// Copyright 2022 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package ecdh implements Elliptic Curve Diffie-Hellman over
// NIST curves and Curve25519.
package ecdh

import (
	"crypto"
	"crypto/subtle"
	"io"
	"sync"
)

type Curve interface {
	// ECDH performs a ECDH exchange and returns the shared secret.
	//
	// For NIST curves, this performs ECDH as specified in SEC 1, Version 2.0,
	// Section 3.3.1, and returns the x-coordinate encoded according to SEC 1,
	// Version 2.0, Section 2.3.5. In particular, if the result is the point at
	// infinity, ECDH returns an error. (Note that for NIST curves, that's only
	// possible if the private key is the all-zero value.)
	//
	// For X25519, this performs ECDH as specified in RFC 7748, Section 6.1. If
	// the result is the all-zero value, ECDH returns an error.
	ECDH(local *PrivateKey, remote *PublicKey) ([]byte, error)

	// GenerateKey generates a new PrivateKey from rand.
	GenerateKey(rand io.Reader) (*PrivateKey, error)

	// NewPrivateKey checks that key is valid and returns a PrivateKey.
	//
	// For NIST curves, this follows SEC 1, Version 2.0, Section 2.3.6, which
	// amounts to decoding the bytes as a fixed length big endian integer and
	// checking that the result is lower than the order of the curve. The zero
	// private key is also rejected, as the encoding of the corresponding public
	// key would be irregular.
	//
	// For X25519, this only checks the scalar length. Adversarially selected
	// private keys can cause ECDH to return an error.
	NewPrivateKey(key []byte) (*PrivateKey, error)

	// NewPublicKey checks that key is valid and returns a PublicKey.
	//
	// For NIST curves, this decodes an uncompressed point according to SEC 1,
	// Version 2.0, Section 2.3.4. Compressed encodings and the point at
	// infinity are rejected.
	//
	// For X25519, this only checks the u-coordinate length. Adversarially
	// selected public keys can cause ECDH to return an error.
	NewPublicKey(key []byte) (*PublicKey, error)

	// privateKeyToPublicKey converts a PrivateKey to a PublicKey. It's exposed
	// as the PrivateKey.PublicKey method.
	//
	// This method always succeeds: for X25519, it might output the all-zeroes
	// value (unlike the ECDH method); for NIST curves, it would only fail for
	// the zero private key, which is rejected by NewPrivateKey.
	//
	// The private method also allow us to expand the ECDH interface with more
	// methods in the future without breaking backwards compatibility.
	privateKeyToPublicKey(*PrivateKey) *PublicKey
}

// PublicKey is an ECDH public key, usually a peer's ECDH share sent over the wire.
type PublicKey struct {
	curve     Curve
	publicKey []byte
}

// Bytes returns a copy of the encoding of the public key.
func (k *PublicKey) Bytes() []byte {
	// Copy the public key to a fixed size buffer that can get allocated on the
	// caller's stack after inlining.
	var buf [133]byte
	return append(buf[:0], k.publicKey...)
}

// Equal returns whether x represents the same public key as k.
//
// Note that there can be equivalent public keys with different encodings which
// would return false from this check but behave the same way as inputs to ECDH.
//
// This check is performed in constant time as long as the key types and their
// curve match.
func (k *PublicKey) Equal(x crypto.PublicKey) bool {
	xx, ok := x.(*PublicKey)
	if !ok {
		return false
	}
	return k.curve == xx.curve &&
		subtle.ConstantTimeCompare(k.publicKey, xx.publicKey) == 1
}

func (k *PublicKey) Curve() Curve {
	return k.curve
}

// PrivateKey is an ECDH private key, usually kept secret.
type PrivateKey struct {
	curve      Curve
	privateKey []byte
	// publicKey is set under publicKeyOnce, to allow loading private keys with
	// NewPrivateKey without having to perform a scalar multiplication.
	publicKey     *PublicKey
	publicKeyOnce sync.Once
}

// Bytes returns a copy of the encoding of the private key.
func (k *PrivateKey) Bytes() []byte {
	// Copy the private key to a fixed size buffer that can get allocated on the
	// caller's stack after inlining.
	var buf [66]byte
	return append(buf[:0], k.privateKey...)
}

// Equal returns whether x represents the same private key as k.
//
// Note that there can be equivalent private keys with different encodings which
// would return false from this check but behave the same way as inputs to ECDH.
//
// This check is performed in constant time as long as the key types and their
// curve match.
func (k *PrivateKey) Equal(x crypto.PrivateKey) bool {
	xx, ok := x.(*PrivateKey)
	if !ok {
		return false
	}
	return k.curve == xx.curve &&
		subtle.ConstantTimeCompare(k.privateKey, xx.privateKey) == 1
}

func (k *PrivateKey) Curve() Curve {
	return k.curve
}

func (k *PrivateKey) PublicKey() *PublicKey {
	k.publicKeyOnce.Do(func() {
		k.publicKey = k.curve.privateKeyToPublicKey(k)
	})
	return k.publicKey
}

// Public implements the implicit interface of all standard library private
// keys. See the docs of crypto.PrivateKey.
func (k *PrivateKey) Public() crypto.PublicKey {
	return k.PublicKey()
}