k256: get rid of eager computation in mul_shift_vartime()
#638
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Fortunately since it was an internal method only used with shift=272, it never made any difference.
…t is shift-dependent It is expected to be vartime on the value of `shift`, and calculating some of the paths eagerly when they are not required leads to panics.
tarcieri
approved these changes
Aug 10, 2022
Merged
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mul_shift_vartime()in places where it is shift-dependent. It is expected to be vartime on the value ofshift, and calculating some of the paths eagerly when they are not required leads to panics. Fixes K256: panic 'attempt to shift left with overflow' in mul_shift_vartime #624mul_shift_vartime()(incorrect constant, it was448instead of384in the expression forr1). Fortunately, since it was only ever used withshift == 272, it never mattered.