# subtle [](https://crates.io/crates/subtle) [](https://doc.dalek.rs/subtle) [](https://travis-ci.org/dalek-cryptography/subtle)
**Pure-Rust traits and utilities for constant-time cryptographic implementations.**
This crate represents a “best-effort” attempt, since side-channels
are ultimately a property of a deployed cryptographic system
including the hardware it runs on, not just of software.
It consists of a `Choice` type, and a collection of traits using `Choice`
instead of `bool` which are intended to execute in constant-time. The `Choice`
type is a wrapper around a `u8` that holds a `0` or `1`.
The traits are implemented using bitwise operations, and should execute in
constant time provided that a) the bitwise operations are constant-time and b)
the operations are not optimized into a branch.
To prevent this, when using the `nightly` feature (recommended), the crate
attempts to hide the value of a `Choice`'s inner `u8` from the optimizer, by
passing it through an inline assembly block. For more information, see the
_About_ section below.
When not using the `nightly` feature, there is no protection against b). This
is unfortunate, but is no worse than C code.
```toml
[dependencies.subtle]
version = "^0.6"
features = ["nightly"]
```
## Features
* The `nightly` feature enables the use of
an optimization barrier to protect the `Choice` type.
_Using the `nightly` feature is recommended for security_.
* The `generic-impls` feature (enabled by default) provides generic
impls of some traits. It can be disabled to allow specialized impls
without violating the orphan rules.
## Documentation
Documentation is available [here](docs).
## About
This library aims to be the Rust equivalent of Go’s `crypto/subtle` module.
The optimization barrier in `impl From<u8> for Choice` was based on Tim
Maclean's [work on `rust-timing-shield`][rust-timing-shield], which attempts to
provide a more comprehensive approach for preventing software side-channels in
Rust code.
## Warning
This code is a low-level library, intended for specific use-cases implementing
cryptographic protocols. It represents a best-effort attempt to protect
against some software side-channels.
**USE AT YOUR OWN RISK**
[docs]: https://doc.dalek.rs/subtle
[rust-timing-shield]: https://www.chosenplaintext.ca/open-source/rust-timing-shield/security