EC2M Lopez-Dahab ladder implementation

This commit uses the new ladder scaffold to implement a specialized
ladder step based on differential addition-and-doubling in mixed
Lopez-Dahab projective coordinates, modified to independently blind the
operands.

The arithmetic in `ladder_pre`, `ladder_step` and `ladder_post` is
auto generated with tooling:
- see, e.g., "Guide to ECC" Alg 3.40 for reference about the
  `ladder_pre` implementation;
- see https://www.hyperelliptic.org/EFD/g12o/auto-code/shortw/xz/ladder/mladd-2003-s.op3
  for the differential addition-and-doubling formulas implemented in
  `ladder_step`;
- see, e.g., "Fast Multiplication on Elliptic Curves over GF(2**m)
  without Precomputation" (Lopez and Dahab, CHES 1999) Appendix Alg Mxy
  for the `ladder_post` implementation to recover the `(x,y)` result in
  affine coordinates.

Co-authored-by: Billy Brumley <bbrumley@gmail.com>
Co-authored-by: Sohaib ul Hassan <soh.19.hassan@gmail.com>

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6690)

1 files changed, 6 insertions, 0 deletions

diff --git a/CHANGES b/CHANGES
index bfa73aeec3..c1d4c2d5ba 100644
--- a/CHANGES
+++ b/CHANGES
@@ -9,6 +9,12 @@
 
  Changes between 1.1.0h and 1.1.1 [xx XXX xxxx]
 
+  *) Use the new ec_scalar_mul_ladder scaffold to implement a specialized ladder
+     step for binary curves. The new implementation is based on formulas from
+     differential addition-and-doubling in mixed Lopez-Dahab projective
+     coordinates, modified to independently blind the operands.
+     [Billy Bob Brumley, Sohaib ul Hassan, Nicola Tuveri]
+
   *) Add a scaffold to optionally enhance the Montgomery ladder implementation
      for `ec_scalar_mul_ladder` (formerly `ec_mul_consttime`) allowing
      EC_METHODs to implement their own specialized "ladder step", to take