# secp256k1

> Due to the terrible state of the btcec library, and the ethical character of
> dcred, the two main secp256k1 EC libraries in the Go language, it has become
> necessary to refactor and clean up the mess of btcec's module versioning.
>
> In addition, the code has been updated to use several new features of Go that
> were added to the language since these libraries were first created, notably the
> precomps are here directly generated as binary data instead of nasty base64
> source code.

Package secp256k1 implements optimized secp256k1 elliptic curve operations.

This package provides an optimized pure Go implementation of elliptic curve
cryptography operations over the secp256k1 curve as well as data structures and
functions for working with public and secret secp256k1 keys. See
https://www.secg.org/sec2-v2.pdf for details on the standard.

In addition, sub packages are provided to produce, verify, parse, and serialize
ECDSA signatures and EC-Schnorr-DCRv0 (a custom Schnorr-based signature scheme
specific to Decred) signatures. See the README.md files in the relevant sub
packages for more details about those aspects.

An overview of the features provided by this package are as follows:

- Secret key generation, serialization, and parsing
- Public key generation, serialization and parsing per ANSI X9.62-1998
  - Parses uncompressed, compressed, and hybrid public keys
  - Serializes uncompressed and compressed public keys
- Specialized types for performing optimized and constant time field operations
  - `FieldVal` type for working modulo the secp256k1 field prime
  - `ModNScalar` type for working modulo the secp256k1 group order
- Elliptic curve operations in Jacobian projective coordinates
  - Point addition
  - Point doubling
  - Scalar multiplication with an arbitrary point
  - Scalar multiplication with the base point (group generator)
- Point decompression from a given x coordinate
- Nonce generation via RFC6979 with support for extra data and version
  information that can be used to prevent nonce reuse between signing algorithms

It also provides an implementation of the Go standard library `crypto/elliptic`
`Curve` interface via the `S256` function so that it may be used with other
packages in the standard library such as `crypto/tls`, `crypto/x509`, and
`crypto/ecdsa`. However, in the case of ECDSA, it is highly recommended to use
the `ecdsa` sub package of this package instead since it is optimized
specifically for secp256k1 and is significantly faster as a result.

Although this package was primarily written for dcrd, it has intentionally been
designed so it can be used as a standalone package for any projects needing to
use optimized secp256k1 elliptic curve cryptography.

Finally, a comprehensive suite of tests is provided to provide a high level of
quality assurance.
