PSA_ALG_ECDH macro

The elliptic curve Diffie-Hellman (ECDH) key agreement algorithm. The shared secret produced by key agreement is the x-coordinate of the shared secret point. It is always `ceiling(m / 8)` bytes long where `m` is the bit size associated with the curve, i.e. the bit size of the order of the curve's coordinate field. When `m` is not a multiple of 8, the byte containing the most significant bit of the shared secret is padded with zero bits. The byte order is either little-endian or big-endian depending on the curve type. - For Montgomery curves (curve types `PSA_ECC_FAMILY_CURVEXXX`), the shared secret is the x-coordinate of `d_A Q_B = d_B Q_A` in little-endian byte order. The bit size is 448 for Curve448 and 255 for Curve25519. - For Weierstrass curves over prime fields (curve types `PSA_ECC_FAMILY_SECPXXX` and `PSA_ECC_FAMILY_BRAINPOOL_PXXX`), the shared secret is the x-coordinate of `d_A Q_B = d_B Q_A` in big-endian byte order. The bit size is `m = ceiling(log_2(p))` for the field `F_p`. - For Weierstrass curves over binary fields (curve types `PSA_ECC_FAMILY_SECTXXX`), the shared secret is the x-coordinate of `d_A Q_B = d_B Q_A` in big-endian byte order. The bit size is `m` for the field `F_{2^m}`.