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I am trying to figure out the process behind the Speck block cipher. I understand how XOR works (Exclusive-or) when you take 2 strings of bits and you want to XOR them together.

However, in the key schedule of the cipher, there is a point where XOR takes place but I don t understand what is being XORed.

Link: https://en.wikipedia.org/wiki/Speck_(cipher)#/media/File:Speck_block_cipher_0_indexing.svg

For example, in the key schedule in the link, the result of the binary addition is being XOR d with 0?

Can someone explain what this means? Thanks.

Ella Rose
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Marc Hughes
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1 Answers1

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For example, in the key schedule in the link, the result of the binary addition is being XOR d with 0?

It's very simple: In the key schedule, the exclusive-or step after the addition is done with the round counter, and the round counter starts at $0$ rather than $1$.

So for round number $0$, the first exclusive-or is done with the value $0$. In the next round, the exclusive-or is done with the value $1$, and so forth.

The supplied reference code might be easier to read than the block diagram, depending on your programming skills.

Ella Rose
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