#include <stdio.h>

/* F_k(in) = k ^ in */

void F(unsigned char *key, unsigned char *in, unsigned char *out){
  /* assumes |key| = |in| = |out| = 8 bytes */

  int i;

  for (i=0; i<8; i++)
    out[i] = key[i] ^ in[i];

  return;
}

main(){
  FILE *randfile, *fpIn, *fpOut;
  unsigned char key[8], in[8], out[8], rand[8];
  int i;

  fpIn = fopen("msg.txt", "r");
  randfile = fopen("/dev/urandom", "r");
  /* don't use /dev/urandom for real-world crypto... */
  fpOut = fopen("ctext.txt", "w");

  fread(key, 1, 8, randfile); // load a random key

  while (fread(in, 1, 8, fpIn) == 8) {
    // read the next block of plaintext

    fread(rand, 1, 8, randfile);
    // rand is a random 8-byte string

    F(key, rand, out);
    // out = F(key, rand)

    // encrypt the current plaintext block;
    // note that each plaintext block corresponds to two ciphertext blocks.
    // plaintext block m is encrypted as r, F_k(r) ^ m as in class
    for (i=0; i<8; i++)
      fprintf(fpOut, "%02X", rand[i]);

    for (i=0; i<8; i++)
      fprintf(fpOut, "%02X", in[i]^out[i]);
  }


  fclose(fpIn);
  fclose(randfile);
  fclose(fpOut);

  return;
}