tor-cell

Coding and decoding for the cell types that make up Tor's protocol

Overview

Tor's primary network protocol is oriented around a set of messages called "Cells". They exist at two primary layers of the protocol: the channel-cell layer, and the relay-cell layer.

Channel cells are sent between relays, or between a client and a relay, over a TLS connection. Each of them encodes a single Channel Message. Channel messages can affect the channel itself (such as those used to negotiate and authenticate the channel), but more frequently are used with respect to a given multi-hop circuit.

Channel message that refer to a circuit do so with a channel-local identifier called a Circuit ID. These messages include CREATE2 (used to extend a circuit to a first hop) and DESTROY (used to tear down a circuit). But the most frequently used channel message is RELAY, which is used to send a message to a given hop along a circuit.

Each RELAY cell is encrypted and decrypted (according to protocols not implemented in this crate) until it reaches its target. When it does, it is decoded into a single Relay Message. Some of these relay messages are used to manipulate circuits (e.g., by extending the circuit to a new hop); others are used to manipulate anonymous data-streams (by creating them, ending them, or sending data); and still others are used for protocol-specific purposes (like negotiating with an onion service.)

For a list of most of the cell types used in Tor, see tor-spec.txt. Other cell types are defined in rend-spec-v3.txt (for onion services) and padding-spec.txt (for padding negotiation).

This crate is part of Arti, a project to implement Tor in Rust.

Compile-time features

• hs -- Types relating to Tor Hidden Services (.onion services).

• hs-pow-v1 -- Types relating to Tor Hidden Services Proof of Work (v1 scheme only)

• hs-pow-full -- Types relating to Tor Hidden Services Proof of Work (metafeature, enables all non-experimental schemes: currently, implies only hs-pow-v1).

• full -- Enable all features above.

Experimental and unstable features

Note that the APIs enabled by these features are NOT covered by semantic versioning¹ guarantees: we might break them or remove them between patch versions.

• experimental-udp: Types for conveying UDP over Tor.

• testing: Additional APIs for testing, used in our whole-workspace tests.

• experimental: Enable all the above experimental features.

License: MIT OR Apache-2.0