device: uniformly check ECDH output for zeros

For some reason, this was omitted for response messages.

Reported-by: z <dzm@unexpl0.red>
Fixes: 8c34c4c ("First set of code review patches")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

device/device.go

@@ -265,7 +265,7 @@ func (device *Device) SetPrivateKey(sk NoisePrivateKey) error {
 	expiredPeers := make([]*Peer, 0, len(device.peers.keyMap))
 	for _, peer := range device.peers.keyMap {
 		handshake := &peer.handshake
-		handshake.precomputedStaticStatic = device.staticIdentity.privateKey.sharedSecret(handshake.remoteStatic)
+		handshake.precomputedStaticStatic, _ = device.staticIdentity.privateKey.sharedSecret(handshake.remoteStatic)
 		expiredPeers = append(expiredPeers, peer)
 	}
 

device/noise-helpers.go

@@ -9,6 +9,7 @@ import (
 	"crypto/hmac"
 	"crypto/rand"
 	"crypto/subtle"
+	"errors"
 	"hash"
 
 	"golang.org/x/crypto/blake2s"
@@ -94,9 +95,14 @@ func (sk *NoisePrivateKey) publicKey() (pk NoisePublicKey) {
 	return
 }
 
-func (sk *NoisePrivateKey) sharedSecret(pk NoisePublicKey) (ss [NoisePublicKeySize]byte) {
+var errInvalidPublicKey = errors.New("invalid public key")
+
+func (sk *NoisePrivateKey) sharedSecret(pk NoisePublicKey) (ss [NoisePublicKeySize]byte, err error) {
 	apk := (*[NoisePublicKeySize]byte)(&pk)
 	ask := (*[NoisePrivateKeySize]byte)(sk)
 	curve25519.ScalarMult(&ss, ask, apk)
-	return ss
+	if isZero(ss[:]) {
+		return ss, errInvalidPublicKey
+	}
+	return ss, nil
 }

device/noise-protocol.go

@@ -175,8 +175,6 @@ func init() {
 }
 
 func (device *Device) CreateMessageInitiation(peer *Peer) (*MessageInitiation, error) {
-	errZeroECDHResult := errors.New("ECDH returned all zeros")
-
 	device.staticIdentity.RLock()
 	defer device.staticIdentity.RUnlock()
 
@@ -204,9 +202,9 @@ func (device *Device) CreateMessageInitiation(peer *Peer) (*MessageInitiation, e
 	handshake.mixHash(msg.Ephemeral[:])
 
 	// encrypt static key
-	ss := handshake.localEphemeral.sharedSecret(handshake.remoteStatic)
-	if isZero(ss[:]) {
-		return nil, errZeroECDHResult
+	ss, err := handshake.localEphemeral.sharedSecret(handshake.remoteStatic)
+	if err != nil {
+		return nil, err
 	}
 	var key [chacha20poly1305.KeySize]byte
 	KDF2(
@@ -221,7 +219,7 @@ func (device *Device) CreateMessageInitiation(peer *Peer) (*MessageInitiation, e
 
 	// encrypt timestamp
 	if isZero(handshake.precomputedStaticStatic[:]) {
-		return nil, errZeroECDHResult
+		return nil, errInvalidPublicKey
 	}
 	KDF2(
 		&handshake.chainKey,
@@ -264,11 +262,10 @@ func (device *Device) ConsumeMessageInitiation(msg *MessageInitiation) *Peer {
 	mixKey(&chainKey, &InitialChainKey, msg.Ephemeral[:])
 
 	// decrypt static key
-	var err error
 	var peerPK NoisePublicKey
 	var key [chacha20poly1305.KeySize]byte
-	ss := device.staticIdentity.privateKey.sharedSecret(msg.Ephemeral)
-	if isZero(ss[:]) {
+	ss, err := device.staticIdentity.privateKey.sharedSecret(msg.Ephemeral)
+	if err != nil {
 		return nil
 	}
 	KDF2(&chainKey, &key, chainKey[:], ss[:])
@@ -384,12 +381,16 @@ func (device *Device) CreateMessageResponse(peer *Peer) (*MessageResponse, error
 	handshake.mixHash(msg.Ephemeral[:])
 	handshake.mixKey(msg.Ephemeral[:])
 
