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package sasquatch
import (
"crypto/ed25519"
"crypto/rand"
"crypto/sha256"
"crypto/sha512"
"errors"
"fmt"
"io"
"math/big"
"golang.org/x/crypto/chacha20poly1305"
"golang.org/x/crypto/curve25519"
"golang.org/x/crypto/hkdf"
"golang.org/x/crypto/ssh"
)
const ed25519Label = "charm.sh/v1/ssh-ed25519"
type Ed25519Recipient struct {
sshKey ssh.PublicKey
theirPublicKey []byte
}
var _ Recipient = &Ed25519Recipient{}
func (*Ed25519Recipient) Type() string { return "ssh-ed25519" }
func NewEd25519Recipient(pk ssh.PublicKey) (*Ed25519Recipient, error) {
if pk.Type() != "ssh-ed25519" {
return nil, errors.New("SSH public key is not an Ed25519 key")
}
r := &Ed25519Recipient{
sshKey: pk,
}
if pk, ok := pk.(ssh.CryptoPublicKey); ok {
if pk, ok := pk.CryptoPublicKey().(ed25519.PublicKey); ok {
r.theirPublicKey = ed25519PublicKeyToCurve25519(pk)
} else {
return nil, errors.New("unexpected public key type")
}
} else {
return nil, errors.New("pk does not implement ssh.CryptoPublicKey")
}
return r, nil
}
var curve25519P, _ = new(big.Int).SetString("57896044618658097711785492504343953926634992332820282019728792003956564819949", 10)
func ed25519PublicKeyToCurve25519(pk ed25519.PublicKey) []byte {
// ed25519.PublicKey is a little endian representation of the y-coordinate,
// with the most significant bit set based on the sign of the x-coordinate.
bigEndianY := make([]byte, ed25519.PublicKeySize)
for i, b := range pk {
bigEndianY[ed25519.PublicKeySize-i-1] = b
}
bigEndianY[0] &= 0b0111_1111
// The Montgomery u-coordinate is derived through the bilinear map
//
// u = (1 + y) / (1 - y)
//
// See https://blog.filippo.io/using-ed25519-keys-for-encryption.
y := new(big.Int).SetBytes(bigEndianY)
denom := big.NewInt(1)
denom.ModInverse(denom.Sub(denom, y), curve25519P) // 1 / (1 - y)
u := y.Mul(y.Add(y, big.NewInt(1)), denom)
u.Mod(u, curve25519P)
out := make([]byte, curve25519.PointSize)
uBytes := u.Bytes()
for i, b := range uBytes {
out[len(uBytes)-i-1] = b
}
return out
}
func (r *Ed25519Recipient) Wrap(fileKey []byte) (*Stanza, error) {
ephemeral := make([]byte, curve25519.ScalarSize)
if _, err := rand.Read(ephemeral); err != nil {
return nil, err
}
ourPublicKey, err := curve25519.X25519(ephemeral, curve25519.Basepoint)
if err != nil {
return nil, err
}
sharedSecret, err := curve25519.X25519(ephemeral, r.theirPublicKey)
if err != nil {
return nil, err
}
tweak := make([]byte, curve25519.ScalarSize)
tH := hkdf.New(sha256.New, nil, r.sshKey.Marshal(), []byte(ed25519Label))
if _, err := io.ReadFull(tH, tweak); err != nil {
return nil, err
}
sharedSecret, _ = curve25519.X25519(tweak, sharedSecret)
l := &Stanza{
Type: "ssh-ed25519",
Args: []string{sshFingerprint(r.sshKey),
EncodeToString(ourPublicKey[:])},
}
salt := make([]byte, 0, len(ourPublicKey)+len(r.theirPublicKey))
salt = append(salt, ourPublicKey...)
salt = append(salt, r.theirPublicKey...)
h := hkdf.New(sha256.New, sharedSecret, salt, []byte(ed25519Label))
wrappingKey := make([]byte, chacha20poly1305.KeySize)
if _, err := io.ReadFull(h, wrappingKey); err != nil {
return nil, err
}
wrappedKey, err := aeadEncrypt(wrappingKey, fileKey)
if err != nil {
return nil, err
}
l.Body = wrappedKey
return l, nil
}
type Ed25519Identity struct {
secretKey, ourPublicKey []byte
sshKey ssh.PublicKey
}
var _ Identity = &Ed25519Identity{}
func (*Ed25519Identity) Type() string { return "ssh-ed25519" }
func NewEd25519Identity(key ed25519.PrivateKey) (*Ed25519Identity, error) {
s, err := ssh.NewSignerFromKey(key)
if err != nil {
return nil, err
}
i := &Ed25519Identity{
sshKey: s.PublicKey(),
secretKey: ed25519PrivateKeyToCurve25519(key),
}
i.ourPublicKey, _ = curve25519.X25519(i.secretKey, curve25519.Basepoint)
return i, nil
}
func ed25519PrivateKeyToCurve25519(pk ed25519.PrivateKey) []byte {
h := sha512.New()
_, _ = h.Write(pk.Seed())
out := h.Sum(nil)
return out[:curve25519.ScalarSize]
}
func (i *Ed25519Identity) Unwrap(block *Stanza) ([]byte, error) {
if block.Type != "ssh-ed25519" {
return nil, ErrIncorrectIdentity
}
if len(block.Args) != 2 {
return nil, errors.New("invalid ssh-ed25519 recipient block")
}
publicKey, err := DecodeString(block.Args[1])
if err != nil {
return nil, fmt.Errorf("failed to parse ssh-ed25519 recipient: %v", err)
}
if len(publicKey) != curve25519.PointSize {
return nil, errors.New("invalid ssh-ed25519 recipient block")
}
if block.Args[0] != sshFingerprint(i.sshKey) {
return nil, ErrIncorrectIdentity
}
sharedSecret, err := curve25519.X25519(i.secretKey, publicKey)
if err != nil {
return nil, fmt.Errorf("invalid X25519 recipient: %v", err)
}
tweak := make([]byte, curve25519.ScalarSize)
tH := hkdf.New(sha256.New, nil, i.sshKey.Marshal(), []byte(ed25519Label))
if _, err := io.ReadFull(tH, tweak); err != nil {
return nil, err
}
sharedSecret, _ = curve25519.X25519(tweak, sharedSecret)
salt := make([]byte, 0, len(publicKey)+len(i.ourPublicKey))
salt = append(salt, publicKey...)
salt = append(salt, i.ourPublicKey...)
h := hkdf.New(sha256.New, sharedSecret, salt, []byte(ed25519Label))
wrappingKey := make([]byte, chacha20poly1305.KeySize)
if _, err := io.ReadFull(h, wrappingKey); err != nil {
return nil, err
}
fileKey, err := aeadDecrypt(wrappingKey, block.Body)
if err != nil {
return nil, fmt.Errorf("failed to decrypt file key: %v", err)
}
return fileKey, nil
}
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