tree eb1b8173f84a71735e94330042c5a53e03c6a743
parent 701742500ddf9cbae6875efa344e21a8a013dd62
author Janos Follath <janos.follath@arm.com> 1624545299 +0100
committer Janos Follath <janos.follath@arm.com> 1624629555 +0100

Reject low-order points on Curve448 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: Janos Follath <janos.follath@arm.com>