[New in v20.0]
Support is provided for Elliptic Curve Diffie-Hellman (ECDH) operations using the ECC_DHSharedSecret and
.NET Ecc.DHSharedSecret Method.
These functions enable you to compute the shared secret given your own private EC key and the other party's public EC key. Note this shared secret (often denoted ZZ or Z) is usually not used directly itself, but is passed to another function such as a key derivation function, perhaps with other agreed parameters. These subsequent operations are out of scope here.
Note also that there are two ways to compute the shared secret using the NIST/SEC curves. One way using the cofactor and one without. The former is referred to as "ECC Cofactor Diffie-Hellman (ECC CDH)" in [SP800-56A] and "ECSVDP-DHC" in [IEEE1363], and the latter as "ECSVDP-DH" in [IEEE1363] and [RFC5349]. BUT all the NIST/SEC curves in this toolkit have a cofactor of one, so you get the same result with either calculation.
There is only one accepted way to compute the shared secret using the safe curves X25519 and X448 - see [RFC7748].