# RustCrypto: ChaCha20Poly1305

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Pure Rust implementation of **ChaCha20Poly1305** ([RFC 8439][1]): an
[Authenticated Encryption with Associated Data (AEAD)][2] cipher amenable to
fast, constant-time implementations in software, based on the [ChaCha20][3]
stream cipher and [Poly1305][4] universal hash function.

This crate also contains an implementation of **XChaCha20Poly1305**: a variant
of ChaCha20Poly1305 with an extended 192-bit (24-byte) nonce.

[Documentation][docs-link]

## About

ChaCha20Poly1305 is notable for being simple and fast when implemented in pure
software. The underlying ChaCha20 stream cipher uses a simple combination of
add, rotate, and XOR instructions (a.k.a. "ARX"), and the Poly1305 hash
function is likewise extremely simple.

While it hasn't received approval from certain standards bodies (i.e. NIST)
the algorithm is widely used and deployed. Notably it's mandatory to implement
in the Transport Layer Security (TLS) protocol. The underlying ChaCha20 cipher
is also widely used as a cryptographically secure random number generator,
including internal use by the Rust standard library.

## Security Notes

This crate has received one [security audit by NCC Group][5], with no significant
findings. We would like to thank [MobileCoin][6] for funding the audit.

All implementations contained in the crate are designed to execute in constant
time, either by relying on hardware intrinsics (i.e. AVX2 on x86/x86_64), or
using a portable implementation which is only constant time on processors which
implement constant-time multiplication.

It is not suitable for use on processors with a variable-time multiplication
operation (e.g. short circuit on multiply-by-zero / multiply-by-one, such as
certain 32-bit PowerPC CPUs and some non-ARM microcontrollers).

## License

Licensed under either of:

 * [Apache License, Version 2.0](http://www.apache.org/licenses/LICENSE-2.0)
 * [MIT license](http://opensource.org/licenses/MIT)

at your option.

### Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
dual licensed as above, without any additional terms or conditions.

[//]: # (badges)

[crate-image]: https://img.shields.io/crates/v/chacha20poly1305.svg
[crate-link]: https://crates.io/crates/chacha20poly1305
[docs-image]: https://docs.rs/chacha20poly1305/badge.svg
[docs-link]: https://docs.rs/chacha20poly1305/
[license-image]: https://img.shields.io/badge/license-Apache2.0/MIT-blue.svg
[rustc-image]: https://img.shields.io/badge/rustc-1.51+-blue.svg
[chat-image]: https://img.shields.io/badge/zulip-join_chat-blue.svg
[chat-link]: https://rustcrypto.zulipchat.com/#narrow/stream/260038-AEADs
[downloads-image]: https://img.shields.io/crates/d/chacha20poly1305.svg
[build-image]: https://github.com/RustCrypto/AEADs/workflows/chacha20poly1305/badge.svg?branch=master&event=push
[build-link]: https://github.com/RustCrypto/AEADs/actions

[//]: # (general links)

[1]: https://tools.ietf.org/html/rfc8439
[2]: https://en.wikipedia.org/wiki/Authenticated_encryption
[3]: https://github.com/RustCrypto/stream-ciphers/tree/master/chacha20
[4]: https://github.com/RustCrypto/universal-hashes/tree/master/poly1305
[5]: https://research.nccgroup.com/2020/02/26/public-report-rustcrypto-aes-gcm-and-chacha20poly1305-implementation-review/
[6]: https://www.mobilecoin.com/
