Hi Steve,

if you want not to program the whole code inside the micro, please look at the C8051F000 (Cygnal).

It has an access limit register, which can protect all adresses below from reading and writing.
So you can program only the key in the lower code space area.
And your manufacturer can program the actual application in the upper unprotected area.

I think, an Ibutton is not secure, since you can read it out and program a micro to act similiar.
I think, also all other serial number chips can be simulated.

But you can program a little micro (e.g. PIC, ATtiny in 8-pin-case) with your own secret key algorithm.
Then you must first check, if the program locking of these chips are useful (means not easy to remove).


Peter