Implement coordinate blinding for EC_POINT

This commit implements coordinate blinding, i.e., it randomizes the
representative of an elliptic curve point in its equivalence class, for
prime curves implemented through EC_GFp_simple_method,
EC_GFp_mont_method, and EC_GFp_nist_method.

This commit is derived from the patch
https://marc.info/?l=openssl-dev&m=131194808413635 by Billy Brumley.

Coordinate blinding is a generally useful side-channel countermeasure
and is (mostly) free. The function itself takes a few field
multiplicationss, but is usually only necessary at the beginning of a
scalar multiplication (as implemented in the patch). When used this way,
it makes the values that variables take (i.e., field elements in an
algorithm state) unpredictable.

For instance, this mitigates chosen EC point side-channel attacks for
settings such as ECDH and EC private key decryption, for the
aforementioned curves.

For EC_METHODs using different coordinate representations this commit
does nothing, but the corresponding coordinate blinding function can be
easily added in the future to extend these changes to such curves.

Co-authored-by: Nicola Tuveri <nic.tuv@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/6501)

1 files changed, 5 insertions, 0 deletions

diff --git a/CHANGES b/CHANGES
index fe3e13aa0d..a4beda66dd 100644
--- a/CHANGES
+++ b/CHANGES
@@ -8,6 +8,11 @@
  release branch.
 
  Changes between 1.1.0h and 1.1.1 [xx XXX xxxx]
+  *) Add coordinate blinding for EC_POINT and implement projective
+     coordinate blinding for generic prime curves as a countermeasure to
+     chosen point SCA attacks.
+     [Sohaib ul Hassan, Nicola Tuveri, Billy Bob Brumley]
+
   *) Add blinding to an ECDSA signature to protect against side channel attacks
      discovered by Keegan Ryan (NCC Group).
      [Matt Caswell]