diff --git a/internal/authentication/jwt_test.go b/internal/authentication/jwt_test.go
index 01d7ae6..8057b72 100644
--- a/internal/authentication/jwt_test.go
+++ b/internal/authentication/jwt_test.go
@@ -6,8 +6,6 @@ import (
 	"github.com/golang-jwt/jwt/v5"
 )
 
-const testVIN = "0123456789abcdefX"
-
 func TestVerify(t *testing.T) {
 	pkey := []byte{
 		0x04, 0x77, 0x5e, 0x2e, 0xf5, 0x70, 0xd2, 0x92, 0xdf, 0x42, 0x4c, 0x09,
diff --git a/internal/authentication/peer.go b/internal/authentication/peer.go
index 6b10539..0bd35a5 100644
--- a/internal/authentication/peer.go
+++ b/internal/authentication/peer.go
@@ -78,3 +78,40 @@ func (p *Peer) hmacTag(message *universal.RoutableMessage, hmacData *signatures.
 	}
 	return meta.Checksum(message.GetProtobufMessageAsBytes()), nil
 }
+
+func RequestID(message *universal.RoutableMessage) []byte {
+	sigData := message.GetSignatureData()
+	if sigData.GetSigType() == nil {
+		return nil
+	}
+
+	switch s := sigData.GetSigType().(type) {
+	case *signatures.SignatureData_AES_GCM_PersonalizedData:
+		return append(
+			[]byte{byte(signatures.SignatureType_SIGNATURE_TYPE_AES_GCM_PERSONALIZED)},
+			s.AES_GCM_PersonalizedData.GetTag()...)
+	case *signatures.SignatureData_HMAC_PersonalizedData:
+		tag := s.HMAC_PersonalizedData.GetTag()
+		if message.GetToDestination().GetDomain() == universal.Domain_DOMAIN_VEHICLE_SECURITY {
+			tag = tag[:16]
+		}
+		return append(
+			[]byte{byte(signatures.SignatureType_SIGNATURE_TYPE_HMAC_PERSONALIZED)}, tag...)
+	default:
+		return nil
+	}
+}
+
+func (p *Peer) responseMetadata(message *universal.RoutableMessage, id []byte, counter uint32) ([]byte, error) {
+	meta := newMetadata()
+	meta.Add(signatures.Tag_TAG_SIGNATURE_TYPE, []byte{byte(signatures.SignatureType_SIGNATURE_TYPE_AES_GCM_RESPONSE)})
+	meta.Add(signatures.Tag_TAG_DOMAIN, []byte{byte(message.GetFromDestination().GetDomain())})
+	if err := meta.Add(signatures.Tag_TAG_PERSONALIZATION, p.verifierName); err != nil {
+		return nil, err
+	}
+	meta.AddUint32(signatures.Tag_TAG_COUNTER, counter)
+	meta.AddUint32(signatures.Tag_TAG_FLAGS, message.Flags)
+	meta.Add(signatures.Tag_TAG_REQUEST_HASH, id)
+	meta.AddUint32(signatures.Tag_TAG_FAULT, uint32(message.GetSignedMessageStatus().GetSignedMessageFault()))
+	return meta.Checksum(nil), nil
+}
diff --git a/internal/authentication/peer_test.go b/internal/authentication/peer_test.go
index b29e956..1dd3e1e 100644
--- a/internal/authentication/peer_test.go
+++ b/internal/authentication/peer_test.go
@@ -1,8 +1,10 @@
 package authentication
 
 import (
+	"bytes"
 	"testing"
 
+	"github.com/teslamotors/vehicle-command/pkg/protocol/protobuf/signatures"
 	universal "github.com/teslamotors/vehicle-command/pkg/protocol/protobuf/universalmessage"
 )
 
@@ -84,3 +86,47 @@ func checkError(t *testing.T, err error, expectedCode universal.MessageFault_E)
 		t.Errorf("Got unexpected error type: %s", err)
 	}
 }
+
+func TestRequestID(t *testing.T) {
+	tag := []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18}
+	message := universal.RoutableMessage{
+		ToDestination: &universal.Destination{
+			SubDestination: &universal.Destination_Domain{
+				Domain: universal.Domain_DOMAIN_VEHICLE_SECURITY,
+			},
+		},
+		SubSigData: &universal.RoutableMessage_SignatureData{
+			SignatureData: &signatures.SignatureData{
+				SigType: &signatures.SignatureData_HMAC_PersonalizedData{
+					HMAC_PersonalizedData: &signatures.HMAC_Personalized_Signature_Data{
+						Tag: append([]byte{}, tag...),
+					},
+				},
+			},
+		},
+	}
+	id := RequestID(&message)
+	if len(id) != 17 {
+		t.Errorf("Expected 17-byte id, but got %d bytes", len(id))
+	}
+	if id[0] != byte(signatures.SignatureType_SIGNATURE_TYPE_HMAC_PERSONALIZED) {
+		t.Errorf("Invalid first byte of request ID: %02x", id[0])
+	}
+	if !bytes.Equal(id[1:], tag[:16]) {
+		t.Errorf("Expected: %02x", tag[:16])
+		t.Errorf("Observed: %02x", id[1:])
+	}
+
+	message.ToDestination.SubDestination = &universal.Destination_Domain{
+		Domain: universal.Domain_DOMAIN_INFOTAINMENT,
+	}
+
+	id = RequestID(&message)
+	if id[0] != byte(signatures.SignatureType_SIGNATURE_TYPE_HMAC_PERSONALIZED) {
+		t.Errorf("Invalid first byte of request ID: %02x", id[0])
+	}
+	if !bytes.Equal(id[1:], tag) {
+		t.Errorf("Expected: %02x", tag)
+		t.Errorf("Observed: %02x", id[1:])
+	}
+}
diff --git a/internal/authentication/signer.go b/internal/authentication/signer.go
index 8445b40..8c558ee 100644
--- a/internal/authentication/signer.go
+++ b/internal/authentication/signer.go
@@ -211,3 +211,32 @@ func (s *Signer) AuthorizeHMAC(message *universal.RoutableMessage, expiresIn tim
 	}
 	return nil
 }
+
+// Decrypt a Verifier message in place.
+//
+// Returns the anti-replay counter included in the message, which the client must verify increases
+// monotonically for a given id or is inside of a sliding window.
+func (s *Signer) Decrypt(message *universal.RoutableMessage, id []byte) (uint32, error) {
+	gcmInfo := message.GetSignatureData().GetAES_GCM_ResponseData()
+	if gcmInfo == nil {
+		return 0, newError(errCodeBadParameter, "missing AES-GCM data")
+	}
+	authenticatedData, err := s.responseMetadata(message, id, gcmInfo.Counter)
+	if err != nil {
+		return 0, nil
+	}
+	plaintext, err := s.session.Decrypt(
+		gcmInfo.Nonce,
+		message.GetProtobufMessageAsBytes(),
+		authenticatedData,
+		gcmInfo.Tag,
+	)
+	if err != nil {
+		return 0, err
+	}
+	message.Payload = &universal.RoutableMessage_ProtobufMessageAsBytes{
+		ProtobufMessageAsBytes: plaintext,
+	}
+	message.SubSigData = nil
+	return gcmInfo.Counter, nil
+}
diff --git a/internal/authentication/verifier.go b/internal/authentication/verifier.go
index 3a728d5..05e1d7d 100644
--- a/internal/authentication/verifier.go
+++ b/internal/authentication/verifier.go
@@ -19,6 +19,7 @@ type Verifier struct {
 	Peer
 	lock   sync.Mutex
 	window uint64
+	handle uint32
 }
 
 // NewVerifier returns a Verifier.
@@ -72,6 +73,12 @@ func (v *Verifier) rotateEpochIfNeeded(force bool) error {
 	return nil
 }
 
+func (v *Verifier) AssignHandle(handle uint32) {
+	v.lock.Lock()
+	v.handle = handle
+	v.lock.Unlock()
+}
+
 func (v *Verifier) sessionInfo() (*signatures.SessionInfo, error) {
 	if err := v.adjustClock(); err != nil {
 		return nil, err
@@ -81,6 +88,7 @@ func (v *Verifier) sessionInfo() (*signatures.SessionInfo, error) {
 		PublicKey: v.session.LocalPublicBytes(),
 		Epoch:     v.epoch[:],
 		ClockTime: v.timestamp(),
+		Handle:    v.handle,
 	}
 	return info, nil
 }
@@ -217,55 +225,6 @@ func (v *Verifier) Verify(message *universal.RoutableMessage) (plaintext []byte,
 	return
 }
 
