RFC7748 states that it is entirely optional for X25519 Diffie-Hellman
implementations to check whether or not the result is the all-zero
value (indicating that an attacker sent a malicious public key with a
small order). RFC8422 states that implementations in TLS must abort
the handshake if the all-zero value is obtained.
Return an error if the all-zero value is obtained, so that the TLS
code will not require knowledge specific to the X25519 curve.
Signed-off-by: Michael Brown <mcb30@ipxe.org>
27398f1360