ecdh.go

// 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/internal/boring"
	"crypto/subtle"
	"errors"
	"io"
	"sync"
)

type Curve interface {
	// 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.
	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)

	// ecdh performs a ECDH exchange and returns the shared secret. It's exposed
	// as the PrivateKey.ECDH method.
	//
	// The private method also allow us to expand the ECDH interface with more
	// methods in the future without breaking backwards compatibility.
	ecdh(local *PrivateKey, remote *PublicKey) ([]byte, error)

	// privateKeyToPublicKey converts a PrivateKey to a PublicKey. It's exposed
	// as the PrivateKey.PublicKey method.
	//
	// This method always succeeds: for X25519, the zero key can't be
	// constructed due to clamping; for NIST curves, it is rejected by
	// NewPrivateKey.
	privateKeyToPublicKey(*PrivateKey) *PublicKey
}

// PublicKey is an ECDH public key, usually a peer's ECDH share sent over the wire.
//
// These keys can be parsed with [crypto/x509.ParsePKIXPublicKey] and encoded
// with [crypto/x509.MarshalPKIXPublicKey]. For NIST curves, they then need to
// be converted with [crypto/ecdsa.PublicKey.ECDH] after parsing.
type PublicKey struct {
	curve     Curve
	publicKey []byte
	boring    *boring.PublicKeyECDH
}

// 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.
//
// These keys can be parsed with [crypto/x509.ParsePKCS8PrivateKey] and encoded
// with [crypto/x509.MarshalPKCS8PrivateKey]. For NIST curves, they then need to
// be converted with [crypto/ecdsa.PrivateKey.ECDH] after parsing.
type PrivateKey struct {
	curve      Curve
	privateKey []byte
	boring     *boring.PrivateKeyECDH
	// publicKey is set under publicKeyOnce, to allow loading private keys with
	// NewPrivateKey without having to perform a scalar multiplication.
	publicKey     *PublicKey
	publicKeyOnce sync.Once
}

// ECDH performs a ECDH exchange and returns the shared secret. The PrivateKey
// and PublicKey must use the same curve.
//
// 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. The result is never the point at infinity.
//
// 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.
func (k *PrivateKey) ECDH(remote *PublicKey) ([]byte, error) {
	if k.curve != remote.curve {
		return nil, errors.New("crypto/ecdh: private key and public key curves do not match")
	}
	return k.curve.ecdh(k, remote)
}

// 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() {
		if k.boring != nil {
			// Because we already checked in NewPrivateKey that the key is valid,
			// there should not be any possible errors from BoringCrypto,
			// so we turn the error into a panic.
			// (We can't return it anyhow.)
			kpub, err := k.boring.PublicKey()
			if err != nil {
				panic("boringcrypto: " + err.Error())
			}
			k.publicKey = &PublicKey{
				curve:     k.curve,
				publicKey: kpub.Bytes(),
				boring:    kpub,
			}
		} else {
			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()
}