-// updateSlidingWindow takes the current counter value (i.e., the highest
-// counter value of any authentic message received so far), the current sliding
-// window, and the newCounter value from an incoming message. The function
-// returns the updated counter and window values and sets ok to true if it
-// could confirm that newCounter has never been previously used. If ok is
-// false, then updatedCounter = counter and updatedWindow = window.
-func updateSlidingWindow(counter uint32, window uint64, newCounter uint32) (updatedCounter uint32, updatedWindow uint64, ok bool) {
-	// If we exit early due to an error, we want to leave the counter/window
-	// state unchanged. Therefore we initialize return values to the current
-	// state.
-	updatedCounter = counter
-	updatedWindow = window
-	ok = false
-
-	if counter == newCounter {
-		// This counter value has been used before.
-		return
-	}
-
-	if newCounter < counter {
-		// This message arrived out of order.
-		age := counter - newCounter
-		if age > windowSize {
-			// Our history doesn't go back this far, so we can't determine if
-			// we've seen this newCounter value before.
-			return
-		}
-		if window>>(age-1)&1 == 1 {
-			// The newCounter value has been used before.
-			return
-		}
-		// Everything looks good.
-		ok = true
-		updatedWindow |= (1 << (age - 1))
-		return
-	}
-
-	// If we've reached this point, newCounter > counter, so newCounter is valid.
-	ok = true
-	updatedCounter = newCounter
-	// Compute how far we need to shift our sliding window.
-	shiftCount := newCounter - counter
-	updatedWindow <<= shiftCount
-	// We need to set the bit in our window that corresponds to counter (if
-	// newCounter = counter + 1, then this is the first [LSB] of the window).
-	updatedWindow |= uint64(1) << (shiftCount - 1)
-	return
-}
-
 func (v *Verifier) verifyGCM(message *universal.RoutableMessage, gcmData *signatures.AES_GCM_Personalized_Signature_Data) (plaintext []byte, err error) {
 	if err = v.verifySessionInfo(message, gcmData); err != nil {
 		return nil, err
@@ -337,3 +296,34 @@ func (v *Verifier) verifySessionInfo(message *universal.RoutableMessage, info se
 	}
 	return nil
 }
+
+// Encrypt a message response in place.
+//
+// The message id must uniquely identify the Signer's request that prompted the message. The
+// counter must increase monotonically for a given id.
+func (v *Verifier) Encrypt(message *universal.RoutableMessage, id []byte, counter uint32) error {
+	plaintext := message.GetProtobufMessageAsBytes()
+	authenticatedData, err := v.responseMetadata(message, id, counter)
+	if err != nil {
+		return err
+	}
+	nonce, ciphertext, tag, err := v.session.Encrypt(plaintext, authenticatedData)
+	if err != nil {
+		return err
+	}
+	message.SubSigData = &universal.RoutableMessage_SignatureData{
+		SignatureData: &signatures.SignatureData{
+			SigType: &signatures.SignatureData_AES_GCM_ResponseData{
+				AES_GCM_ResponseData: &signatures.AES_GCM_Response_Signature_Data{
+					Counter: counter,
+					Nonce:   nonce,
+					Tag:     tag,
+				},
+			},
+		},
+	}
+	message.Payload = &universal.RoutableMessage_ProtobufMessageAsBytes{
+		ProtobufMessageAsBytes: ciphertext,
+	}
+	return nil
+}
diff --git a/internal/authentication/verifier_test.go b/internal/authentication/verifier_test.go
index 35ec411..ee18952 100644
--- a/internal/authentication/verifier_test.go
+++ b/internal/authentication/verifier_test.go
@@ -550,86 +550,46 @@ func TestNoSignatureData(t *testing.T) {
 	runVerifyTest(t, verifier, message, errCodeBadParameter, false)
 }
 
