* Make secp256k1_gej_add_var and secp256k1_gej_double return the
Z ratio to go from a.z to r.z.
* Use these Z ratios to speed up batch point conversion to affine
coordinates, and to speed up batch conversion of points to a
common Z coordinate.
* Add a point addition function that takes a point with a known
Z inverse.
* Due to secp256k1's endomorphism, all additions in the EC
multiplication code can work on affine coordinate (with an
implicit common Z coordinate), correcting the Z coordinate of
the result afterwards.
Refactoring by Pieter Wuille:
* Move more global-z logic into the group code.
* Separate code for computing the odd multiples from the code to bring it
to either storage or globalz format.
* Rename functions.
* Make all addition operations return Z ratios, and test them.
* Make the zr table format compatible with future batch chaining
(the first entry in zr becomes the ratio between the input and the
first output).
Original idea and code by Peter Dettman.
- 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
- Uses a similar approach to the latest 64bit _normalize.
- Add one useful optimization back into the 64bit _normalize too.
Performance of 'bench' improved by around 0.5% for the 32bit field (but tested on a 64-bit machine).
- secp256k1_fe_sqrt now checks that the value it calculated is actually a square root.
- Add return values to secp256k1_fe_sqrt and secp256k1_ge_set_xo.
- Callers of secp256k1_ge_set_xo can use return value instead of explicit validity checks
- Add random value tests for secp256k1_fe_sqrt
Peter Dettman