fiat-p256 contains primitives for ECDH key exchange algorithm over NIST curve P-256.

It internally uses bindings to C code generated using the correct-by-construction implementations from
fiat-crypto.

Please be aware that cryptographic primitives should not be used in end applications, they are better
used as part of a higher level cryptographic library.

Installation

fiat-p256 is available on opam and can be install as follows:

opam install fiat-p256

Usage

The entry point to this library is the Fiat_p256 module and the main function is dh which let
you perform a key exchange given your private key scalar and the other party's public key point:

let secret = Fiat_p256.dh ~scalar ~point

Note that the point values built or parsed using Fiat_p256's interface are checked
according to NIST's
Recommendation for Pair-Wise Key Establishment Schemes Using Discrete Logarithm Cryptography
section 5.6.2.3.2 so that you shouldn't be able to provide an invalid point or the point at
infinity to the functions exposed in this module.

You can also compute the public P-256 key corresponding to your private key scalar using the public
function:

let public_key = Fiat_p256.public ~scalar

Note that the scalar values parsed using Fiat_p256's interface must be within P-256's generator
subgroup order range so that your public key can't be the point at infinity. Any scalar not in the
range [1 - (n-1)], n being the group order, will be rejected.