tree beeda491f5382f438756e3fccf54dc1634a03aae
parent 92f387777d684a124e79380defe7ea216d249e53
author Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com> 1624436098 +0200
committer Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com> 1624436098 +0200

Reject low-order points on Curve25519 early

We were already rejecting them at the end, due to the fact that with the
usual (x, z) formulas they lead to the result (0, 0) so when we want to
normalize at the end, trying to compute the modular inverse of z will
give an error.

If we wanted to support those points, we'd a special case in
ecp_normalize_mxz(). But it's actually permitted by all sources
(RFC 7748 say we MAY reject 0 as a result) and recommended by some to
reject those points (either to ensure contributory behaviour, or to
protect against timing attack when the underlying field arithmetic is
not constant-time).

Since our field arithmetic is indeed not constant-time, let's reject
those points before they get mixed with sensitive data (in
ecp_mul_mxz()), in order to avoid exploitable leaks caused by the
special cases they would trigger. (See the "May the Fourth" paper
https://eprint.iacr.org/2017/806.pdf)

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>