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In their paper about the AEAD scheme NORX, the authors mention that $a+b$ can be "well approximated" (this is a bit a hand-wavy statement) by $$a+b \; "="\; a\oplus b \oplus ((a\land b) \ll 1).$$

(Note that the purpose of $((a \land b) \ll 1)$ is to simulate the "carry bit operation.")

Is there such a neat approximation for multiplication $a\cdot b$?

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  • $\begingroup$ While I can recommend the whole paper by Aumasson et al linked in the question, let me add that the approximation of $+$ is mentioned at the top of page 2. $\endgroup$ May 30, 2021 at 17:53
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    $\begingroup$ While I have no literature references, it looks like that approximation comes from representing $a+b$ via an adder (via something like $a + b = (a\oplus b) + ((a\land b)\gg 1)$, and then approximating $+\mapsto \oplus$. To derive something similar for $a\cdot b$ I would try to apply some similar process to a binary multiplier, and then try to approximate the resulting expression. $\endgroup$
    – Mark
    May 30, 2021 at 22:41
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    $\begingroup$ I can't think of anything that wouldn't result in a lot of operations. Even using $\mathbb{F}_2[x]$ multiplication as a primitive, there doesn't seem to be some simple relation between the two operations to make a useful compromise. The obvious thing is to do shift-and-add multiplication using the NORX operation as the "add", but that's not exactly "neat". $\endgroup$ May 31, 2021 at 4:10
  • $\begingroup$ Thanks to both commenters; the way @Mark suggests is the natural thing to do. I wondered whether there is some nifty shortcut I keep missing $\endgroup$ May 31, 2021 at 8:39

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