Re: Security for embedded device

From: Michael Amling (nospam_at_nospam.com)
Date: 06/29/04 

Date: Tue, 29 Jun 2004 14:07:58 GMT

Francois Grieu wrote:
> In article <40e120e0$0$828$a344fe98@news.wanadoo.nl>,
> "Joris Dobbelsteen" <none.of@your.business> wrote:
>
>
>>Does anyone know a simple cipher for a 8-bit processor with only a few bytes
>>of available memory, doing its job without about 1K cycles?

   If you can fit it in, AES is the best. One of it designers works for
a smart card company, and tried to make it implementable with few
resources. It does need at least 16 bytes for the key and 16 for the data.

>
>
> TEA
> <http://www.ftp.cl.cam.ac.uk/ftp/papers/djw-rmn/djw-rmn-tea.html>

  Or XTEA. Code and test vectors for both are at
http://www.cix.co.uk/~klockstone/teavect.htm.

>
> Be sure you understand the issue of equivalent keys; BTW,
> it can't strike you with a fixed key, only in fancy feedback modes.
>
>
>>Perhaps some advice on generating a initial key. I don't have EEPROM,
>>only RAM and two timers available. The key is needed when somebody
>>begins to talk to the chip (UART).
>
>
> TEA's 128 bits key is wide enough that its can be used without fear of
> brute-forcing. So maybe you can have a fixed key in ROM, and forget
> about the problem of key generation altogether. It'll save RAM, too.
> If you have no EEPROM or Flash, you have little options anyway.
>
> If you need a random value, it could be a CBC-MAC, using a ROM key, of
> anything you get that gives entropy; value of timer while in an iddle
> loop, and right in the interrupt when a character is received, are
> often good sources. If you have an ADC or RTC with independent XTall,
> mix that in.

--Mike Amling

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