On Mon, Mar 13, 2000 at 03:38:43PM -0000, Alan Pearce wrote:
> It shouldn't be too hard to make your own display if all you want is digits.
> Taken to the extreme make your own display using SMD LEDs. I have seen clear
> keycaps available for switches. the display angle problem could be sorted with a
> shroud around the keyboard.

Thanks for all the technical hints. However the main point is that the
key (let's give it a code name "PICKEY" - sounds good) should be as small
as possible (width and height of credit card, only a little thicker).

The main problem however is the choice of proper microcontroller.
The simplest RSA implementation requires holding of "challenge" and
result of it's exponentiantion in RAM. If "challenge" is L-bits long,
and the RSA's N coefficient is M-bits long, than I need L-bits for
"challenge" and L+M bits for temporary multiplication result.
For 512-bit key (it is a minimum for practical use) the M is 1024.
If L is 512 I need the 1536 bits = 192 bytes of RAM.
It can't be done with 16F84 :-(. For 1024-bit key results are even worse
and probably even the 16F87x is not sufficient.
However maybe there is a tricky RSA implementation better suited for
microcontroller implementation. Does anybody knows about it?

There is an additional problem: To defend the PICKEY against the timing attack
(a "feature" specific to RSA algorithm) the encrytpion time should be
probably hidden (eg. by outputing the results after the constant time
<longer than the longest possible calculation time>, independently on
the real calculation time).

--
        Thanks for all asnwers (both to the list and to my e-mail)
                              Wojciech M. Zabolotny
        http://www.ise.pw.edu.pl/~wzab  <--> wzab@ise.pw.edu.pl

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