# tor-proto

Implementations for the core Tor protocol

## Overview

The `tor-proto` crate lies at the core of
[Arti](https://gitlab.torproject.org/tpo/core/arti/), a project to
implement [Tor](https://www.torproject.org/) in Rust.
Most people shouldn't use this crate directly,
since its APIs are needlessly low-level for most purposes, and it is
easy to misuse them in an insecure or privacy-violating way.

Most people should use the [`arti-client`] crate instead.  This crate is
of interest mainly for those that want to access the Tor protocols at
a low level.

### Core concepts

At its essence, Tor makes connections called "channels" to other
Tor instances.  These channels are implemented using TLS.  Each of
these channels multiplexes a number of anonymized multihop
"circuits" that act as reliable transports for "relay messages"
that are sent between clients and the different relays on the
circuits.  Finally, each circuit multiplexes a number of "streams",
each corresponding roughly to an application-level request.

This crate implements the logic, protocols, and cryptography that
implement these [`channel::Channel`]s, [`circuit::ClientCirc`]s, and
[`stream::DataStream`]s.  It uses rust async code and future-related
traits, and is intended to work with (nearly) any executor
implementation that complies with the futures API.  It should also
work with nearly any TLS implementation that exposes AsyncRead and
AsyncWrite traits.

### Not in this crate

This crate does _not_ implement higher level protocols, like onion
services or the Tor directory protocol, that are based on the Tor
protocol here.  Nor does it decide _when_, _how_, or _where_ to
build channels and circuits: that's the role of higher-level crates.

This crate also has no support for timeouts, so every network
operation here has the potential to block the current task
indefinitely.  Timeouts are another necessary piece that gets
added at a higher level.

In order to create channels and circuits, you'll need to know
about some Tor relays, and expose their information via
[`tor_linkspec::ChanTarget`] and [`tor_linkspec::CircTarget`].
Currently, the [`tor-netdir`] crate is the easiest way to do so.

For an example of this crate in action, see the [`arti-client`]
library, or the `arti` CLI.

## Design notes

This crate's APIs are structured to limit usage of an asynchronous runtime:
It doesn't launch tasks or create timers except when necessary.

To the extent possible, this crate avoids doing public-key
cryptography in the same functions it uses for network activity.
This makes it easier for higher-level code to parallelize or yield
around public-key operations.

Also, this crate tries to avoid knowing or encoding information about what
its objects (channels, circuits, streams) are "used for".  That is, whenever
possible, we encode _how an object should behave_, not _the reason that it
should behave that way_.  For example, the `Circuit` object in this crate
remembers the path through which the circuit was built, but _not_ the
purpose that the circuit serves, or what it may be used for.  It's the
responsibility of other crates to enforce that kind of rule.

Why separate behavior from purpose in this way?
We do so in order to prevent a kind of logical overloading that we ran into
with the C tor implementation, where usage information is _not_ separate
from behavioral settings.  Since usage information is available, at all
points in the codebase the C tor code has converged in many places on
complex logic involving that usage information in order to set individual
behaviors.  Because of that, adding a new kinds usage or behavior in C tor
has become quite complex.  We're trying to avoid that kind of complexity in
Arti.

## Limitations

This is all a work in progress, and will need severe refactoring
before it's done.

This is a client-only implementation; there is no support the
operations that Relays need.

There are too many missing features to list.

There isn't enough documentation or examples.

This crate was my first attempt to use async in rust, and is probably
pretty kludgy.

I bet that there are deadlocks somewhere in this code.  I fixed
all the ones I could find or think of, but it would be great to
find a good way to eliminate every lock that we have.

License: MIT OR Apache-2.0
