Secure PIC programming

[Keith Wootten]

Hi

We have customers using PICs who sometimes need to re-program them with
updated firmware. It's inconvenient to have to ship the equipment back
to a service centre, and most customers are quite capable of doing it
themselves.

The problem is that the PIC hex or bin file is easily disassembled, and
this may give away some proprietary secrets. If we could supply our
customers with a USB programmer and PC software which would allow them
to program a PIC from encrypted files we'd be a lot happier. Obviously,
a determined person could capture the ICSP data as it's loaded to the
PIC, but we think the risk is small and is one we'd be prepared to take.

Does anyone make a USB PIC ICSP programmer capable of using encrypted
files?

Thanks

 

[Luhan]

We have customers using PICs who sometimes need to re-program them with
updated firmware. It's inconvenient to have to ship the equipment back
to a service centre, and most customers are quite capable of doing it
themselves.

I have the same issue with one of my clients. If the customer can
program the PIC, they can replace one. So, we ship them an updated
PIC and let them put it in the equipment themselves.

Luhan

 

[Keith Wootten]

I have the same issue with one of my clients. If the customer can
program the PIC, they can replace one. So, we ship them an updated
PIC and let them put it in the equipment themselves.

Luhan

Thanks Luhan,

I'd rather not - it can take a long time for a chip to get to a far
flung place, and we don't really want customers soldering surface mount
chips on site, at least not often. Sockets are out for environmental
reasons.

Cheers

 

[Luhan]

I'd rather not - it can take a long time for a chip to get to a far
flung place, and we don't really want customers soldering surface mount
chips on site, at least not often. Sockets are out for environmental
reasons.

Ok, then how about a 'bootloader' program in a memory protected PIC?
Some PICs can write to their own program space. Put a decryption
algorythm in the bootloader and send it serial data from a PC com port.
You ship your customer the encrypted program.

Luhan

 

[Luhan]

I guess you don't use a lot of QFN or TQFP-100 parts..

You betcha, I like all my DIP chips in sockets! Also, all the circuit
boards are hot-swapable from the front of the machine! There are no
pots, switches, or jumpers and only ordinary common parts are used.
The units (reflow solder machines) work well in Malasia, China, and
places I dont even know about.

Luhan

 

[Spehro Pefhany]

I have the same issue with one of my clients. If the customer can
program the PIC, they can replace one. So, we ship them an updated
PIC and let them put it in the equipment themselves.

Luhan

I guess you don't use a lot of QFN or TQFP-100 parts..


Best regards,
Spehro Pefhany

 

[Ulf Samuelsson]

Hi

We have customers using PICs who sometimes need to re-program them
with updated firmware. It's inconvenient to have to ship the
equipment back to a service centre, and most customers are quite
capable of doing it themselves.

The problem is that the PIC hex or bin file is easily disassembled,
and this may give away some proprietary secrets. If we could supply
our customers with a USB programmer and PC software which would allow
them to program a PIC from encrypted files we'd be a lot happier.
Obviously, a determined person could capture the ICSP data as it's
loaded to the PIC, but we think the risk is small and is one we'd be
prepared to take.

Does anyone make a USB PIC ICSP programmer capable of using encrypted

files?

You can get an AES bootloader for the AVR
Download the encrypted application and let the CPU
decrypt and program, based on the key hidden in internal BootROM.

Thanks

--
Best Regards,
Ulf Samuelsson
[email protected]
This message is intended to be my own personal view and it
may or may not be shared by my employer Atmel Nordic AB

 

[[email protected]]

Ok, then how about a 'bootloader' program in a memory protected PIC?
Some PICs can write to their own program space. Put a decryption
algorythm in the bootloader and send it serial data from a PC com port.
You ship your customer the encrypted program.

This is a good idea since even snooping the serial port while
downloading will not give away the raw program code. Blowfish is quite
simple to implement and is easily embeddable. You can get PIC
implementation of DES and SKIPJACK at:

http://www.brouhaha.com/~eric/crypto/

Personally I think something like DES is overkill for a bootloader. A
common "good enough", weak encryption method is to simply XOR the
plaintext with the secret key. Something like:

#define KEY_LEN 22
static char secret[] = "THiS is tHE sECrEt KeY";

void encrypt_decrypt (char *data,int len)
{
for (i=len; i>0; i--)
{
data = data ^ secret[i%KEY_LEN];
}
}

takes very,very little program memory and can be easily included in a
bootloader.

