get_mac.c

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <openssl/des.h>

const int DES_BLOCKSIZE = 8;
const int HEAD_LEN = 16;
const int MAC_LEN = 6;
const int TOTAL_LEN = 24;
unsigned char KEY[] = "360028C9";

void des_ecb_decrypt(unsigned char* s, int len, unsigned char* key_buf)
{
    DES_key_schedule key;
    DES_set_key_unchecked((unsigned char (*)[DES_BLOCKSIZE])key_buf,&key);
    int i;
    for (i = 0; i < len; i+=DES_BLOCKSIZE)
        DES_encrypt1((DES_LONG *)&s[i],&key,0);
}

int main(int argc, char** argv)
{
    unsigned char buf[TOTAL_LEN];
    read(STDIN_FILENO, buf, TOTAL_LEN);

    des_ecb_decrypt(buf,HEAD_LEN,KEY);
    des_ecb_decrypt(&buf[HEAD_LEN],DES_BLOCKSIZE,buf);

    int i;
    for (i = 0; i < MAC_LEN; i++)
        printf("%02X",buf[i + HEAD_LEN]);
    printf("\n");
}