# Questions tagged [mds]

An MDS matrix (Maximum Distance Separable) is a matrix representing a function with certain diffusion properties that have useful applications in cryptography.

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### How to find the AES branch number?

By definition, branch number Definition: The branch number of a linear transformation $F$ is $$min_{a\neq0}(W(a) + W(F(a)))$$ Source here (7.3.1) For AES MixColumns $a \in GF(2^8)^4$ since ...
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### How can I calculate the Rijndael SBox?

I would like to implement the Rijndael subBytes() operation using calculation instead of tables, because I like to play with this on different wordsizes, as an ...
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### How 2 rounds in AES achieve full diffusion?

I have read somewhere that 2 rounds is AES provide full diffusion. So I looked it up to find out what it exactly meant. In The Design of Rijndael page 41, section 3.5 and it states that: Two ...
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### What is the branch number of this matrix?

We have the following matrix: $$\begin{pmatrix}0&1&1&1\\ 1&0&1&1\\ 1&1&0&1\\ 1&1&1&0\end{pmatrix}$$ What is the branch number? Is this a MDS marix?
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### How to calculate active s-boxes from branch number?

If MDS in AES has branch number 5 (so 5 active s-boxes in 2 rounds), wouldn't that mean 4 rounds of AES has $5*2=10$ active s-boxes? AES paper says it has 25 ($5^2$?) active s-boxes in 4 rounds. How ...
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### How was the MDS matrix used in AES chosen?

$$\begin{pmatrix}2&3&1&1\\ 1&2&3&1\\ 1&1&2&3\\ 3&1&1&2\end{pmatrix}$$ In the above MDS matrix used in AES encryption, why are the numbers $2$,$3$ and $1$...
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