Result is the point Scalar*Point on Curve25519, as mandated by
X25519 (RFC 7748). Scalar is an integer between 0 and 2^256-1, or
32 bytes. Fails if Point has small order, i.e., if the result
would be the point at infinity.
Alice and Bob can use this to establish a shared secret, where
Generator is obtained with curve25519_generator/1:
- Alice creates a random integer a and sends As = a*Generator
to Bob.
- Bob creates a random integer b and sends Bs = b*Generator
to Alice.
- Alice computes Rs = a*Bs.
- Bob computes Rs = b*As.
- Alice and Bob use crypto_data_hkdf/4 on Rs with suitable (same)
parameters to obtain keys and initialization vectors for
symmetric encryption.
If a and b are kept secret, this method is considered very
secure.