Implement MontgomeryPoint::to_edwards via 4-isogeny inversion #1504
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The implementation in this PR is very much variable-time. Does it have prior art somewhere? I'm wondering if a more straightforward constant-time implementation is possible. #1350 implements the 4-isogeny. I'm wondering if instead you could first convert from Montgomery to twisted Edwards, then leverage that to convert to untwisted Edwards? cc @daxpedda |
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It seems like at least an initial implementation of this could use a similar method to FWIW, I guess their implementation handles the exceptional cases in variable-time. |
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This change implements the previously unimplemented MontgomeryPoint::to_edwards() using the 4-isogeny inverse consistent with the existing forward map in EdwardsPoint::to_montgomery(). It solves the quadratic for y^2 from u, checks discriminant and square roots in the field, and uses the provided sign bit to select the x-sign, returning None on non-residues or invalid denominators. This removes a todo!() panic point, enables Montgomery to Edwards conversion needed for interoperability, and follows existing project style and primitives without changing the public API.