 

[Keith Wootten]

Ok, then how about a 'bootloader' program in a memory protected PIC?
Some PICs can write to their own program space. Put a decryption
algorythm in the bootloader and send it serial data from a PC com port.
You ship your customer the encrypted program.

Luhan

Another good idea, but this is pre-existing kit, none of it uses
self-programming PICs. It tends to be mostly old 16F84 chips and
similar.

Cheers

 

[Ulf Samuelsson]

Personally I think something like DES is overkill for a bootloader. A

common "good enough", weak encryption method is to simply XOR the
plaintext with the secret key. Something like:

#define KEY_LEN 22
static char secret[] = "THiS is tHE sECrEt KeY";

void encrypt_decrypt (char *data,int len)
{
for (i=len; i>0; i--)
{
data = data ^ secret[i%KEY_LEN ];
}
}

takes very,very little program memory and can be easily included in a
bootloader.

If you really want to break in, it is not so secure.

If it is compiled code, you can
* Make sure you get
* search for combinations normally generated by a compiler

The larger the program is, the higher is the risk that you get a repeated
string

Assume you have the following code in several places:
pop r<n>
pop r<n>
ret

and several happen to be located aligned with the "secret" key.
Then they happen to be at location 44 and 176.
After factoring 44 = 2 * 2 * 11
176 = 2 * 2 * 2 * 11
you can probably soon find out the length of the key.
Then you know that the start of the code is (often) the exception vectors.
Search for valid keys generating subroutine call instructions.

Methods like this compbined with cheap CPU power limits the number of keys
very quickly.

Your suggestion is only safe if noone tries or if the value of the code is
small
enough that it costs more to develop the code than to crack it.

You would not, with any success, protect a settop box with your suggestion.
DES/AES cost very little code in the bootloader compared to its value.

--
Best Regards,
Ulf Samuelsson
[email protected]
This message is intended to be my own personal view and it
may or may not be shared by my employer Atmel Nordic AB

 

[Robert Scott]

...Obviously, a determined person could capture the ICSP data as it's loaded to the
PIC, but we think the risk is small and is one we'd be prepared to take..

Then you should be willing to take the much smaller risk that they would be able
to make any sense of the hex file.


Robert Scott
Ypsilanti, Michigan

 

[Keith Wootten]

Then you should be willing to take the much smaller risk that they
would be able
to make any sense of the hex file.

Disassembly is trivial with freeware tools and interpreting the result
may even be fun. There are parts of the code which would give away the
secrets of a proprietary protocol. Capturing the ICSP data requires,
comparatively, a lot of effort and equipment.

Making the code hard to interpret with all the usual tricks isn't an
option as it's pre-existing and tight.

If we can't find something off the shelf, then we may try to commission
a USB PIC programmer maker to do what should be a fairly easy
enhancement.

Cheers

 

[Luhan]

Here is a real simple methode for PIC code. You generate a 'keysteam'
of bytes which you XOR for both the encode and decode process. Look
for "Better, Faster, random numbers" on this page...

http://members.cox.net/berniekm/tricks.html

Luhan

 

[Chris Jones]

Disassembly is trivial with freeware tools and interpreting the result
may even be fun. There are parts of the code which would give away the
secrets of a proprietary protocol. Capturing the ICSP data requires,
comparatively, a lot of effort and equipment.

Not really, an old 80386 PC with a printer port could probably do it, or it
would be trivial with a 20 year old logic analyser or a $50 FPGA eval
board.

Chris

 

[Tom Lucas]

We have customers using PICs who sometimes need to re-program them with
updated firmware. It's inconvenient to have to ship the equipment back to
a service centre, and most customers are quite capable of doing it
themselves.

Perhaps it would be easier to try a legal solution rather than a technical
one. Supply the code under a license that forbids the disassembly of it and
the use of any knowledge gleaned from doing so.