# Proving that two points on elliptic curve are within range

Is it possible to prove that a point on an elliptic curve falls within a given range of another point, without revealing the distance between them. For example:

Let's say $$X$$ and $$Y$$ are two points on an elliptic curve such that $$X = x \cdot G$$ and $$Y = y \cdot G$$, where $$G$$ is the generator. The proover knows values of $$x$$ and $$y$$, and wants to prove to the verifier that $$|x - y| < z$$, where $$z$$ is some value supplied by the verifier.

Another option: prove that $$\frac{x}{y} < z$$, where $$z$$ can be a relatively small number (e.g. 2).

Is either of these possible?

• Two step Yao's Millionaires' problem? – kelalaka Dec 23 '18 at 12:40

The proof showing that, given $$G$$ and $$x \cdot G$$ and a range $$[A,B]$$, $$x$$ is in the range $$[A,B]$$ is known as the range proof. In your case, we can convert your statement as the range proof as, given $$G$$ and $$H = xG - yG$$, showing $$x-y$$ is in the range $$[-z,z]$$.