arti/crates/tor-llcrypto/README.md

# tor-llcrypto

`tor-llcrypto`: Low-level cryptographic implementations for Tor.

## Overview

The `tor-llcrypto` crate wraps lower-level cryptographic primitives that Tor
needs, and provides a few smaller pieces of cryptographic functionality that
are commonly required to implement Tor correctly.

This crate is part of [Arti](https://gitlab.torproject.org/tpo/core/arti/),
a project to implement [Tor](https://www.torproject.org/) in Rust. Many
other crates in Arti depend on it.

You probably wouldn't want to use this crate for implementing non-Tor-based
protocols; instead you should probably use the other crates that it depends
on if you have a low-level protocol to implement, or a higher-level
cryptographic system if you want to add security to something else.  It is
easy to accidentally put these functions together in ways that are unsafe.

### Why a separate crate?

Why do we collect and re-export our cryptography here in `tor-llcrypto`,
instead of having the different crates in Arti use underlying cryptographic
crates directly?

By wrapping our cryptography in this crate, we ensure that we're using the
same implementations across our ecosystem, and provide a single place to
upgrade and test our cryptography.

### Adding to `tor-llcrypto`

Any low-level cryptographic algorithm that is used by at least two other
crates in Arti is a candidate for inclusion in `tor-llcrypto`, especially if
that algorithm's purpose is not specific to any single piece of the Tor
algorithm.

Cryptographic _traits_ (like those from RustCrypto) don't have to go in
`tor-llcrypto`, since they are interfaces rather than implementations.

## Contents

Encryption is implemented in [`cipher`]: Currently only AES is exposed or
needed.

Cryptographic digests are in [`d`]: The Tor protocol uses several digests in
different places, and these are all collected here.

Public key cryptography (including signatures, encryption, and key
agreement) are in [`pk`].  Older parts of the Tor protocol require RSA;
newer parts are based on Curve25519 and Ed25519. There is also functionality
here for _key manipulation_ for the keys used in these symmetric algorithms.

The [`util`] module has some miscellaneous compatibility utilities for
manipulating cryptography-related objects and code.

## Features

### API features

`relay` -- enable cryptography that's only used on relays.

`hsv3-client` -- enable cryptography that's only needed when running as a v3
onion service client.

### Acceleration features

These features should never be enabled by default from libraries, since they
are not "strictly additive": they disable one implementation in order to
enable another.

`with-openssl` -- Use `openssl` as the backend for those cryptographic
features it supports.

`with-sha1-asm` -- Use an assembly implementation of the sha1 algorithm, if
one is enabled.

License: MIT OR Apache-2.0