-func TestSlidingWindow(t *testing.T) {
-	type windowTest struct {
-		counter                uint32
-		window                 uint64
-		newCounter             uint32
-		expectedUpdatedCounter uint32
-		expectedUpdatedWindow  uint64
-		expectedOk             bool
-	}
-	tests := []windowTest{
-		// Update should succeed because newCounter is greater than all previous counters.
-		windowTest{
-			counter:                100,
-			window:                 uint64((1 << 0) | (1 << 5)),
-			newCounter:             101,
-			expectedUpdatedCounter: 101,
-			expectedUpdatedWindow:  uint64(1 | (1 << 1) | (1 << 6)),
-			expectedOk:             true,
-		},
-		// Update should succeed because newCounter is greater than all previous counters.
-		// In this test, some messages were skipped and so the expectedUpdatedWindow shifts further.
-		windowTest{
-			counter:                100,
-			window:                 uint64((1 << 0) | (1 << 5)),
-			newCounter:             103,
-			expectedUpdatedCounter: 103,
-			expectedUpdatedWindow:  uint64((1 << 2) | (1 << 3) | (1 << 8)),
-			expectedOk:             true,
-		},
-		// Update should succeed because newCounter is greater than all previous counters.
-		// In this test, the previous counter doesn't fit in sliding window.
-		windowTest{
-			counter:                100,
-			window:                 uint64((1 << 0) | (1 << 5)),
-			newCounter:             500,
-			expectedUpdatedCounter: 500,
-			expectedUpdatedWindow:  0,
-			expectedOk:             true,
-		},
-		// Update should succeed because newCounter falls in window but isn't set.
-		windowTest{
-			counter:                100,
-			window:                 uint64((1 << 0) | (1 << 5)),
-			newCounter:             98,
-			expectedUpdatedCounter: 100,
-			expectedUpdatedWindow:  uint64((1 << 0) | (1 << 1) | (1 << 5)),
-			expectedOk:             true,
-		},
-		// Update should fail because newCounter falls in window and is already set.
-		windowTest{
-			counter:                100,
-			window:                 uint64((1 << 0) | (1 << 5)),
-			newCounter:             99,
-			expectedUpdatedCounter: 100,
-			expectedUpdatedWindow:  uint64((1 << 0) | (1 << 5)),
-			expectedOk:             false,
-		},
-		// Update should fail because newCounter falls outside of window and freshness cannot be validated.
-		windowTest{
-			counter:                100,
-			window:                 uint64((1 << 0) | (1 << 5)),
-			newCounter:             3,
-			expectedUpdatedCounter: 100,
-			expectedUpdatedWindow:  uint64((1 << 0) | (1 << 5)),
-			expectedOk:             false,
-		},
-		// Update should fail because newCounter == counter.
-		windowTest{
-			counter:                100,
-			window:                 uint64((1 << 0) | (1 << 5)),
-			newCounter:             100,
-			expectedUpdatedCounter: 100,
-			expectedUpdatedWindow:  uint64((1 << 0) | (1 << 5)),
-			expectedOk:             false,
-		},
-	}
-	for _, test := range tests {
-		counter, window, ok := updateSlidingWindow(test.counter, test.window, test.newCounter)
-		if counter != test.expectedUpdatedCounter || window != test.expectedUpdatedWindow || ok != test.expectedOk {
-			t.Errorf("Failed window test %+v, got counter=%d, window=%d, ok=%v", test, counter, window, ok)
-		}
+func TestProvideHandle(t *testing.T) {
+	verifier, _ := getGCMVerifierAndSigner(t)
+	handle := uint32(0xDEADBEEF)
+	verifier.AssignHandle(handle)
+	info, err := verifier.SessionInfo()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if info.GetHandle() != handle {
+		t.Fatalf("Invalid handle value: %s", info)
+	}
+}
+
+func TestVerifierEncryption(t *testing.T) {
+	verifier, signer := getGCMVerifierAndSigner(t)
+	message := getTestMessage()
+	message.FromDestination = &universal.Destination{
+		SubDestination: &universal.Destination_Domain{Domain: testVerifierDomain},
+	}
+	plaintext := append([]byte{}, message.GetProtobufMessageAsBytes()...)
+	id := []byte{1, 2, 3}
+	counter := uint32(4)
+	t.Logf("Original message: %+v", message)
+	if err := verifier.Encrypt(message, id, counter); err != nil {
+		t.Fatalf("Error encrypting message: %s", err)
+	}
+	t.Logf("Encrypted message: %+v", message)
+	if bytes.Equal(plaintext, message.GetProtobufMessageAsBytes()) {
+		t.Fatalf("Encryption didn't change payload")
+	}
+
+	id[0] ^= 1
+	if _, err := signer.Decrypt(message, id); err == nil {
+		t.Fatalf("Decryption should have failed")
+	}
+	id[0] ^= 1
+	if rxCounter, err := signer.Decrypt(message, id); err != nil || rxCounter != counter {
+		t.Fatalf("Decryption failed: %s", err)
+	}
+	if !bytes.Equal(plaintext, message.GetProtobufMessageAsBytes()) {
+		t.Fatalf("Decryption failed to recover original plaintext")
 	}
 }
diff --git a/internal/authentication/window.go b/internal/authentication/window.go
new file mode 100644
index 0000000..7368ff0
--- /dev/null
+++ b/internal/authentication/window.go
@@ -0,0 +1,68 @@
+package authentication
+
+// updateSlidingWindow takes the current counter value (i.e., the highest
+// counter value of any authentic message received so far), the current sliding
+// window, and the newCounter value from an incoming message. The function
+// returns the updated counter and window values and sets ok to true if it
+// could confirm that newCounter has never been previously used. If ok is
+// false, then updatedCounter = counter and updatedWindow = window.
+func updateSlidingWindow(counter uint32, window uint64, newCounter uint32) (updatedCounter uint32, updatedWindow uint64, ok bool) {
+	// If we exit early due to an error, we want to leave the counter/window
+	// state unchanged. Therefore we initialize return values to the current
+	// state.
+	updatedCounter = counter
+	updatedWindow = window
+	ok = false
+
+	if counter == newCounter {
+		// This counter value has been used before.
+		return
+	}
+
+	if newCounter < counter {
+		// This message arrived out of order.
+		age := counter - newCounter
+		if age > windowSize {
+			// Our history doesn't go back this far, so we can't determine if
+			// we've seen this newCounter value before.
+			return
+		}
+		if window>>(age-1)&1 == 1 {
+			// The newCounter value has been used before.
+			return
+		}
+		// Everything looks good.
+		ok = true
+		updatedWindow |= (1 << (age - 1))
+		return
+	}
+
+	// If we've reached this point, newCounter > counter, so newCounter is valid.
+	ok = true
+	updatedCounter = newCounter
+	// Compute how far we need to shift our sliding window.
+	shiftCount := newCounter - counter
+	updatedWindow <<= shiftCount
+	// We need to set the bit in our window that corresponds to counter (if
+	// newCounter = counter + 1, then this is the first [LSB] of the window).
+	updatedWindow |= uint64(1) << (shiftCount - 1)
+	return
+}
+
+type SlidingWindow struct {
+	history uint64
+	counter uint32
+	used    bool
+}
+
+func (w *SlidingWindow) Update(counter uint32) bool {
+	if !w.used {
+		w.used = true
+		w.counter = counter
+		return true
+	}
+	var ok bool
+
+	w.counter, w.history, ok = updateSlidingWindow(w.counter, w.history, counter)
+	return ok
+}
diff --git a/internal/authentication/window_test.go b/internal/authentication/window_test.go
new file mode 100644
index 0000000..b799e4b
--- /dev/null
+++ b/internal/authentication/window_test.go
@@ -0,0 +1,97 @@
+package authentication
+
+import (
+	"testing"
+)
+
+func TestSlidingWindow(t *testing.T) {
+	type windowTest struct {
+		counter                uint32
+		window                 uint64
+		newCounter             uint32
+		expectedUpdatedCounter uint32
+		expectedUpdatedWindow  uint64
+		expectedOk             bool
+	}
+	tests := []windowTest{
+		// Update should succeed because newCounter is greater than all previous counters.
+		windowTest{
+			counter:                100,
+			window:                 uint64((1 << 0) | (1 << 5)),
+			newCounter:             101,
+			expectedUpdatedCounter: 101,
+			expectedUpdatedWindow:  uint64(1 | (1 << 1) | (1 << 6)),
+			expectedOk:             true,
+		},
+		// Update should succeed because newCounter is greater than all previous counters.
+		// In this test, some messages were skipped and so the expectedUpdatedWindow shifts further.
+		windowTest{
+			counter:                100,
+			window:                 uint64((1 << 0) | (1 << 5)),
+			newCounter:             103,
+			expectedUpdatedCounter: 103,
+			expectedUpdatedWindow:  uint64((1 << 2) | (1 << 3) | (1 << 8)),
+			expectedOk:             true,
+		},
+		// Update should succeed because newCounter is greater than all previous counters.
+		// In this test, the previous counter doesn't fit in sliding window.
+		windowTest{
+			counter:                100,
+			window:                 uint64((1 << 0) | (1 << 5)),
+			newCounter:             500,
+			expectedUpdatedCounter: 500,
+			expectedUpdatedWindow:  0,
+			expectedOk:             true,
+		},
+		// Update should succeed because newCounter falls in window but isn't set.
+		windowTest{
+			counter:                100,
+			window:                 uint64((1 << 0) | (1 << 5)),
+			newCounter:             98,
+			expectedUpdatedCounter: 100,
+			expectedUpdatedWindow:  uint64((1 << 0) | (1 << 1) | (1 << 5)),
+			expectedOk:             true,
+		},
+		// Update should fail because newCounter falls in window and is already set.
+		windowTest{
+			counter:                100,
+			window:                 uint64((1 << 0) | (1 << 5)),
+			newCounter:             99,
+			expectedUpdatedCounter: 100,
+			expectedUpdatedWindow:  uint64((1 << 0) | (1 << 5)),
+			expectedOk:             false,
+		},
+		// Update should fail because newCounter falls outside of window and freshness cannot be validated.
+		windowTest{
+			counter:                100,
+			window:                 uint64((1 << 0) | (1 << 5)),
+			newCounter:             3,
+			expectedUpdatedCounter: 100,
+			expectedUpdatedWindow:  uint64((1 << 0) | (1 << 5)),
+			expectedOk:             false,
+		},
+		// Update should fail because newCounter == counter.
+		windowTest{
+			counter:                100,
+			window:                 uint64((1 << 0) | (1 << 5)),
+			newCounter:             100,
+			expectedUpdatedCounter: 100,
+			expectedUpdatedWindow:  uint64((1 << 0) | (1 << 5)),
+			expectedOk:             false,
+		},
+	}
+	for _, test := range tests {
+		counter, window, ok := updateSlidingWindow(test.counter, test.window, test.newCounter)
+		if counter != test.expectedUpdatedCounter || window != test.expectedUpdatedWindow || ok != test.expectedOk {
+			t.Errorf("Failed window test %+v, got counter=%d, window=%d, ok=%v", test, counter, window, ok)
+		}
+		w := SlidingWindow{
+			history: test.window,
+			counter: test.counter,
+			used:    true,
+		}
+		if w.Update(test.newCounter) != test.expectedOk {
+			t.Errorf("Failed window test %+v", test)
+		}
+	}
+}
diff --git a/internal/dispatcher/dispatcher.go b/internal/dispatcher/dispatcher.go
index ad6a9ea..12b2628 100644
--- a/internal/dispatcher/dispatcher.go
+++ b/internal/dispatcher/dispatcher.go
@@ -141,7 +141,7 @@ func (d *Dispatcher) StartSessions(ctx context.Context, domains []universal.Doma
 	return err
 }
 
-func (d *Dispatcher) createHandler(key *receiverKey) *receiver {
+func (d *Dispatcher) createHandler(key *receiverKey, id []byte) *receiver {
 	d.handlerLock.Lock()
 	defer d.handlerLock.Unlock()
 
@@ -152,6 +152,7 @@ func (d *Dispatcher) createHandler(key *receiverKey) *receiver {
 		dispatcher:    d,
 		requestSentAt: now,
 		lastActive:    now,
+		requestID:     id,
 	}
 
 	d.handlers[*key] = recv
@@ -207,6 +208,25 @@ func (d *Dispatcher) checkForSessionUpdate(message *universal.RoutableMessage, h
 	log.Info("[%02x] Updated session info for %s", message.GetRequestUuid(), domain)
 }
 
