= xor =
== 問題 ==
712249146f241d31651a504a1a7372384d173f7f790c2b115f47

Source Code:
{{{#!highlight c
#include <stdio.h>
#include <string.h>

int main() {
  char flag[] = "ADCTF_XXXXXXXXXXXXXXXXXXXX";
  int len = strlen(flag);
  for (int i = 0; i < len; i++) {
    if (i > 0) flag[i] ^= flag[i-1];
    flag[i] ^= flag[i] >> 4;
    flag[i] ^= flag[i] >> 3;
    flag[i] ^= flag[i] >> 2;
    flag[i] ^= flag[i] >> 1;
    printf("%02x", (unsigned char)flag[i]);
  }
  return 0;
}
}}}

== メモ ==

== 解法 ==
{{{#!highlight c
#include <stdio.h>
#include <string.h>

int main() {
  char ct[] = "\x71\x22\x49\x14\x6f\x24\x1d\x31\x65\x1a\x50\x4a\x1a\x73\x72\x38\x4d\x17\x3f\x7f\x79\x0c\x2b\x11\x5f\x47";
  char flag[] = "ADCTF_XXXXXXXXXXXXXXXXXXXX";
  char pt[256];
  int len = strlen(flag);
  for (int i = 0; i < len; i++) {
    for (int c = 0; c < 256; c++) {
      flag[i] = c;
      if (i > 0) flag[i] ^= flag[i-1];
      flag[i] ^= flag[i] >> 4;
      flag[i] ^= flag[i] >> 3;
      flag[i] ^= flag[i] >> 2;
      flag[i] ^= flag[i] >> 1;
      if (flag[i] == ct[i]) {
        pt[i] = c;
        break;
      }
    }
  }
  pt[len] = 0;
  printf("%s\n", pt);
  return 0;
}
}}}
{{{
% ./solve 
ADCTF_51mpl3_X0R_R3v3r51n6                                                     
}}}