# tor-hscrypto
`tor-hscrypto`: Basic cryptography used by onion services
## Overview
This crate is part of
[Arti](https://gitlab.torproject.org/tpo/core/arti/), a project to
implement [Tor](https://www.torproject.org/) in Rust.
Onion services and the clients that connect to them need a few cryptographic
operations not used by the rest of Tor. These include:
* A set of key-blinding operations to derive short-term public keys
from long-term public keys.
* An ad-hoc SHA3-based message authentication code.
* Operations to encode and decode public keys as `.onion` addresses.
* A set of operations to divide time into different "periods". These periods
are used as inputs to the DHT-style hash ring, and to the key-blinding
operations.
* Proof of work schemes for resisting denial of service attacks
This crate implements those operations, along with a set of wrapper types to
keep us from getting confused about the numerous keys and nonces used for the
onion services.
## Compile-time features
* `memquota-memcost` -- implement `tor_memquota::HasMemoryCost` for many types.
(Does not actually force compiling in memory quota tracking;
that's `memquota` in `tor-memquota` and higher-level crates.)
* `ope` -- support for Order Preserving Encryption
* `full` -- Enable all features above.
### Experimental and unstable features
Note that the APIs enabled by these features are NOT covered by
semantic versioning[^1] guarantees: we might break them or remove
them between patch versions.
* `hs-pow-full` -- Tor Hidden Services Proof of Work.
* `experimental`: Enable all the above experimental features.
features work reliably. To be explicit: if you want `cargo update`
to _only_ make safe changes, then you cannot enable these
features.
## License
MIT OR Apache-2.0
When the `hs-pow-full` feature is used, we link with LGPL licensed dependencies.