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Expression for Galois Fiels(2^8) multiplication:-

Which of the following expression is the correct one? please suggest..

Exp[ ( Log A + Log B )Mod 255]
OR
[Exp( Log A + Log B )] Mod 255
Posted 14-Feb-12 19:11pm
Updated 15-Feb-12 3:55am
v3
digimanus 15-Feb-12 1:16am

why don't you try it yourself?
Ratika Agarwal 15-Feb-12 1:18am

try what?? Try to run? Above is a formula.
Ratika Agarwal 15-Feb-12 10:01am

do you have an idea that how to store a very long stream of bits of about 2^256 length in any variable??
Varun Sareen 15-Feb-12 1:32am

Try it to find it
Ratika Agarwal 15-Feb-12 2:53am

I have come here to ask for help and discuss on its issues to the concerned peoples and experts, that I do not find in other sources. I request you that if you know the solution then please post here. If you do not know the solution then please do not down vote the question and let others help me.
Varun Sareen 16-Feb-12 13:02pm

sorry dear ratika i accidentally down voted your question. I was trying to up vote it :)
Ratika Agarwal 15-Feb-12 2:57am

I have already searched it on internet. But there is an issue. The formula given in 2-3 sources is that- the firth one is right...i.e.,
Exp have to be done after taking mod of addition.
Exp[ (Log A + Log B) Mod 255].

But Result of addition is so small that it doesn't need Mod, instead, result of Exponent is much long upto 2-4 bytes, which needs Mod.

What should be done in this case??
Ratika Agarwal 15-Feb-12 3:00am

Now do you have any optimistic comments on my problem??
Varun Sareen 16-Feb-12 13:04pm

have you tried any of the formulae atleast once??
Varun Sareen 16-Feb-12 13:05pm

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## Solution 1

Ratika you can use Galois Field GF(2m) Calculator with this link

Galois Field GF(2m) Calculator
Ratika Agarwal 15-Feb-12 4:50am

thanks. but I do not just want the calculated value.But I need to implement GF(2^8) multiplication for AES Mix Columns Operation. Therefore, to use GF(2^m) and to implement its code is compulsion.
krumia 15-Feb-12 10:34am

Read more about GF's. Read more theory. Then you will be able to answer yourself. Ask yourself! What should happen when multiplying. Should you get the Exp after Mod, or vice versa.

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