Code proctection and firmware updates?

[Trampas]

I am trying to figure out the best way to do code proctection and allow a firmware update.

First I have been reading the data sheets on the code protection bits, did anyone ever mention that Microchip data sheets are not clear? So here is what I can figure out:

The Code Protection bits (CPx) do not allow external read and writes of the program memory.

The Write-Protect bits (WRTx) do not allow program to write to program memory.

The External Block Table Read Bit (EBTRx) do not allow a block of program memory to be read from outside of it's block.

So I was thinking that I will need to set the CPx bits, but not WRTx and EBTRx bits. This way my loader can read/write program memory.

Now assuming that I have encryption of the data and that the encrypted updates are loaded into external flash memory. Then on power up the PIC checks external flash for vaild program. Then decrypts data and loads(program) into PIC. What is the risk that the program can be compromised? I am looking for a difficulty rating from 0-easy to 10-impossible, assuming my encryption algorithm is 128-bit encryption. With the keys stored in program memory with the bootloader.

Basically I am looking for something that will take cost more than $10k to crack. Heck for $10k I would consider selling the code. Is the CPx bits relativly secure?

Thanks
Trampas

[Yashu]

[Haplo]

[Yashu]

[quote="Haplo"]