# Re: Computing Rijndael's S-Box?

**From:** Björn Fay (*mail_at_bfay.de*)

**Date:** 08/30/05

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Date: Tue, 30 Aug 2005 12:04:25 +0200

cobussteyn@absamail.co.za wrote:

*> I'm a newbie when it comes to cryptography and currently I'm hooked on
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*> Rijndael. I know most of the implementations of it have the S-box
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*> hard-coded into the app for speed gain, but I would like to compute
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*> that s-box myself.
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*>
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*> You probably know it is defined by the finite field mapping in GF(2^8)
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*> onto its inverse, and then it is transformed by a certain matrix (i
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*> have that one) and then 0x63 is added.
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*>
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*> Can someone help me to compute that first mapping of GF(2^8) onto its
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*> inverse? How do you compute the inverse?
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*>
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*> Thanx !
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You have to use the Extended Euclidean Algorithm an explanation can be found in books for discrete mthematics or in wikipedia:

http://en.wikipedia.org/wiki/Extended_Euclidean_algorithm

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