# RustCrypto: secp256k1 (K-256) elliptic curve
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[secp256k1] (a.k.a. K-256) elliptic curve library written in pure Rust with
support for [ECDSA] signing/verification (including Ethereum-style signatures
with public-key recovery), Elliptic Curve Diffie-Hellman (ECDH), and general
purpose arithmetic which can be used to implement arbitrary protocols.
Uses traits and base types from the [`elliptic-curve`] crate.
Optionally includes a secp256k1 [`arithmetic`] feature providing scalar and
point types (projective/affine) with support for constant-time scalar
multiplication. Additionally, implements traits from the [`group`] crate
which can be used to generically construct group-based protocols.
[Documentation][docs-link]
## ⚠️ Security Warning
The secp256k1 elliptic curve arithmetic contained in this crate has never been
independently audited!
This crate has been designed with the goal of ensuring that secret-dependent
secp256k1 operations are performed in constant time (using the `subtle` crate
and constant-time formulas). However, it has not been thoroughly assessed to
ensure that generated assembly is constant time on common CPU architectures.
USE AT YOUR OWN RISK!
## Supported Algorithms
- [Elliptic Curve Diffie-Hellman (ECDH)][ECDH]: gated under the `ecdh` feature.
Note that this is technically ephemeral secp256k1 Diffie-Hellman
(a.k.a. ECDHE)
- [Elliptic Curve Digital Signature Algorithm (ECDSA)][ECDSA]: gated under the
`ecdsa` feature. Support for ECDSA/secp256k1 signing and verification,
applying [low-S normalization (BIP 0062)][BIP0062] as used in
consensus-critical applications, and additionally supports secp256k1
public-key recovery from ECDSA signatures (as used by e.g. Ethereum)
## About secp256k1 (K-256)
[secp256k1] is a Koblitz curve commonly used in cryptocurrency applications.
The "K-256" name follows NIST notation where P = prime fields,
B = binary fields, and K = Koblitz curves.
The curve is specified as `secp256k1` by Certicom's SECG in
"SEC 2: Recommended Elliptic Curve Domain Parameters":
<https://www.secg.org/sec2-v2.pdf>
secp256k1 is primarily notable for usage in Bitcoin and other cryptocurrencies,
particularly in conjunction with the
[Elliptic Curve Digital Signature Algorithm (ECDSA)][ECDSA].
Owing to its wide deployment in these applications, secp256k1 is one of the
most popular and commonly used elliptic curves.
## Minimum Supported Rust Version
Rust **1.51** or higher.
Minimum supported Rust version can be changed in the future, but it will be
done with a minor version bump.
## SemVer Policy
- All on-by-default features of this library are covered by SemVer
- MSRV is considered exempt from SemVer as noted above
## License
All crates 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/k256.svg
[crate-link]: https://crates.io/crates/k256
[docs-image]: https://docs.rs/k256/badge.svg
[docs-link]: https://docs.rs/k256/
[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/260040-elliptic-curves
[build-image]: https://github.com/RustCrypto/elliptic-curves/workflows/k256/badge.svg?branch=master&event=push
[build-link]: https://github.com/RustCrypto/elliptic-curves/actions?query=workflow%3Ak256
[//]: # (general links)
[secp256k1]: https://en.bitcoin.it/wiki/Secp256k1
[`elliptic-curve`]: https://github.com/RustCrypto/traits/tree/master/elliptic-curve
[`arithmetic`]: https://docs.rs/k256/latest/k256/arithmetic/index.html
[`group`]: https://github.com/zkcrypto/group
[ECDH]: https://en.wikipedia.org/wiki/Elliptic-curve_Diffie-Hellman
[ECDSA]: https://en.wikipedia.org/wiki/Elliptic_Curve_Digital_Signature_Algorithm
[BIP0062]: https://github.com/bitcoin/bips/blob/master/bip-0062.mediawiki