+func (d *Dispatcher) decrypt(message *universal.RoutableMessage, handler *receiver) error {
+	decryptionData := message.GetSignatureData().GetAES_GCM_ResponseData()
+	if decryptionData == nil {
+		return nil
+	}
+
+	domain := handler.key.domain
+
+	d.sessionLock.Lock()
+	defer d.sessionLock.Unlock()
+
+	session, ok := d.sessions[domain]
+	if !ok {
+		return protocol.ErrNoDecryptionContext
+	}
+
+	return session.decrypt(message, handler)
+}
+
 func (d *Dispatcher) process(message *universal.RoutableMessage) {
 	var key receiverKey
 
@@ -263,6 +283,15 @@ func (d *Dispatcher) process(message *universal.RoutableMessage) {
 	// session info.
 	d.checkForSessionUpdate(message, handler)
 
+	// Decryption is a no-op for plaintext messages
+	if err := d.decrypt(message, handler); err == protocol.ErrReplayedResponse {
+		log.Info("[%02x] Dropping duplicate vehicle response", requestUUID)
+		return
+	} else if err != nil {
+		log.Warning("[%02x] Error decrypting vehicle response: %s", requestUUID, err)
+		return
+	}
+
 	select {
 	case handler.ch <- message:
 	default:
@@ -386,7 +415,7 @@ func (d *Dispatcher) Send(ctx context.Context, message *universal.RoutableMessag
 		}
 	}
 
-	resp := d.createHandler(&key)
+	resp := d.createHandler(&key, authentication.RequestID(message))
 	encodedMessage, err := proto.Marshal(message)
 	if err != nil {
 		return nil, err
diff --git a/internal/dispatcher/receiver.go b/internal/dispatcher/receiver.go
index 4b4170a..f3a94e3 100644
--- a/internal/dispatcher/receiver.go
+++ b/internal/dispatcher/receiver.go
@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"time"
 
+	"github.com/teslamotors/vehicle-command/internal/authentication"
 	universal "github.com/teslamotors/vehicle-command/pkg/protocol/protobuf/universalmessage"
 )
 
@@ -32,6 +33,8 @@ type receiver struct {
 	dispatcher    *Dispatcher
 	requestSentAt time.Time
 	lastActive    time.Time
+	antireplay    authentication.SlidingWindow
+	requestID     []byte
 }
 
 // Recv returns a channel that receives responses to the command that created the receiver.
diff --git a/internal/dispatcher/session.go b/internal/dispatcher/session.go
index 2173f87..1b5406c 100644
--- a/internal/dispatcher/session.go
+++ b/internal/dispatcher/session.go
@@ -8,6 +8,7 @@ import (
 
 	"github.com/teslamotors/vehicle-command/internal/authentication"
 	"github.com/teslamotors/vehicle-command/pkg/connector"
+	"github.com/teslamotors/vehicle-command/pkg/protocol"
 
 	universal "github.com/teslamotors/vehicle-command/pkg/protocol/protobuf/universalmessage"
 )
@@ -44,6 +45,19 @@ func NewSession(private authentication.ECDHPrivateKey, vin string) (*session, er
 	}, nil
 }
 
+func (s *session) decrypt(message *universal.RoutableMessage, handler *receiver) error {
+	s.lock.Lock()
+	defer s.lock.Unlock()
+	counter, err := s.ctx.Decrypt(message, handler.requestID)
+	if err != nil {
+		return err
+	}
+	if !handler.antireplay.Update(counter) {
+		return protocol.ErrReplayedResponse
+	}
+	return nil
+}
+
 func (s *session) Authorize(ctx context.Context, command *universal.RoutableMessage, method connector.AuthMethod) error {
 	var err error
 	lifetime := defaultExpiration
diff --git a/pkg/protocol/error.go b/pkg/protocol/error.go
index 672da85..a9ab2ed 100644
--- a/pkg/protocol/error.go
+++ b/pkg/protocol/error.go
@@ -51,6 +51,11 @@ var (
 	ErrProtocolNotSupported = errors.New("vehicle does not support protocol -- use REST API")
 	ErrRequiresBLE          = errors.New("command can only be sent over BLE")
 	ErrRequiresEncryption   = errors.New("command should not be sent in plaintext or encrypted with an unauthenticated public key")
+	ErrNoDecryptionContext  = errors.New("could not decrypt vehicle response without a session")
+	// ErrReplayedResponse indicates the client received multiple responses from the vehicle with
+	// the same response counter. This could be benign, as the network may have reattempted
+	// transmission.
+	ErrReplayedResponse = errors.New("received vehicle response with duplicate counter")
 )
 
 type CommandError struct {
diff --git a/pkg/protocol/protobuf/carserver/car_server.pb.go b/pkg/protocol/protobuf/carserver/car_server.pb.go
index 86cc95a..5a7c08b 100644
--- a/pkg/protocol/protobuf/carserver/car_server.pb.go
+++ b/pkg/protocol/protobuf/carserver/car_server.pb.go
@@ -328,7 +328,6 @@ type Action struct {
 	unknownFields protoimpl.UnknownFields
 
 	// Types that are assignable to ActionMsg:
-	//
 	//	*Action_VehicleAction
 	ActionMsg isAction_ActionMsg `protobuf_oneof:"action_msg"`
 }
@@ -395,7 +394,6 @@ type VehicleAction struct {
 	unknownFields protoimpl.UnknownFields
 
 	// Types that are assignable to VehicleActionMsg:
-	//
 	//	*VehicleAction_ChargingSetLimitAction
 	//	*VehicleAction_ChargingStartStopAction
 	//	*VehicleAction_DrivingClearSpeedLimitPinAction
@@ -1196,7 +1194,6 @@ type Response struct {
 
 	ActionStatus *ActionStatus `protobuf:"bytes,1,opt,name=actionStatus,proto3" json:"actionStatus,omitempty"`
 	// Types that are assignable to ResponseMsg:
-	//
 	//	*Response_GetSessionInfoResponse
 	//	*Response_GetNearbyChargingSites
 	//	*Response_Ping
@@ -1353,7 +1350,6 @@ type ResultReason struct {
 	unknownFields protoimpl.UnknownFields
 
 	// Types that are assignable to Reason:
-	//
 	//	*ResultReason_PlainText
 	Reason isResultReason_Reason `protobuf_oneof:"reason"`
 }
@@ -1530,7 +1526,6 @@ type ChargingStartStopAction struct {
 	unknownFields protoimpl.UnknownFields
 
 	// Types that are assignable to ChargingAction:
-	//
 	//	*ChargingStartStopAction_Unknown
 	//	*ChargingStartStopAction_Start
 	//	*ChargingStartStopAction_StartStandard
@@ -2519,7 +2514,6 @@ type MediaUpdateVolume struct {
 	unknownFields protoimpl.UnknownFields
 
 	// Types that are assignable to MediaVolume:
-	//
 	//	*MediaUpdateVolume_VolumeDelta
 	//	*MediaUpdateVolume_VolumeAbsoluteFloat
 	MediaVolume isMediaUpdateVolume_MediaVolume `protobuf_oneof:"media_volume"`
@@ -3053,12 +3047,10 @@ type VehicleControlSunroofOpenCloseAction struct {
 	unknownFields protoimpl.UnknownFields
 
 	// Types that are assignable to SunroofLevel:
-	//
 	//	*VehicleControlSunroofOpenCloseAction_AbsoluteLevel
 	//	*VehicleControlSunroofOpenCloseAction_DeltaLevel
 	SunroofLevel isVehicleControlSunroofOpenCloseAction_SunroofLevel `protobuf_oneof:"sunroof_level"`
 	// Types that are assignable to Action:
-	//
 	//	*VehicleControlSunroofOpenCloseAction_Vent
 	//	*VehicleControlSunroofOpenCloseAction_Close
 	//	*VehicleControlSunroofOpenCloseAction_Open
@@ -3247,7 +3239,6 @@ type VehicleControlWindowAction struct {
 	unknownFields protoimpl.UnknownFields
 
 	// Types that are assignable to Action:
-	//
 	//	*VehicleControlWindowAction_Unknown
 	//	*VehicleControlWindowAction_Vent
 	//	*VehicleControlWindowAction_Close
@@ -4281,7 +4272,6 @@ type HvacSeatHeaterActions_HvacSeatHeaterAction struct {
 	unknownFields protoimpl.UnknownFields
 
