# Converting a point in a finite field to its real (x, y) coordinate [closed]

Let curve $$A: y^2 = x^3 + 7$$ and curve $$B: y^2 \equiv x^3 + 7 \pmod{p}$$

Curve $$B$$ is secp256k1, assume the usual parameters for that curve.

Let $$k$$ be any private key, and compute the corresponding public key point $$[k]G$$ in curve $$B$$.

Now, from this point $$[k]G$$ in curve $$B$$, how can we find what the point $$[k]G$$ in the "real" curve $$A$$ is? You don't know the value for $$k$$, but you do know all other parameters.

Point $$G$$ in curve $$A$$ is the positive $$y$$ solution to the curve $$A$$ with $$x$$ equal to the $$x$$ value of $$G$$ in curve $$B$$.

• Use Discrete Log. I think you were asking X and wanting Y, we call it X/Y? DLog has complexity $\mathcal{O}(\sqrt{n})$ where $n$ is the group order and for this curve, it has 13 points! – kelalaka Jan 22 at 8:19
• @kelalaka I'm not exactly sure how that could be helpful. I'm asking for the equivalent $G*r$ point in the curve over all real numbers given the $G*r$ point in the curve over the finite field. – enriquejr99 Jan 22 at 8:37
• That is scalar multiplication, then use the addition laws, with double-and-add algorithm. – kelalaka Jan 22 at 8:40
• The general answer is that you can't since the modulo operation is destructive. Since you don't know about $k$ then you don't know how many times the scalar multiplication uses the $\mod p$. There are cases that one can lift the curve $B$ and solve the DLog problem easily there, however, secp256k1 has no such known lifting. – kelalaka Jan 22 at 10:33
• I’m voting to close this question because it is cross-posted with Math. This is not considered good in Is cross-posting a question on multiple Stack Exchange sites permitted if the question is on-topic for each site? – kelalaka Jan 22 at 16:08