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Using $x$-coordinates only, given a point $G = (x_0,y_0)$ on an elliptic curve $y^2 = x^3 + ax + b$ and a scalar $r$, the Montgomery ladder outputs the $x$-coordinate of $rG$. However, a closer look at the algorithm shows that the other accumulator used in the computation contains the $x$-coordinate of $(r+1)G$. Letting $x_1$ the $x$-coordinate of $rG$ and ...