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package x25519
import (
"bytes"
"crypto"
cryptorand "crypto/rand"
"fmt"
"io"
"golang.org/x/crypto/curve25519"
)
// This mirrors ed25519's structure for private/public "keys". jwx
// requires dedicated types for these as they drive
// serialization/deserialization logic, as well as encryption types.
//
// Note that with the x25519 scheme, the private key is a sequence of
// 32 bytes, while the public key is the result of X25519(private,
// basepoint).
//
// Portions of this file are from Go's ed25519.go, which is
// Copyright 2016 The Go Authors. All rights reserved.
const (
// PublicKeySize is the size, in bytes, of public keys as used in this package.
PublicKeySize = 32
// PrivateKeySize is the size, in bytes, of private keys as used in this package.
PrivateKeySize = 64
// SeedSize is the size, in bytes, of private key seeds. These are the private key representations used by RFC 8032.
SeedSize = 32
)
// PublicKey is the type of X25519 public keys
type PublicKey []byte
// Any methods implemented on PublicKey might need to also be implemented on
// PrivateKey, as the latter embeds the former and will expose its methods.
// Equal reports whether pub and x have the same value.
func (pub PublicKey) Equal(x crypto.PublicKey) bool {
xx, ok := x.(PublicKey)
if !ok {
return false
}
return bytes.Equal(pub, xx)
}
// PrivateKey is the type of X25519 private key
type PrivateKey []byte
// Public returns the PublicKey corresponding to priv.
func (priv PrivateKey) Public() crypto.PublicKey {
publicKey := make([]byte, PublicKeySize)
copy(publicKey, priv[SeedSize:])
return PublicKey(publicKey)
}
// Equal reports whether priv and x have the same value.
func (priv PrivateKey) Equal(x crypto.PrivateKey) bool {
xx, ok := x.(PrivateKey)
if !ok {
return false
}
return bytes.Equal(priv, xx)
}
// Seed returns the private key seed corresponding to priv. It is provided for
// interoperability with RFC 7748. RFC 7748's private keys correspond to seeds
// in this package.
func (priv PrivateKey) Seed() []byte {
seed := make([]byte, SeedSize)
copy(seed, priv[:SeedSize])
return seed
}
// NewKeyFromSeed calculates a private key from a seed. It will return
// an error if len(seed) is not SeedSize. This function is provided
// for interoperability with RFC 7748. RFC 7748's private keys
// correspond to seeds in this package.
func NewKeyFromSeed(seed []byte) (PrivateKey, error) {
privateKey := make([]byte, PrivateKeySize)
if len(seed) != SeedSize {
return nil, fmt.Errorf("unexpected seed size: %d", len(seed))
}
copy(privateKey, seed)
public, err := curve25519.X25519(seed, curve25519.Basepoint)
if err != nil {
return nil, fmt.Errorf(`failed to compute public key: %w`, err)
}
copy(privateKey[SeedSize:], public)
return privateKey, nil
}
// GenerateKey generates a public/private key pair using entropy from rand.
// If rand is nil, crypto/rand.Reader will be used.
func GenerateKey(rand io.Reader) (PublicKey, PrivateKey, error) {
if rand == nil {
rand = cryptorand.Reader
}
seed := make([]byte, SeedSize)
if _, err := io.ReadFull(rand, seed); err != nil {
return nil, nil, err
}
privateKey, err := NewKeyFromSeed(seed)
if err != nil {
return nil, nil, err
}
publicKey := make([]byte, PublicKeySize)
copy(publicKey, privateKey[SeedSize:])
return publicKey, privateKey, nil
}
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