EC point multiplication: add `ladder` scaffold

for specialized Montgomery ladder implementations

PR #6009 and #6070 replaced the default EC point multiplication path for
prime and binary curves with a unified Montgomery ladder implementation
with various timing attack defenses (for the common paths when a secret
scalar is feed to the point multiplication).
The newly introduced default implementation directly used
EC_POINT_add/dbl in the main loop.

The scaffolding introduced by this commit allows EC_METHODs to define a
specialized `ladder_step` function to improve performances by taking
advantage of efficient formulas for differential addition-and-doubling
and different coordinate systems.

- `ladder_pre` is executed before the main loop of the ladder: by
  default it copies the input point P into S, and doubles it into R.
  Specialized implementations could, e.g., use this hook to transition
  to different coordinate systems before copying and doubling;
- `ladder_step` is the core of the Montgomery ladder loop: by default it
  computes `S := R+S; R := 2R;`, but specific implementations could,
  e.g., implement a more efficient formula for differential
  addition-and-doubling;
- `ladder_post` is executed after the Montgomery ladder loop: by default
  it's a noop, but specialized implementations could, e.g., use this
  hook to transition back from the coordinate system used for optimizing
  the differential addition-and-doubling or recover the y coordinate of
  the result point.

This commit also renames `ec_mul_consttime` to `ec_scalar_mul_ladder`,
as it better corresponds to what this function does: nothing can be
truly said about the constant-timeness of the overall execution of this
function, given that the underlying operations are not necessarily
constant-time themselves.
What this implementation ensures is that the same fixed sequence of
operations is executed for each scalar multiplication (for a given
EC_GROUP), with no dependency on the value of the input scalar.

Co-authored-by: Sohaib ul Hassan <soh.19.hassan@gmail.com>
Co-authored-by: Billy Brumley <bbrumley@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, 5 insertions, 0 deletions

diff --git a/crypto/err/openssl.txt b/crypto/err/openssl.txt
index 007560a..99bd118 100644
--- a/crypto/err/openssl.txt
+++ b/crypto/err/openssl.txt
@@ -630,6 +630,7 @@ EC_F_EC_POINT_SET_JPROJECTIVE_COORDINATES_GFP:126:\
 	EC_POINT_set_Jprojective_coordinates_GFp
 EC_F_EC_POINT_SET_TO_INFINITY:127:EC_POINT_set_to_infinity
 EC_F_EC_PRE_COMP_NEW:196:ec_pre_comp_new
+EC_F_EC_SCALAR_MUL_LADDER:284:ec_scalar_mul_ladder
 EC_F_EC_WNAF_MUL:187:ec_wNAF_mul
 EC_F_EC_WNAF_PRECOMPUTE_MULT:188:ec_wNAF_precompute_mult
 EC_F_I2D_ECPARAMETERS:190:i2d_ECParameters
@@ -2130,6 +2131,9 @@ EC_R_INVALID_PRIVATE_KEY:123:invalid private key
 EC_R_INVALID_TRINOMIAL_BASIS:137:invalid trinomial basis
 EC_R_KDF_PARAMETER_ERROR:148:kdf parameter error
 EC_R_KEYS_NOT_SET:140:keys not set
+EC_R_LADDER_POST_FAILURE:136:ladder post failure
+EC_R_LADDER_PRE_FAILURE:153:ladder pre failure
+EC_R_LADDER_STEP_FAILURE:162:ladder step failure
 EC_R_MISSING_PARAMETERS:124:missing parameters
 EC_R_MISSING_PRIVATE_KEY:125:missing private key
 EC_R_NEED_NEW_SETUP_VALUES:157:need new setup values
@@ -2144,6 +2148,7 @@ EC_R_PEER_KEY_ERROR:149:peer key error
 EC_R_PKPARAMETERS2GROUP_FAILURE:127:pkparameters2group failure
 EC_R_POINT_ARITHMETIC_FAILURE:155:point arithmetic failure
 EC_R_POINT_AT_INFINITY:106:point at infinity
+EC_R_POINT_COORDINATES_BLIND_FAILURE:163:point coordinates blind failure
 EC_R_POINT_IS_NOT_ON_CURVE:107:point is not on curve
 EC_R_RANDOM_NUMBER_GENERATION_FAILED:158:random number generation failed
 EC_R_SHARED_INFO_ERROR:150:shared info error