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Im reading the CRYSTALS kyber paper and am stuck on the PKE correctness proof on page 5. I can't see how the triangle inequality would help to get to the result $|| \lceil q / 2 \rfloor \cdot (m - m') ||_\infty < 2 \cdot \lceil q / 4 \rfloor$.

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They must be using the $\|a\|-\|b\| \le \|a+b\|$ variant of the triangle inequality (see Wolfram MathWorld).

For those of you following along, this is all at the end of page 5. They start with the following fact:

$$ \bigl\| w + \lceil q/2 \rfloor \cdot m - \lceil q/2 \rfloor \cdot m' \bigr\|_\infty \le \lceil q/4 \rfloor. $$

Apply the triangle inequality that I wrote above (with $b=w$), to get:

$$ -\bigl\| w \bigr\|_\infty + \bigl\| \lceil q/2 \rfloor \cdot m - \lceil q/2 \rfloor \cdot m' \bigr\|_\infty \le \lceil q/4 \rfloor. $$

Then move $\|w\|_\infty$ to the right hand side, and use the fact that $\|w\|_\infty < \lceil q/4\rfloor$ to finally get:

$$ \bigl\| \lceil q/2 \rfloor \cdot m - \lceil q/2 \rfloor \cdot m' \bigr\|_\infty < 2 \lceil q/4 \rfloor. $$

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  • $\begingroup$ The authors then go on to write that for all odd $q$ it implies that $m = m'$. Why is this the case? Does this follow from a "coefficient comparison"? $\endgroup$
    – P_Gate
    Nov 9, 2022 at 17:47

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