582a6c2db4
Two edge cases that were mentioned in the docs are actually impossible:
* For NIST curves, ECDH can't fail, since the zero scalar is rejected
by NewPrivateKey, the identity point is rejected by NewPublicKey,
and NIST curves are a prime-order group.
Let's call the inputs to scalar multiplication k and P, and the
order of the group q. If k[P] is the identity, and also q[P] is the
identity by definition, then P's order is a divisor of q-k, because
k[P] + [q-k]P = q[P] = I
P's order is either 1 or q, and can only be a divisor of q-k if it's
1 (so P is the identity), or if k is zero.
* For X25519, PrivateKey.PublicKey can't return the all-zero value,
since no value is equivalent to zero after clamping.
Clamping unsets the lowest three bit, sets the second-to-highest
bit, and unsets the top bit; this means that a scalar equivalent to
zero needs to be a multiple of 8*q, and needs to be between 2**254
and 2**255-1, but 8*p > 2**255-1.
Tests for other exotic edge cases such as non-canonical point encodings,
clamping, points on the twist, and low-order components are covered by
x/crypto/wycheproof.
Change-Id: I731a878c58bd59aee5636211dc0f19ad8cfae9db
Reviewed-on: https://go-review.googlesource.com/c/go/+/425463
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
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