tamper resistant messages on 8051

I need to send short RF messages (approx: 2 bytes) between 8051 devices. How do I make sure that the messages are not tampered and that the message really came from the device?

I was thinking of using CMAC AES. But found out that AES isn't efficient on 8051.

How do I generate untamperable messages in 8051?


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Regardless of the CPU, you are looking at "message authentication" (aka "signing"). It's usually done by taking some cryptographic function of the message, & sending that along with it. The first thing you need to determine is, what level of threat do you face? A bored student filling in a rainy night, or a major government agency which *really* wants to hack your system? A large spectrum in between. Where you are on it, determines how much security (read, CPU cycles) you need to provide.

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David R Brooks

If you only have two bytes of bandwidth, your attacker only has to try

65536 messages and he's in, even if you have the most robust cryptosystem known to man.

An application like this would possibly work well with a rolling-code algorithm like Microchip's Keeloq (note that it is patented, and you can't field this commercially unless you either use a Microchip part, or license the algorithm).

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and why would they want to hack your system?

In which case there are other methods and they have the resources to try other methods from "interviewing" you to a small air strike :-). So the crypto becomes pointless

You probably don't need AES. Just enough to be a bit more difficult that it is worth or making it a bit harder than alternative possible attacks.

It comes back down to the threat level.

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Chris Hills

What distance are all of the 8051 devices from each other?

Maybe you could get away with using really low power. Any hacker would have to get within a few metres to receive your RF messages. If they are stuck outside a building in a car then they simply wouldn't be able to receive your RF messages.

Obscure your RF messages in a sea of noise. Transmit more than just 2 bytes at a time, say, 64. Make the other bogus bytes look like they might actually be something when they aren't. Eg. add in a few valid checksums.

Send more transmissions than are necessary. Not every transmission actually has to mean something.

Lead your hacker up many blind alleys

The only sure way is this:

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Although I guess it will take a while for this technology to "trickle down" ;)

Reply to
Rob Horton

An algorithm is a recipe and you cannot patent recipes (in Europe). You can patent a means of implementing one, like a chip, but not the recipe itself. Do you have a pointer to the Microchip patent?


Reply to
Ian Bell

how would a secure 8051 suit this requirment, like the DS2252(T)? expensive. rw

Reply to
Ryan Weihl

They only restrict encoder implementations, the decoder is free.

The encoder datasheets (e.g. HCS300) specify: Code hopping encoder patents issued in Europe, USA and RSA - USA:

5,517,187; Europe: 0459781; RSA: ZA93/4726 Secure learning patents issued in the USA and RSA - USA: 5,686,904 - RSA: 95/5429.

Here is the license agreement for Keeloq:

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use a secure 8051 which prevents anybody to change code. we use the DS2252T for that purpose rw

Reply to
Ryan Weihl

How fast do you need it to be? If you write the code in assembler you can do AES encryption in about 1ms on an 8051 (I'd have to find my old notes to check what speed part that was, but it wasn't anything fancy, just a Phillips LPC of some sort).

- Charles

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