char.c

#include <stdio.h>
#define ID_SIZE 6

// this is the array of authorized key strings that
// reader input is checked against
char *authorized_ids[] = {
  "000013579EF3",
  "\0"
};

// turn string input into 6 byte array, and reverse
void str2bytes(char *input, unsigned char *into) {
  int i,q;
  char buf[3]; buf[2] = '\0';

  for(i=ID_SIZE*2-1,q=0; i>0; i-=2,q++) {
    buf[0] = input[i-1];
    buf[1] = input[i];
    into[q] = strtol(buf,0,16);
  }
}

// compare a known id's bytes with those from the reader
int matches_reader(unsigned char *id, unsigned char *reader) {
  int i;
  for(i = 0; i < ID_SIZE; i++) {
    if(id[i] != reader[i+1])
      return 0;
  }
  return 1;
}

// check input from reader against authorized IDs
int is_authorized(unsigned char *input) {
  int i = 0;
  char *checkstr;
  unsigned char checkbytes[ID_SIZE];

  while(*(checkstr = authorized_ids[i++]) != '\0') {
    str2bytes(checkstr, checkbytes);
    if(matches_reader(checkbytes, input))
      return 1;
  }

  return 0;
}

int main(int argc, char **argv)
{

  unsigned char from_reader[] = {0x01, 0xF3, 0x9E, 0x57, 0x13, 0x00, 0x00, 0xF3};

  if(is_authorized(from_reader)) {
    printf("Authorized\n");
  } else {
    printf("Not Authorized\n");
  }

  return 0;
}