Re: Bar code choice

From: Edward Cherlin <edward_dot_cherlin_at_etssg_dot_com>
Date: Sat Sep 13 2003 - 13:32:58 CDT

On Saturday 13 September 2003 01:46 am, Jan Karrman wrote:
> As Edward Cherlin suggested, one can use bitwise xor to
> obscure the bar code:
>
> http://gnosis.python-hosting.com/voting-project/August.2003/02
>86.html
>
> I suppose there is not a problem if the ballot number can be
> visually read from the bar code by a person that recognizes
> Code 128. Then one can xor the rest of the bit string with the
> ballot number (repeated).

Not exactly. There are people who can read data xored with a
fixed value. You use the key (the ballot number) to generate a
pseudo-random bitstring to xor the data with. Every pragramming
language has a pseudo-random number generator built in. In APL
you can write

   ? 1 1 1 1 1 1 1 1
and get something like
0 1 1 0 1 0 1 0

Some, including APL, let you set the seed value for the
generator. With this capability, the bitstrings generated from
similar ballot numbers will be, on average, completely different
from each other, that is, they will differ in about half the
bits in a pseudo-random distribution.

> This will not increase the length of
> the bar code at all.

Exactly.

-- 
Edward Cherlin, Simputer Evangelist
Encore Technologies (S) Pte. Ltd.
Computers for all of us
http://www.simputerland.com, http://cherlin.blogspot.com
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Received on Tue Sep 30 23:17:04 2003

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