Source: Daily AlpacaHack
Author: chokorusk

フラグチェッカーのような動作を行うバイナリが配布される。

Ghidraでデコンパイルしてmain関数を読む。

undefined8 main(void)

{
  char cVar1;
  int iVar2;
  undefined8 uVar3;
  long in_FS_OFFSET;
  long local_188;
  ulong local_180;
  undefined1 local_178 [176];
  char local_c8 [48];
  char local_98 [136];
  long local_10;
  
  local_10 = *(long *)(in_FS_OFFSET + 0x28);
  local_188 = 0;
  printf("flag: ");
  cVar1 = read_flag_input(local_98,0x80,&local_188);
  if (cVar1 == '\x01') {
    if (local_188 == 0x20) {
      local_c8[0] = -0x52;
      local_c8[1] = '=';
      local_c8[2] = '\x10';
      local_c8[3] = 'l';
      local_c8[4] = -0x4c;
      local_c8[5] = -0x7e;
      local_c8[6] = '\x10';
      local_c8[7] = -0x75;
      local_c8[8] = -0x61;
      local_c8[9] = 'z';
      local_c8[10] = -0x37;
      local_c8[0xb] = '\x01';
      local_c8[0xc] = -0x52;
      local_c8[0xd] = -0x69;
      local_c8[0xe] = -0x5e;
      local_c8[0xf] = -0x50;
      local_c8[0x10] = '@';
      local_c8[0x11] = '/';
      local_c8[0x12] = 'J';
      local_c8[0x13] = -0x5b;
      local_c8[0x14] = -0x2a;
      local_c8[0x15] = '+';
      local_c8[0x16] = 'B';
      local_c8[0x17] = '<';
      local_c8[0x18] = -0x34;
      local_c8[0x19] = '~';
      local_c8[0x1a] = -0x41;
      local_c8[0x1b] = -0x5c;
      local_c8[0x1c] = '\x14';
      local_c8[0x1d] = -0x7d;
      local_c8[0x1e] = -0x7d;
      local_c8[0x1f] = '3';
      init_cipher(local_178,key);
      for (local_180 = 0; local_180 < 0x20; local_180 = local_180 + 0x10) {
        decrypt_block(local_178,local_c8 + local_180);
      }
      local_c8[0x20] = 0;
      iVar2 = strcmp(local_98,local_c8);
      if (iVar2 == 0) {
        puts("correct!");
      }
      else {
        puts("wrong...");
      }
      uVar3 = 0;
    }
    else {
      puts("wrong...");
      uVar3 = 0;
    }
  }
  else {
    uVar3 = 1;
  }
  if (local_10 != *(long *)(in_FS_OFFSET + 0x28)) {
                    /* WARNING: Subroutine does not return */
    __stack_chk_fail();
  }
  return uVar3;
}

本質はここで、入力の長さが32の時、decryptしたflagと思しき文字列をstrcmpで比較している。

if (local_188 == 0x20) {
    local_c8[0] = -0x52;
    ...(略)
    local_c8[0x1f] = '3';
    init_cipher(local_178,key);
    for (local_180 = 0; local_180 < 0x20; local_180 = local_180 + 0x10) {
        decrypt_block(local_178,local_c8 + local_180);
    }
    local_c8[0x20] = 0;
    iVar2 = strcmp(local_98,local_c8);

strcmpを実行する時点でlocal_c8にはflagが入っていることが期待できるので、これを動的解析で抜き取る。
今回はltraceコマンドを使用し、適当な32文字の文字列を入力することでメモリ上に復元されたflagを確認できた。

$ ltrace -e strcmp ./chal
flag: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
chal->strcmp("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"..., "Alpaca{m3ccha_rand0m_5b0x_d4yo!}"...) = 32
wrong...
+++ exited (status 0) +++

Alpaca{m3ccha_rand0m_5b0x_d4yo!}