[zer0pts CTF] easy strcmp

tags: zer0pts CTF

overview

64bit ELF. its main function is pretty simple.

__int64 __fastcall main(int a1, char **a2, char **a3)
{
  if ( a1 > 1 )
  {
    if ( !strcmp(a2[1], "zer0pts{********CENSORED********}") )
      puts("Correct!");
    else
      puts("Wrong!");
  }
  else
  {
    printf("Usage: %s <FLAG>\n", *a2);
  }
  return 0LL;
}

solution

There was a suspicious function sub_6EA. Let's decompile with IDA demo.

_int64 __fastcall sub_6EA(__int64 a1, __int64 a2)
{
  int i; // [rsp+18h] [rbp-8h]
  int v4; // [rsp+18h] [rbp-8h]
  int j; // [rsp+1Ch] [rbp-4h]

  for ( i = 0; *(_BYTE *)(i + a1); ++i )
    ;
  v4 = (i >> 3) + 1;
  for ( j = 0; j < v4; ++j )
    *(_QWORD *)(8 * j + a1) -= qword_201060[j];
  return qword_201090(a1, a2);
}

This function process char[] as long[] to get difference with qword_201060. To recover the correct input, we can recover the difference from zer0pts{********CENSORED********}.

from struct import pack, unpack

bs = bytes([0x00,(...snip...),0x00]) # qword_201060
flag = "zer0pts{********CENSORED********}"
while len(flag) % 8 != 0:
    flag += "\0"
size = len(flag) // 8

# intepreted bytes array as long[] 
xs = unpack("q" * size, bs[: size * 8])
ys = unpack("q" * size, flag[: size * 8].encode())

# adding two long values, then cast to bytes
result = b""
for x, y in zip(xs, ys):
    result += b"".join(unpack("8c", int.to_bytes(y + x, 8, "little")))
print(result)

then got the flag zer0pts{l3ts_m4k3_4_DETOUR_t0d4y}