Division by zero in blueprint_algebra_curves_plonk_variable_base_scalar_mul_test

[martun]

We always had a division by zero in blueprint_algebra_curves_plonk_variable_base_scalar_mul_test, near

assignment.witness(component.W(7), i + 1) = (P[0].Y - Q.Y) * (P[0].X - Q.X).inversed();

in include/nil/blueprint/components/algebra/curves/pasta/plonk/variable_base_scalar_mul.hpp.

P[0].X - Q.X is zero because both P[0].X and Q.X are zero.

We never noticed it because of in multiprecision A / 0 = 0, here:

So all this time when asking for the inverse element of zero, we were getting zero back... With changes to multiprecision we get a segfault...

[SK0M0R0H]

Not sure that I understand:

P[0].X - Q.X is zero because both P[0].X and Q.X are zero.

DO you mean P[0].X and Q.X are always zero or do you consider particular case when they're zeros?

[martun]

@SK0M0R0H No, they are not always zero, they are sometimes zero for a particular input.