SWI-Prolog -- curve25519_scalar

Documentation
- Reference manual
- Packages
  - SWI-Prolog SSL Interface
    - library(crypto): Cryptography and authentication library
      - Curve25519
        
        curve25519_generator/1
        
        curve25519_scalar_mult/3

Availability::- use_module(library(crypto)).(can be autoloaded)

[semidet]curve25519_scalar_mult(+Scalar, +Point, -Result)

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.