Dreamhack CTF Season 6 Round #10 (🌱Div2) Recover

__int64 __fastcall main(int a1, char **a2, char **a3)
{
  char ptr; // [rsp+Bh] [rbp-25h] BYREF
  int v5; // [rsp+Ch] [rbp-24h]
  _BYTE *v6; // [rsp+10h] [rbp-20h]
  FILE *stream; // [rsp+18h] [rbp-18h]
  FILE *s; // [rsp+20h] [rbp-10h]
  unsigned __int64 v9; // [rsp+28h] [rbp-8h]

  v9 = __readfsqword(0x28u);
  v6 = &unk_2004;
  stream = fopen("flag.png", "rb");
  if ( !stream )
  {
    puts("fopen() error");
    exit(1);
  }
  s = fopen("encrypted", "wb");
  if ( !s )
  {
    puts("fopen() error");
    fclose(stream);
    exit(1);
  }
  v5 = 0;
  while ( fread(&ptr, 1uLL, 1uLL, stream) == 1 )
  {
    ptr ^= v6[v5 % 4];
    ptr += 19;
    fwrite(&ptr, 1uLL, 1uLL, s);
    ++v5;
  }
  fclose(stream);
  fclose(s);
  return 0LL;
}

// Name: dec.c
// Compile: gcc -Wall dec.c -o dec
#include <stdlib.h>
#include <stdio.h>

int main(void) {
    FILE *encrypted_file;
    FILE *flag_file;
    char byte;
    char *key = "\xde\xad\xbe\xef";
    int i;

    encrypted_file = fopen("encrypted", "rb");
    if (encrypted_file == NULL) {
        puts("fopen() error");
        exit(1);
    }

    flag_file = fopen("flag.png", "wb");
    if (flag_file == NULL) {
        puts("fopen() error");
        fclose(encrypted_file);
        exit(1);
    }

    i = 0;

    while (fread(&byte, 1, 1, encrypted_file) == 1) {
        byte -= 19;
        byte ^= key[i % 4];
        fwrite(&byte, 1, 1, flag_file);
        i++;
    }

    fclose(encrypted_file);
    fclose(flag_file);

    return 0;
}

리눅스에서 nano dec.c

gcc dec-c

./dec

ls