lib25519: Install

Prerequisites:

- `python3` (for: build, test)
- recommended `python3-capstone` (for: build)
- `gcc` and/or `clang` (for: build)
- `valgrind` (for: build, test)
- `libcpucycles` (for: build, bench)
- `librandombytes` (for: build, test, bench, run)

Make sure to test the compiled library. The tests check for subtle security problems that can be created by compilers. A compiled version of lib25519 is not supported unless it passes the full test suite.

### For sysadmins

To install in `/usr/local/{include,lib,bin,man}`:

    ./configure && make -j8 install

### For developers with an unprivileged account

Typically you'll already have

    export LD_LIBRARY_PATH="$HOME/lib"
    export LIBRARY_PATH="$HOME/lib"
    export CPATH="$HOME/include"
    export MANPATH="$HOME/man"
    export PATH="$HOME/bin:$PATH"

in `$HOME/.profile`. To install in `$HOME/{include,lib,bin,man}`:

    ./configure --prefix=$HOME && make -j8 install

### For distributors creating a package

Run

    ./configure --prefix=/usr && make -j8

and then follow your usual packaging procedures for the `build/0/package` files:

    build/0/package/include/lib25519.h
    build/0/package/lib/lib25519*
    build/0/package/bin/lib25519*
    build/0/package/man/man3/*.3
    build/0/package/man/man1/*.1

### More options, part 1: s2n-bignum

Before `./configure` you can run `./use-s2n-bignum` to download various assembly implementations from s2n-bignum and integrate them into lib25519. These implementations save time on some CPUs, but the more important feature of these implementations is that they are formally verified to work correctly on all inputs. The implementations cover, for 64-bit Intel/AMD/ARM, the main `nG` and `nP` subroutines used in X25519 key generation and shared-secret generation, along with a lower-level `pow/inv25519` subroutine having further applications.

You can run

    ./configure --prioritizeverified

to prioritize implementations marked as verified even when they are slower. Beware that this will still fall back to unverified implementations for primitives and CPUs that do not have verified implementations.

### More options, part 2: architectures

You can run

    ./configure --host=amd64

to override `./configure`'s guess of the architecture that it should compile for.

Inside the `build` directory, `0` is symlinked to `amd64` for `--host=amd64`. Running `make clean` removes `build/amd64`. Re-running `./configure` automatically starts with `make clean`.

A subsequent `./configure --host=arm64` will create `build/arm64` and symlink `0 -> arm64`, without touching an existing `build/amd64`. However, cross-compilers aren't yet selected automatically.

### More options, part 3

One further `./configure` option is supported for developers: `--no-trim`.

All `./configure` options not listed above are experimental and not supported. Use them at your own risk.

Version: This is version 2026.06.13 of the "Install" web page.