 	// Types that are assignable to SeatHeaterLevel:
-	//
 	//	*HvacSeatHeaterActions_HvacSeatHeaterAction_SEAT_HEATER_UNKNOWN
 	//	*HvacSeatHeaterActions_HvacSeatHeaterAction_SEAT_HEATER_OFF
 	//	*HvacSeatHeaterActions_HvacSeatHeaterAction_SEAT_HEATER_LOW
@@ -4289,7 +4279,6 @@ type HvacSeatHeaterActions_HvacSeatHeaterAction struct {
 	//	*HvacSeatHeaterActions_HvacSeatHeaterAction_SEAT_HEATER_HIGH
 	SeatHeaterLevel isHvacSeatHeaterActions_HvacSeatHeaterAction_SeatHeaterLevel `protobuf_oneof:"seat_heater_level"`
 	// Types that are assignable to SeatPosition:
-	//
 	//	*HvacSeatHeaterActions_HvacSeatHeaterAction_CAR_SEAT_UNKNOWN
 	//	*HvacSeatHeaterActions_HvacSeatHeaterAction_CAR_SEAT_FRONT_LEFT
 	//	*HvacSeatHeaterActions_HvacSeatHeaterAction_CAR_SEAT_FRONT_RIGHT
@@ -4628,7 +4617,6 @@ type HvacTemperatureAdjustmentAction_Temperature struct {
 	unknownFields protoimpl.UnknownFields
 
 	// Types that are assignable to Type:
-	//
 	//	*HvacTemperatureAdjustmentAction_Temperature_TEMP_UNKNOWN
 	//	*HvacTemperatureAdjustmentAction_Temperature_TEMP_MIN
 	//	*HvacTemperatureAdjustmentAction_Temperature_TEMP_MAX
@@ -4726,7 +4714,6 @@ type HvacTemperatureAdjustmentAction_HvacTemperatureZone struct {
 	unknownFields protoimpl.UnknownFields
 
 	// Types that are assignable to Type:
-	//
 	//	*HvacTemperatureAdjustmentAction_HvacTemperatureZone_TEMP_ZONE_UNKNOWN
 	//	*HvacTemperatureAdjustmentAction_HvacTemperatureZone_TEMP_ZONE_FRONT_LEFT
 	//	*HvacTemperatureAdjustmentAction_HvacTemperatureZone_TEMP_ZONE_FRONT_RIGHT
diff --git a/pkg/protocol/protobuf/carserver/common.pb.go b/pkg/protocol/protobuf/carserver/common.pb.go
index d2e4c82..2e3eb51 100644
--- a/pkg/protocol/protobuf/carserver/common.pb.go
+++ b/pkg/protocol/protobuf/carserver/common.pb.go
@@ -162,7 +162,6 @@ type PreconditioningTimes struct {
 	unknownFields protoimpl.UnknownFields
 
 	// Types that are assignable to Times:
-	//
 	//	*PreconditioningTimes_AllWeek
 	//	*PreconditioningTimes_Weekdays
 	Times isPreconditioningTimes_Times `protobuf_oneof:"times"`
@@ -243,7 +242,6 @@ type OffPeakChargingTimes struct {
 	unknownFields protoimpl.UnknownFields
 
 	// Types that are assignable to Times:
-	//
 	//	*OffPeakChargingTimes_AllWeek
 	//	*OffPeakChargingTimes_Weekdays
 	Times isOffPeakChargingTimes_Times `protobuf_oneof:"times"`
diff --git a/pkg/protocol/protobuf/signatures.proto b/pkg/protocol/protobuf/signatures.proto
index b22a591..f9eb4d1 100644
--- a/pkg/protocol/protobuf/signatures.proto
+++ b/pkg/protocol/protobuf/signatures.proto
@@ -15,6 +15,8 @@ enum Tag
     TAG_COUNTER         = 5;
     TAG_CHALLENGE       = 6;
     TAG_FLAGS           = 7;
+    TAG_REQUEST_HASH    = 8;
+    TAG_FAULT           = 9;
     TAG_END             = 255;
 }
 
@@ -25,6 +27,7 @@ enum SignatureType
     SIGNATURE_TYPE_HMAC                 = 6;
     reserved 7;
     SIGNATURE_TYPE_HMAC_PERSONALIZED    = 8;
+    SIGNATURE_TYPE_AES_GCM_RESPONSE     = 9;
 }
 
 message KeyIdentity {
@@ -43,6 +46,12 @@ message AES_GCM_Personalized_Signature_Data {
     bytes   tag         = 5;
 }
 
+message AES_GCM_Response_Signature_Data {
+    bytes  nonce   = 1;
+    uint32 counter = 2;
+    bytes  tag     = 3;
+}
+
 message HMAC_Signature_Data {
     bytes tag = 1;
 }
@@ -61,6 +70,7 @@ message SignatureData {
         AES_GCM_Personalized_Signature_Data AES_GCM_Personalized_data = 5;
         HMAC_Signature_Data                 session_info_tag          = 6;
         HMAC_Personalized_Signature_Data    HMAC_Personalized_data    = 8;
+        AES_GCM_Response_Signature_Data     AES_GCM_Response_data     = 9;
     }
 }
 
diff --git a/pkg/protocol/protobuf/signatures/signatures.pb.go b/pkg/protocol/protobuf/signatures/signatures.pb.go
index 3aba59a..f2167a4 100644
--- a/pkg/protocol/protobuf/signatures/signatures.pb.go
+++ b/pkg/protocol/protobuf/signatures/signatures.pb.go
@@ -31,6 +31,8 @@ const (
 	Tag_TAG_COUNTER         Tag = 5
 	Tag_TAG_CHALLENGE       Tag = 6
 	Tag_TAG_FLAGS           Tag = 7
+	Tag_TAG_REQUEST_HASH    Tag = 8
+	Tag_TAG_FAULT           Tag = 9
 	Tag_TAG_END             Tag = 255
 )
 
@@ -45,6 +47,8 @@ var (
 		5:   "TAG_COUNTER",
 		6:   "TAG_CHALLENGE",
 		7:   "TAG_FLAGS",
+		8:   "TAG_REQUEST_HASH",
+		9:   "TAG_FAULT",
 		255: "TAG_END",
 	}
 	Tag_value = map[string]int32{
@@ -56,6 +60,8 @@ var (
 		"TAG_COUNTER":         5,
 		"TAG_CHALLENGE":       6,
 		"TAG_FLAGS":           7,
+		"TAG_REQUEST_HASH":    8,
+		"TAG_FAULT":           9,
 		"TAG_END":             255,
 	}
 )
@@ -94,6 +100,7 @@ const (
 	SignatureType_SIGNATURE_TYPE_AES_GCM_PERSONALIZED SignatureType = 5
 	SignatureType_SIGNATURE_TYPE_HMAC                 SignatureType = 6
 	SignatureType_SIGNATURE_TYPE_HMAC_PERSONALIZED    SignatureType = 8
+	SignatureType_SIGNATURE_TYPE_AES_GCM_RESPONSE     SignatureType = 9
 )
 
 // Enum value maps for SignatureType.
@@ -103,12 +110,14 @@ var (
 		5: "SIGNATURE_TYPE_AES_GCM_PERSONALIZED",
 		6: "SIGNATURE_TYPE_HMAC",
 		8: "SIGNATURE_TYPE_HMAC_PERSONALIZED",
+		9: "SIGNATURE_TYPE_AES_GCM_RESPONSE",
 	}
 	SignatureType_value = map[string]int32{
 		"SIGNATURE_TYPE_AES_GCM":              0,
 		"SIGNATURE_TYPE_AES_GCM_PERSONALIZED": 5,
 		"SIGNATURE_TYPE_HMAC":                 6,
 		"SIGNATURE_TYPE_HMAC_PERSONALIZED":    8,
+		"SIGNATURE_TYPE_AES_GCM_RESPONSE":     9,
 	}
 )
 
@@ -191,7 +200,6 @@ type KeyIdentity struct {
 	unknownFields protoimpl.UnknownFields
 
