This has the effect of making `secp256k1_scalar_mul_shift_var` constant
time in both input scalars. Keep the _var name because it is NOT constant
time in the shift amount.
As used in `secp256k1_scalar_split_lambda_var`, the shift is always
the constant 272, so this function becomes constant time, and it
loses the `_var` suffix.
Designed with clear separation of the wNAF conversion, precomputation
and exponentiation (since the precomp at least we will probably want
to separate in the API for users who reuse points a lot.
Future work:
- actually separate precomp in the API
- do multiexp rather than single exponentiation
Unbraced statements spanning multiple lines has been shown in many
projects to contribute to the introduction of bugs and a failure
to catch them in review, especially for maintenance on infrequently
modified code.
Most, but not all, of the existing practice in the codebase were not
cases that I would have expected to eventually result in bugs but
applying it as a rule makes it easier for other people to safely
contribute.
I'm not aware of any such evidence for the case with the statement
on a single line, but some people strongly prefer to never do that
and the opposite rule of "_always_ use a single line for single
statement blocks" isn't a reasonable rule for formatting reasons.
Might as well brace all these too, since that's more universally
acceptable.
[In any case, I seem to have introduced the vast majority of the
single-line form (as they're my preference where they fit).]
This also removes a broken test which is no longer needed.
- In secp256k1_gej_split_exp, there are two divisions used. Since the denominator is a constant known at compile-time, each can be replaced by a multiplication followed by a right-shift (and rounding).
- Add the constants g1, g2 for this purpose and rewrite secp256k1_scalar_split_lambda_var accordingly.
- Remove secp256k1_num_div since no longer used
Rebased-by: Pieter Wuille
