# [RustCrypto]: NIST P-256 (secp256r1) elliptic curve

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Pure Rust implementation of the NIST P-256 (a.k.a. secp256r1, prime256v1)
elliptic curve with support for ECDH, ECDSA signing/verification, and general
purpose curve arithmetic support implemented in terms of traits from the
[`elliptic-curve`] crate.

[Documentation][docs-link]

## ⚠️ Security Warning

The 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
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.
- [Elliptic Curve Digital Signature Algorithm (ECDSA)][ECDSA]: gated under the
  `ecdsa` feature.

## PKCS#8 Key Encoding

PKCS#8 is a private key format with support for multiple algorithms. It can be
encoded as binary DER or text PEM.

You can recognize PEM encoded PKCS#8 private keys because they do not have an
algorithm name in the type label, e.g.:

```text
-----BEGIN PRIVATE KEY-----
```

PKCS#8 support is gated under the `pkcs8` feature. The `pem` feature, which is
enabled by default, adds PEM decoding and also enables `pkcs8`.

The same pattern is used by the other curve crates in this repository which
re-export `pkcs8`.

The following traits can be used to decode/encode secret and public keys as
PKCS#8/SPKI. Note that [`pkcs8`] is re-exported from `p256` when the `pkcs8`
feature is enabled:

- [`pkcs8::DecodePrivateKey`]: decode private keys from PKCS#8
- [`pkcs8::EncodePrivateKey`]: encode private keys to PKCS#8
- [`pkcs8::DecodePublicKey`]: decode public keys from SPKI
- [`pkcs8::EncodePublicKey`]: encode public keys to SPKI

For private keys, [`SecretKey::from_der`] and [`SecretKey::from_pem`] provide
convenience methods which can decode PKCS#8 keys. Use the trait methods above
when the input is expected to be specifically PKCS#8.

### Example

```rust
# fn main() -> Result<(), Box<dyn std::error::Error>> {
# #[cfg(feature = "pem")]
# {
use p256::SecretKey;

// WARNING: Do not hardcode private keys in your source code. This is for demonstration purposes only.
let pem = r#"-----BEGIN PRIVATE KEY-----
MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQgaWJBcVYaYzQN4OfY
afKgVJJVjhoEhotqn4VKhmeIGI2hRANCAAQcrP+1Xy8s79idies3SyaBFSRSgC3u
oJkWBoE32DnPf8SBpESSME1+9mrBF77+g6jQjxVfK1L59hjdRHApBI4P
-----END PRIVATE KEY-----"#;
let secret_key = SecretKey::from_pem(pem)?;
# let _ = secret_key;
# }
# Ok(())
# }
```

## About NIST P-256

NIST P-256 is a Weierstrass curve specified in [SP 800-186]:
Recommendations for Discrete Logarithm-based Cryptography:
Elliptic Curve Domain Parameters.

Also known as prime256v1 (ANSI X9.62) and secp256r1 (SECG), it's included in
the US National Security Agency's "Suite B" and is widely used in protocols
like TLS and the associated X.509 PKI.

## 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/p256?logo=rust
[crate-link]: https://crates.io/crates/p256
[docs-image]: https://docs.rs/p256/badge.svg
[docs-link]: https://docs.rs/p256/
[build-image]: https://github.com/RustCrypto/elliptic-curves/actions/workflows/p256.yml/badge.svg
[build-link]: https://github.com/RustCrypto/elliptic-curves/actions/workflows/p256.yml
[license-image]: https://img.shields.io/badge/license-Apache2.0/MIT-blue.svg
[rustc-image]: https://img.shields.io/badge/rustc-1.85+-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

[//]: # (general links)

[RustCrypto]: https://github.com/rustcrypto/
[`elliptic-curve`]: https://github.com/RustCrypto/traits/tree/master/elliptic-curve
[ECDH]: https://en.wikipedia.org/wiki/Elliptic-curve_Diffie-Hellman
[ECDSA]: https://en.wikipedia.org/wiki/Elliptic_Curve_Digital_Signature_Algorithm
[SP 800-186]: https://csrc.nist.gov/publications/detail/sp/800-186/final