SECCON 2014 - crypto200

solve.c

#include <stdio.h>

int main(int argc, char** argv)
{
  signed int result;
  unsigned int seed;
  FILE *input_file;
  FILE *output_file;
  char buf;

  if ( argc > 2 )
  {
    seed = atoi(argv[3]);
    srand(seed);
    input_file = fopen(argv[1], "rb");
    output_file = fopen(argv[2], "wb");
    while ( fread(&buf, 1, 1, input_file) == 1 )
    {
      buf ^= rand();
      fwrite(&buf, 1, 1, output_file);
    }
    fclose(input_file);
    fclose(output_file);
    result = 0;
  }
  else
  {
    result = 1;
  }
  return result;
}