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Check if an Elliptic Curve point (over finite field) is nearer than another point

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user106244

Is there any algorithm that I can check if point(G^9857) is more near to point(G^54) rather than point(G^448)?

fgrieu avatar
ng flag
It's unusual to consider distance between points on a discrete Elliptic Curve as used in cryptography. Several different distances can be defined. Which one do you want, or what is the actual purpose?
fgrieu avatar
ng flag
Example making some sense: 1) The number of times we must use $G$ to go between points $G^a$ and $G^b$, that is $|G^a,G^b|=|((a-b+\frac{n-1}2)\bmod n)-\frac{n-1}2|$ where $n$ is the order of $G$. 2) "Manhattan/taxicab" distance per Cartesian coordinates, for points other than the point at infinity, that is $|A,B|$ is the integer $|((x_A-x_B+\frac{p-1}2)\bmod p)-\frac{p-1}2|+|((y_A-y_B+\frac{p-1}2)\bmod p)-\frac{p-1}2|$, where $p$ is the field order. 3) "Flying" distance $\sqrt{(((x_A-x_B+\frac{p-1}2)\bmod p)-\frac{p-1}2)^2+(((y_A-y_B+\frac{p-1}2)\bmod p)-\frac{p-1}2)^2}$
fgrieu avatar
ng flag
If by _"private key"_ it's meant what's 9857, 54 and 448 in the question, then one can compute the distance as in 1 above (then compare) only when that distance is below some threshold (e.g. $2^{72}$), or the private keys of the points involved are known. For distances 2/3, only the points are needed (but these other distances do not seem so useful).
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