#include <stdio.h>
#define KEY_LENGTH 5 // Can be anything from 1 to 15

main(){
  unsigned int ch;
  FILE *fpIn;
  int i;
  unsigned char key[KEY_LENGTH] = {0x00, 0x00, 0x00, 0x00, 0x00};
  /* of course, the all-0 key was probably not the one actually used... */

  fpIn = fopen("ctext.txt", "r");

  i=0;

  while (fscanf(fpIn, "%2x", &ch) != EOF) {
    printf("%c", ch ^ key[i % KEY_LENGTH]);
    i++;
  }
  fclose(fpIn);

  return;
}