 	// Types that are assignable to IdentityType:
-	//
 	//	*KeyIdentity_PublicKey
 	//	*KeyIdentity_Handle
 	IdentityType isKeyIdentity_IdentityType `protobuf_oneof:"identity_type"`
@@ -345,6 +353,69 @@ func (x *AES_GCM_Personalized_Signature_Data) GetTag() []byte {
 	return nil
 }
 
+type AES_GCM_Response_Signature_Data struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	Nonce   []byte `protobuf:"bytes,1,opt,name=nonce,proto3" json:"nonce,omitempty"`
+	Counter uint32 `protobuf:"varint,2,opt,name=counter,proto3" json:"counter,omitempty"`
+	Tag     []byte `protobuf:"bytes,3,opt,name=tag,proto3" json:"tag,omitempty"`
+}
+
+func (x *AES_GCM_Response_Signature_Data) Reset() {
+	*x = AES_GCM_Response_Signature_Data{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_signatures_proto_msgTypes[2]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *AES_GCM_Response_Signature_Data) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*AES_GCM_Response_Signature_Data) ProtoMessage() {}
+
+func (x *AES_GCM_Response_Signature_Data) ProtoReflect() protoreflect.Message {
+	mi := &file_signatures_proto_msgTypes[2]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use AES_GCM_Response_Signature_Data.ProtoReflect.Descriptor instead.
+func (*AES_GCM_Response_Signature_Data) Descriptor() ([]byte, []int) {
+	return file_signatures_proto_rawDescGZIP(), []int{2}
+}
+
+func (x *AES_GCM_Response_Signature_Data) GetNonce() []byte {
+	if x != nil {
+		return x.Nonce
+	}
+	return nil
+}
+
+func (x *AES_GCM_Response_Signature_Data) GetCounter() uint32 {
+	if x != nil {
+		return x.Counter
+	}
+	return 0
+}
+
+func (x *AES_GCM_Response_Signature_Data) GetTag() []byte {
+	if x != nil {
+		return x.Tag
+	}
+	return nil
+}
+
 type HMAC_Signature_Data struct {
 	state         protoimpl.MessageState
 	sizeCache     protoimpl.SizeCache
@@ -356,7 +427,7 @@ type HMAC_Signature_Data struct {
 func (x *HMAC_Signature_Data) Reset() {
 	*x = HMAC_Signature_Data{}
 	if protoimpl.UnsafeEnabled {
-		mi := &file_signatures_proto_msgTypes[2]
+		mi := &file_signatures_proto_msgTypes[3]
 		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
 		ms.StoreMessageInfo(mi)
 	}
@@ -369,7 +440,7 @@ func (x *HMAC_Signature_Data) String() string {
 func (*HMAC_Signature_Data) ProtoMessage() {}
 
 func (x *HMAC_Signature_Data) ProtoReflect() protoreflect.Message {
-	mi := &file_signatures_proto_msgTypes[2]
+	mi := &file_signatures_proto_msgTypes[3]
 	if protoimpl.UnsafeEnabled && x != nil {
 		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
 		if ms.LoadMessageInfo() == nil {
@@ -382,7 +453,7 @@ func (x *HMAC_Signature_Data) ProtoReflect() protoreflect.Message {
 
 // Deprecated: Use HMAC_Signature_Data.ProtoReflect.Descriptor instead.
 func (*HMAC_Signature_Data) Descriptor() ([]byte, []int) {
-	return file_signatures_proto_rawDescGZIP(), []int{2}
+	return file_signatures_proto_rawDescGZIP(), []int{3}
 }
 
 func (x *HMAC_Signature_Data) GetTag() []byte {
@@ -406,7 +477,7 @@ type HMAC_Personalized_Signature_Data struct {
 func (x *HMAC_Personalized_Signature_Data) Reset() {
 	*x = HMAC_Personalized_Signature_Data{}
 	if protoimpl.UnsafeEnabled {
-		mi := &file_signatures_proto_msgTypes[3]
+		mi := &file_signatures_proto_msgTypes[4]
 		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
 		ms.StoreMessageInfo(mi)
 	}
@@ -419,7 +490,7 @@ func (x *HMAC_Personalized_Signature_Data) String() string {
 func (*HMAC_Personalized_Signature_Data) ProtoMessage() {}
 
 func (x *HMAC_Personalized_Signature_Data) ProtoReflect() protoreflect.Message {
-	mi := &file_signatures_proto_msgTypes[3]
+	mi := &file_signatures_proto_msgTypes[4]
 	if protoimpl.UnsafeEnabled && x != nil {
 		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
 		if ms.LoadMessageInfo() == nil {
@@ -432,7 +503,7 @@ func (x *HMAC_Personalized_Signature_Data) ProtoReflect() protoreflect.Message {
 
 // Deprecated: Use HMAC_Personalized_Signature_Data.ProtoReflect.Descriptor instead.
 func (*HMAC_Personalized_Signature_Data) Descriptor() ([]byte, []int) {
-	return file_signatures_proto_rawDescGZIP(), []int{3}
+	return file_signatures_proto_rawDescGZIP(), []int{4}
 }
 
 func (x *HMAC_Personalized_Signature_Data) GetEpoch() []byte {
@@ -470,17 +541,17 @@ type SignatureData struct {
 
 	SignerIdentity *KeyIdentity `protobuf:"bytes,1,opt,name=signer_identity,json=signerIdentity,proto3" json:"signer_identity,omitempty"`
 	// Types that are assignable to SigType:
-	//
 	//	*SignatureData_AES_GCM_PersonalizedData
 	//	*SignatureData_SessionInfoTag
 	//	*SignatureData_HMAC_PersonalizedData
+	//	*SignatureData_AES_GCM_ResponseData
 	SigType isSignatureData_SigType `protobuf_oneof:"sig_type"`
 }
 
 func (x *SignatureData) Reset() {
 	*x = SignatureData{}
 	if protoimpl.UnsafeEnabled {
-		mi := &file_signatures_proto_msgTypes[4]
+		mi := &file_signatures_proto_msgTypes[5]
 		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
 		ms.StoreMessageInfo(mi)
 	}
@@ -493,7 +564,7 @@ func (x *SignatureData) String() string {
 func (*SignatureData) ProtoMessage() {}
 
 func (x *SignatureData) ProtoReflect() protoreflect.Message {
-	mi := &file_signatures_proto_msgTypes[4]
+	mi := &file_signatures_proto_msgTypes[5]
 	if protoimpl.UnsafeEnabled && x != nil {
 		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
 		if ms.LoadMessageInfo() == nil {
@@ -506,7 +577,7 @@ func (x *SignatureData) ProtoReflect() protoreflect.Message {
 
 // Deprecated: Use SignatureData.ProtoReflect.Descriptor instead.
 func (*SignatureData) Descriptor() ([]byte, []int) {
-	return file_signatures_proto_rawDescGZIP(), []int{4}
+	return file_signatures_proto_rawDescGZIP(), []int{5}
 }
 
 func (x *SignatureData) GetSignerIdentity() *KeyIdentity {
@@ -544,6 +615,13 @@ func (x *SignatureData) GetHMAC_PersonalizedData() *HMAC_Personalized_Signature_
 	return nil
 }
 
+func (x *SignatureData) GetAES_GCM_ResponseData() *AES_GCM_Response_Signature_Data {
+	if x, ok := x.GetSigType().(*SignatureData_AES_GCM_ResponseData); ok {
+		return x.AES_GCM_ResponseData
+	}
+	return nil
+}
+
 type isSignatureData_SigType interface {
 	isSignatureData_SigType()
 }
@@ -560,12 +638,18 @@ type SignatureData_HMAC_PersonalizedData struct {
 	HMAC_PersonalizedData *HMAC_Personalized_Signature_Data `protobuf:"bytes,8,opt,name=HMAC_Personalized_data,json=HMACPersonalizedData,proto3,oneof"`
 }
 
