#include <stdio.h>
#define KEY_LENGTH 2 // Can be anything from 1 to 15

main(){
  unsigned char ch;
  FILE *fpIn, *fpOut;
  int i;
  unsigned char key[KEY_LENGTH] = {0x00, 0x00};
  /* of course, I probably did not use the all-0s key to encrypt */

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

  i=0;

  while (fscanf(fpIn, "%c", &ch) != EOF) {
    /* avoid encrypting the trailing newline
       at the end of the file */
    if (ch!='\n')
      fprintf(fpOut, "%02X", ch ^ key[i % KEY_LENGTH]); // ^ is logical XOR
    i++;
  }

  fclose(fpIn);
  fclose(fpOut);

  return;
}