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lib25519 is a microlibrary for the X25519 encryption system and the
Ed25519 signature system, both of which use the Curve25519 elliptic
curve. Curve25519 is the fastest curve in TLS 1.3, and the only curve in
Wireguard, Signal, and many other applications (see Nicolai Brown's page
https://ianix.com/pub/curve25519-deployment.html).

lib25519 has a very simple stateless API based on the SUPERCOP API, with
wire-format inputs and outputs, providing functions that directly match
the central cryptographic operations in X25519 and Ed25519:

    lib25519_dh_keypair(pk,sk): X25519 key generation
    lib25519_dh(k,pk,sk): shared-secret generation

    lib25519_sign_keypair(pk,sk): Ed25519 key generation
    lib25519_sign(sm,&smlen,m,mlen,sk): signing
    lib25519_sign_open(m,&mlen,sm,smlen,pk): verification + message recovery

Internally, lib25519 includes implementations designed for performance
on various CPUs, implementations designed to work portably across CPUs,
and automatic run-time selection of implementations.

lib25519 is designed to avoid all data flow from secret data to timing.
Beware, however, that this can be spoiled by CPUs where the most common
instructions take variable time; in particular, there are some embedded
CPUs with variable-time multipliers. Also, lib25519 does not currently
include protection against side-channel attacks beyond timing.

lib25519 is intended to become a central target for verification of full
functional correctness of implementations of X25519 and Ed25519. Beware,
however, that only certain portions have been verified so far.

lib25519 is intended to be called by larger multi-function libraries,
including libraries in other languages via FFI. The idea is that
lib25519 will take responsibility for the details of X25519/Ed25519
computation, including optimization, timing-attack protection, and
eventually verification, freeing up the calling libraries to concentrate
on application-specific needs such as protocol integration. Applications
can also call lib25519 directly.