one_time_pad.c

#include <stdio.h>

// datas
unsigned char data[] = {0x32, 0x7b, 0xe0, 0x51, 0xee, 0x6b, 0x45};
unsigned char other_data[] = {0x2f, 0x71, 0xf6, 0x3, 0xe0, 0x72, 0x5F};

unsigned char pad[] = {0x56, 0x1e, 0x83, 0x23, 0x97, 0x1b, 0x31};
unsigned char out[7];

// works as both an encryptor and decryptor
void crypt(unsigned char *data, unsigned char *pad, unsigned char *out, int size) {
  unsigned char *optr = out;
  unsigned char *dptr = data;
  unsigned char *pptr = pad;
  
  int count = size;
  while (count-- != 0)
    *optr++ = *dptr++ ^ *pptr++;
}

// prints the data bytes in a c-like format
void print_data(unsigned char *dat, int size) {
  int i;
  int buff[7];
  unsigned char *ptr = dat;
  
  printf("{");
  for (i=0; i<size; i++) {
    buff[i] = *ptr++;
    printf("0x%x, ", buff[i]);
  }
  printf("}\n");
}

// entrypoint
int main(int argc, char **argv)
{
  out[6] = '\0';
  
  // encode first message
  sprintf((char*)out, "decrypt");
  decrypt(out, pad, data, 6);
  print_data(data, 6);
  
  // decode first message
  decrypt(data, pad, out, 6);
  printf("First message: %s\n", out);
  
  // encode second message
  sprintf((char*)out, "you win");
  decrypt(out, pad, other_data, 6);
  print_data(other_data, 6);
  
  // decode second message
  decrypt(other_data, pad, out, 6);
  printf("Second message: %s\n", out);

  return 0;
}