Thank you all for your comments to date.

A level of security that would scare the CIA or other code-breaking savy organization is not required. It is simply necessary to prevent the customer from making free copies since we have an exclusive manufacturing agreement but are located in different countries. It is still desirable to be able to allow the customer to reflash new code but not create working new products from blank micros.

The P89C51RB2 allows for in-system reprogramming. It seems that a custom boot loader could be activated by the internal software that would decrypt the new encrypted code data and re-flash the micro.

I do not want the customer to be able to read the old code or use the new code except to reflash updated code into already programmed micros.

Has anyone had any experience doing this simply type of protection? Are there any other suggestions or ideas?

Regards,

Bert