[ucc] Vending machine authentication ideas?

[Bernard Blackham]

So we've finally got the vending machine completely controllable via
a serial port. We've also got loads of room to spare on the ROM.
What would be very nice is to be able to implement some sort of
authenication or integrity checking on the serial protocol, so that
a random passer-by can't simply unplug the vending machine, plug his
own machine in and start dispensing away.

The CPU is a HC11, which can do 16-bit math. 32-bit math can be
emulated by gcc I believe (but it's awfully slow). There is NVRAM on
the machine to store a secret key. We don't require the packets to
be encrypted, just to be certain about their integrity. And even
better, it can be written in C :)

Can anybody suggest something suitable?

Bernard.

-- 
 Bernard Blackham <bernard at blackham dot com dot au>