-	func() {
-		ss := handshake.localEphemeral.sharedSecret(handshake.remoteEphemeral)
-		handshake.mixKey(ss[:])
-		ss = handshake.localEphemeral.sharedSecret(handshake.remoteStatic)
-		handshake.mixKey(ss[:])
-	}()
+	ss, err := handshake.localEphemeral.sharedSecret(handshake.remoteEphemeral)
+	if err != nil {
+		return nil, err
+	}
+	handshake.mixKey(ss[:])
+	ss, err = handshake.localEphemeral.sharedSecret(handshake.remoteStatic)
+	if err != nil {
+		return nil, err
+	}
+	handshake.mixKey(ss[:])
 
 	// add preshared key
 
@@ -406,11 +407,9 @@ func (device *Device) CreateMessageResponse(peer *Peer) (*MessageResponse, error
 
 	handshake.mixHash(tau[:])
 
-	func() {
-		aead, _ := chacha20poly1305.New(key[:])
-		aead.Seal(msg.Empty[:0], ZeroNonce[:], nil, handshake.hash[:])
-		handshake.mixHash(msg.Empty[:])
-	}()
+	aead, _ := chacha20poly1305.New(key[:])
+	aead.Seal(msg.Empty[:0], ZeroNonce[:], nil, handshake.hash[:])
+	handshake.mixHash(msg.Empty[:])
 
 	handshake.state = handshakeResponseCreated
 
@@ -455,17 +454,19 @@ func (device *Device) ConsumeMessageResponse(msg *MessageResponse) *Peer {
 		mixHash(&hash, &handshake.hash, msg.Ephemeral[:])
 		mixKey(&chainKey, &handshake.chainKey, msg.Ephemeral[:])
 
-		func() {
-			ss := handshake.localEphemeral.sharedSecret(msg.Ephemeral)
-			mixKey(&chainKey, &chainKey, ss[:])
-			setZero(ss[:])
-		}()
+		ss, err := handshake.localEphemeral.sharedSecret(msg.Ephemeral)
+		if err != nil {
+			return false
+		}
+		mixKey(&chainKey, &chainKey, ss[:])
+		setZero(ss[:])
 
-		func() {
-			ss := device.staticIdentity.privateKey.sharedSecret(msg.Ephemeral)
-			mixKey(&chainKey, &chainKey, ss[:])
-			setZero(ss[:])
-		}()
+		ss, err = device.staticIdentity.privateKey.sharedSecret(msg.Ephemeral)
+		if err != nil {
+			return false
+		}
+		mixKey(&chainKey, &chainKey, ss[:])
+		setZero(ss[:])
 
 		// add preshared key (psk)
 
@@ -483,7 +484,7 @@ func (device *Device) ConsumeMessageResponse(msg *MessageResponse) *Peer {
 		// authenticate transcript
 
 		aead, _ := chacha20poly1305.New(key[:])
-		_, err := aead.Open(nil, ZeroNonce[:], msg.Empty[:], hash[:])
+		_, err = aead.Open(nil, ZeroNonce[:], msg.Empty[:], hash[:])
 		if err != nil {
 			return false
 		}

device/noise_test.go

@@ -24,10 +24,10 @@ func TestCurveWrappers(t *testing.T) {
 	pk1 := sk1.publicKey()
 	pk2 := sk2.publicKey()
 
-	ss1 := sk1.sharedSecret(pk2)
-	ss2 := sk2.sharedSecret(pk1)
+	ss1, err1 := sk1.sharedSecret(pk2)
+	ss2, err2 := sk2.sharedSecret(pk1)
 
-	if ss1 != ss2 {
+	if ss1 != ss2 || err1 != nil || err2 != nil {
 		t.Fatal("Failed to compute shared secet")
 	}
 }

device/peer.go

@@ -92,7 +92,7 @@ func (device *Device) NewPeer(pk NoisePublicKey) (*Peer, error) {
 	// pre-compute DH
 	handshake := &peer.handshake
 	handshake.mutex.Lock()
-	handshake.precomputedStaticStatic = device.staticIdentity.privateKey.sharedSecret(pk)
+	handshake.precomputedStaticStatic, _ = device.staticIdentity.privateKey.sharedSecret(pk)
 	handshake.remoteStatic = pk
 	handshake.mutex.Unlock()