+type SignatureData_AES_GCM_ResponseData struct {
+	AES_GCM_ResponseData *AES_GCM_Response_Signature_Data `protobuf:"bytes,9,opt,name=AES_GCM_Response_data,json=AESGCMResponseData,proto3,oneof"`
+}
+
 func (*SignatureData_AES_GCM_PersonalizedData) isSignatureData_SigType() {}
 
 func (*SignatureData_SessionInfoTag) isSignatureData_SigType() {}
 
 func (*SignatureData_HMAC_PersonalizedData) isSignatureData_SigType() {}
 
+func (*SignatureData_AES_GCM_ResponseData) isSignatureData_SigType() {}
+
 type GetSessionInfoRequest struct {
 	state         protoimpl.MessageState
 	sizeCache     protoimpl.SizeCache
@@ -577,7 +661,7 @@ type GetSessionInfoRequest struct {
 func (x *GetSessionInfoRequest) Reset() {
 	*x = GetSessionInfoRequest{}
 	if protoimpl.UnsafeEnabled {
-		mi := &file_signatures_proto_msgTypes[5]
+		mi := &file_signatures_proto_msgTypes[6]
 		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
 		ms.StoreMessageInfo(mi)
 	}
@@ -590,7 +674,7 @@ func (x *GetSessionInfoRequest) String() string {
 func (*GetSessionInfoRequest) ProtoMessage() {}
 
 func (x *GetSessionInfoRequest) ProtoReflect() protoreflect.Message {
-	mi := &file_signatures_proto_msgTypes[5]
+	mi := &file_signatures_proto_msgTypes[6]
 	if protoimpl.UnsafeEnabled && x != nil {
 		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
 		if ms.LoadMessageInfo() == nil {
@@ -603,7 +687,7 @@ func (x *GetSessionInfoRequest) ProtoReflect() protoreflect.Message {
 
 // Deprecated: Use GetSessionInfoRequest.ProtoReflect.Descriptor instead.
 func (*GetSessionInfoRequest) Descriptor() ([]byte, []int) {
-	return file_signatures_proto_rawDescGZIP(), []int{5}
+	return file_signatures_proto_rawDescGZIP(), []int{6}
 }
 
 func (x *GetSessionInfoRequest) GetKeyIdentity() *KeyIdentity {
@@ -629,7 +713,7 @@ type SessionInfo struct {
 func (x *SessionInfo) Reset() {
 	*x = SessionInfo{}
 	if protoimpl.UnsafeEnabled {
-		mi := &file_signatures_proto_msgTypes[6]
+		mi := &file_signatures_proto_msgTypes[7]
 		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
 		ms.StoreMessageInfo(mi)
 	}
@@ -642,7 +726,7 @@ func (x *SessionInfo) String() string {
 func (*SessionInfo) ProtoMessage() {}
 
 func (x *SessionInfo) ProtoReflect() protoreflect.Message {
-	mi := &file_signatures_proto_msgTypes[6]
+	mi := &file_signatures_proto_msgTypes[7]
 	if protoimpl.UnsafeEnabled && x != nil {
 		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
 		if ms.LoadMessageInfo() == nil {
@@ -655,7 +739,7 @@ func (x *SessionInfo) ProtoReflect() protoreflect.Message {
 
 // Deprecated: Use SessionInfo.ProtoReflect.Descriptor instead.
 func (*SessionInfo) Descriptor() ([]byte, []int) {
-	return file_signatures_proto_rawDescGZIP(), []int{6}
+	return file_signatures_proto_rawDescGZIP(), []int{7}
 }
 
 func (x *SessionInfo) GetCounter() uint32 {
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 }
 
 var (
@@ -824,31 +925,33 @@ func file_signatures_proto_rawDescGZIP() []byte {
 }
 
 var file_signatures_proto_enumTypes = make([]protoimpl.EnumInfo, 3)
-var file_signatures_proto_msgTypes = make([]protoimpl.MessageInfo, 7)
+var file_signatures_proto_msgTypes = make([]protoimpl.MessageInfo, 8)
 var file_signatures_proto_goTypes = []interface{}{
 	(Tag)(0),                 // 0: Signatures.Tag
 	(SignatureType)(0),       // 1: Signatures.SignatureType
 	(Session_Info_Status)(0), // 2: Signatures.Session_Info_Status
 	(*KeyIdentity)(nil),      // 3: Signatures.KeyIdentity
 	(*AES_GCM_Personalized_Signature_Data)(nil), // 4: Signatures.AES_GCM_Personalized_Signature_Data
-	(*HMAC_Signature_Data)(nil),                 // 5: Signatures.HMAC_Signature_Data
-	(*HMAC_Personalized_Signature_Data)(nil),    // 6: Signatures.HMAC_Personalized_Signature_Data
-	(*SignatureData)(nil),                       // 7: Signatures.SignatureData
-	(*GetSessionInfoRequest)(nil),               // 8: Signatures.GetSessionInfoRequest
-	(*SessionInfo)(nil),                         // 9: Signatures.SessionInfo
+	(*AES_GCM_Response_Signature_Data)(nil),     // 5: Signatures.AES_GCM_Response_Signature_Data
+	(*HMAC_Signature_Data)(nil),                 // 6: Signatures.HMAC_Signature_Data
+	(*HMAC_Personalized_Signature_Data)(nil),    // 7: Signatures.HMAC_Personalized_Signature_Data
+	(*SignatureData)(nil),                       // 8: Signatures.SignatureData
+	(*GetSessionInfoRequest)(nil),               // 9: Signatures.GetSessionInfoRequest
+	(*SessionInfo)(nil),                         // 10: Signatures.SessionInfo
 }
 var file_signatures_proto_depIdxs = []int32{
 	3, // 0: Signatures.SignatureData.signer_identity:type_name -> Signatures.KeyIdentity
 	4, // 1: Signatures.SignatureData.AES_GCM_Personalized_data:type_name -> Signatures.AES_GCM_Personalized_Signature_Data
-	5, // 2: Signatures.SignatureData.session_info_tag:type_name -> Signatures.HMAC_Signature_Data
-	6, // 3: Signatures.SignatureData.HMAC_Personalized_data:type_name -> Signatures.HMAC_Personalized_Signature_Data
-	3, // 4: Signatures.GetSessionInfoRequest.key_identity:type_name -> Signatures.KeyIdentity
-	2, // 5: Signatures.SessionInfo.status:type_name -> Signatures.Session_Info_Status
-	6, // [6:6] is the sub-list for method output_type
-	6, // [6:6] is the sub-list for method input_type
-	6, // [6:6] is the sub-list for extension type_name
-	6, // [6:6] is the sub-list for extension extendee
-	0, // [0:6] is the sub-list for field type_name
+	6, // 2: Signatures.SignatureData.session_info_tag:type_name -> Signatures.HMAC_Signature_Data
+	7, // 3: Signatures.SignatureData.HMAC_Personalized_data:type_name -> Signatures.HMAC_Personalized_Signature_Data
+	5, // 4: Signatures.SignatureData.AES_GCM_Response_data:type_name -> Signatures.AES_GCM_Response_Signature_Data
+	3, // 5: Signatures.GetSessionInfoRequest.key_identity:type_name -> Signatures.KeyIdentity
+	2, // 6: Signatures.SessionInfo.status:type_name -> Signatures.Session_Info_Status
+	7, // [7:7] is the sub-list for method output_type
+	7, // [7:7] is the sub-list for method input_type
+	7, // [7:7] is the sub-list for extension type_name
+	7, // [7:7] is the sub-list for extension extendee
+	0, // [0:7] is the sub-list for field type_name
 }
 
