0x86, 0xC1, 0x1D, 0x9E, 0xE1, 0xF8, 0x98, 0x11, 0x69, 0xD9,
  0x8E, 0x94, 0x9B, 0x1E, 0x87, 0xE9, 0xCE, 0x55, 0x28, 0xDF,
  0x8C, 0xA1, 0x89, 0x0D, 0xBF, 0xE6, 0x42, 0x68, 0x41, 0x99,
  0x2D, 0x0F, 0xB0, 0x54, 0xBB, 0x16
]

for i in range(0,len(enc),2):
    v8 = enc[i]
    v10 = enc[i + 1]
    if ( v8 < 48 or v8 > 57 ):
        v9 = v8 - 87
    else:
        v9 = v8 - 48
    v11 = v9 << 4
    if ( v10 < 48 or v10 > 57 ):
        v12 = v10 - 87
    else:
        v12 = v10 - 48
    for j in range(len(key)):
        if key[j] == (v11 + v12) ^ 0x19:
            print(chr(j), end='')
            break