 func init() { file_signatures_proto_init() }
@@ -882,7 +985,7 @@ func file_signatures_proto_init() {
 			}
 		}
 		file_signatures_proto_msgTypes[2].Exporter = func(v interface{}, i int) interface{} {
-			switch v := v.(*HMAC_Signature_Data); i {
+			switch v := v.(*AES_GCM_Response_Signature_Data); i {
 			case 0:
 				return &v.state
 			case 1:
@@ -894,7 +997,7 @@ func file_signatures_proto_init() {
 			}
 		}
 		file_signatures_proto_msgTypes[3].Exporter = func(v interface{}, i int) interface{} {
-			switch v := v.(*HMAC_Personalized_Signature_Data); i {
+			switch v := v.(*HMAC_Signature_Data); i {
 			case 0:
 				return &v.state
 			case 1:
@@ -906,7 +1009,7 @@ func file_signatures_proto_init() {
 			}
 		}
 		file_signatures_proto_msgTypes[4].Exporter = func(v interface{}, i int) interface{} {
-			switch v := v.(*SignatureData); i {
+			switch v := v.(*HMAC_Personalized_Signature_Data); i {
 			case 0:
 				return &v.state
 			case 1:
@@ -918,7 +1021,7 @@ func file_signatures_proto_init() {
 			}
 		}
 		file_signatures_proto_msgTypes[5].Exporter = func(v interface{}, i int) interface{} {
-			switch v := v.(*GetSessionInfoRequest); i {
+			switch v := v.(*SignatureData); i {
 			case 0:
 				return &v.state
 			case 1:
@@ -930,6 +1033,18 @@ func file_signatures_proto_init() {
 			}
 		}
 		file_signatures_proto_msgTypes[6].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*GetSessionInfoRequest); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
+		file_signatures_proto_msgTypes[7].Exporter = func(v interface{}, i int) interface{} {
 			switch v := v.(*SessionInfo); i {
 			case 0:
 				return &v.state
@@ -946,10 +1061,11 @@ func file_signatures_proto_init() {
 		(*KeyIdentity_PublicKey)(nil),
 		(*KeyIdentity_Handle)(nil),
 	}
-	file_signatures_proto_msgTypes[4].OneofWrappers = []interface{}{
+	file_signatures_proto_msgTypes[5].OneofWrappers = []interface{}{
 		(*SignatureData_AES_GCM_PersonalizedData)(nil),
 		(*SignatureData_SessionInfoTag)(nil),
 		(*SignatureData_HMAC_PersonalizedData)(nil),
+		(*SignatureData_AES_GCM_ResponseData)(nil),
 	}
 	type x struct{}
 	out := protoimpl.TypeBuilder{
@@ -957,7 +1073,7 @@ func file_signatures_proto_init() {
 			GoPackagePath: reflect.TypeOf(x{}).PkgPath(),
 			RawDescriptor: file_signatures_proto_rawDesc,
 			NumEnums:      3,
-			NumMessages:   7,
+			NumMessages:   8,
 			NumExtensions: 0,
 			NumServices:   0,
 		},
diff --git a/pkg/protocol/protobuf/universal_message.proto b/pkg/protocol/protobuf/universal_message.proto
index 4b46739..146ca3f 100644
--- a/pkg/protocol/protobuf/universal_message.proto
+++ b/pkg/protocol/protobuf/universal_message.proto
@@ -69,6 +69,7 @@ message SessionInfoRequest
 
 enum Flags {
     FLAG_USER_COMMAND = 0;
+    FLAG_ENCRYPT_RESPONSE = 1;
 }
 
 message RoutableMessage {
diff --git a/pkg/protocol/protobuf/universalmessage/universal_message.pb.go b/pkg/protocol/protobuf/universalmessage/universal_message.pb.go
index 6a7d807..3d6af71 100644
--- a/pkg/protocol/protobuf/universalmessage/universal_message.pb.go
+++ b/pkg/protocol/protobuf/universalmessage/universal_message.pb.go
@@ -234,16 +234,19 @@ func (MessageFault_E) EnumDescriptor() ([]byte, []int) {
 type Flags int32
 
 const (
-	Flags_FLAG_USER_COMMAND Flags = 0
+	Flags_FLAG_USER_COMMAND     Flags = 0
+	Flags_FLAG_ENCRYPT_RESPONSE Flags = 1
 )
 
 // Enum value maps for Flags.
 var (
 	Flags_name = map[int32]string{
 		0: "FLAG_USER_COMMAND",
+		1: "FLAG_ENCRYPT_RESPONSE",
 	}
 	Flags_value = map[string]int32{
-		"FLAG_USER_COMMAND": 0,
+		"FLAG_USER_COMMAND":     0,
+		"FLAG_ENCRYPT_RESPONSE": 1,
 	}
 )
 
@@ -280,7 +283,6 @@ type Destination struct {
 	unknownFields protoimpl.UnknownFields
 
 	// Types that are assignable to SubDestination:
-	//
 	//	*Destination_Domain
 	//	*Destination_RoutingAddress
 	SubDestination isDestination_SubDestination `protobuf_oneof:"sub_destination"`
@@ -473,13 +475,11 @@ type RoutableMessage struct {
 	ToDestination   *Destination `protobuf:"bytes,6,opt,name=to_destination,json=toDestination,proto3" json:"to_destination,omitempty"`
 	FromDestination *Destination `protobuf:"bytes,7,opt,name=from_destination,json=fromDestination,proto3" json:"from_destination,omitempty"`
 	// Types that are assignable to Payload:
-	//
 	//	*RoutableMessage_ProtobufMessageAsBytes
 	//	*RoutableMessage_SessionInfoRequest
 	//	*RoutableMessage_SessionInfo
 	Payload isRoutableMessage_Payload `protobuf_oneof:"payload"`
 	// Types that are assignable to SubSigData:
-	//
 	//	*RoutableMessage_SignatureData
 	SubSigData          isRoutableMessage_SubSigData `protobuf_oneof:"sub_sigData"`
 	SignedMessageStatus *MessageStatus               `protobuf:"bytes,12,opt,name=signedMessageStatus,proto3" json:"signedMessageStatus,omitempty"`
@@ -782,17 +782,18 @@ var file_universal_message_proto_rawDesc = []byte{
 	0x46, 0x41, 0x55, 0x4c, 0x54, 0x5f, 0x45, 0x52, 0x52, 0x4f, 0x52, 0x5f, 0x43, 0x4f, 0x4d, 0x4d,
 	0x41, 0x4e, 0x44, 0x5f, 0x52, 0x45, 0x51, 0x55, 0x49, 0x52, 0x45, 0x53, 0x5f, 0x41, 0x43, 0x43,
 	0x4f, 0x55, 0x4e, 0x54, 0x5f, 0x43, 0x52, 0x45, 0x44, 0x45, 0x4e, 0x54, 0x49, 0x41, 0x4c, 0x53,
-	0x10, 0x17, 0x2a, 0x1e, 0x0a, 0x05, 0x46, 0x6c, 0x61, 0x67, 0x73, 0x12, 0x15, 0x0a, 0x11, 0x46,
+	0x10, 0x17, 0x2a, 0x39, 0x0a, 0x05, 0x46, 0x6c, 0x61, 0x67, 0x73, 0x12, 0x15, 0x0a, 0x11, 0x46,
 	0x4c, 0x41, 0x47, 0x5f, 0x55, 0x53, 0x45, 0x52, 0x5f, 0x43, 0x4f, 0x4d, 0x4d, 0x41, 0x4e, 0x44,
-	0x10, 0x00, 0x42, 0x75, 0x0a, 0x24, 0x63, 0x6f, 0x6d, 0x2e, 0x74, 0x65, 0x73, 0x6c, 0x61, 0x2e,
-	0x67, 0x65, 0x6e, 0x65, 0x72, 0x61, 0x74, 0x65, 0x64, 0x2e, 0x75, 0x6e, 0x69, 0x76, 0x65, 0x72,
-	0x73, 0x61, 0x6c, 0x6d, 0x65, 0x73, 0x73, 0x61, 0x67, 0x65, 0x5a, 0x4d, 0x67, 0x69, 0x74, 0x68,
-	0x75, 0x62, 0x2e, 0x63, 0x6f, 0x6d, 0x2f, 0x74, 0x65, 0x73, 0x6c, 0x61, 0x6d, 0x6f, 0x74, 0x6f,
-	0x72, 0x73, 0x2f, 0x76, 0x65, 0x68, 0x69, 0x63, 0x6c, 0x65, 0x2d, 0x63, 0x6f, 0x6d, 0x6d, 0x61,
-	0x6e, 0x64, 0x2f, 0x70, 0x6b, 0x67, 0x2f, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x63, 0x6f, 0x6c, 0x2f,
-	0x70, 0x72, 0x6f, 0x74, 0x6f, 0x62, 0x75, 0x66, 0x2f, 0x75, 0x6e, 0x69, 0x76, 0x65, 0x72, 0x73,
-	0x61, 0x6c, 0x6d, 0x65, 0x73, 0x73, 0x61, 0x67, 0x65, 0x62, 0x06, 0x70, 0x72, 0x6f, 0x74, 0x6f,
-	0x33,
+	0x10, 0x00, 0x12, 0x19, 0x0a, 0x15, 0x46, 0x4c, 0x41, 0x47, 0x5f, 0x45, 0x4e, 0x43, 0x52, 0x59,
+	0x50, 0x54, 0x5f, 0x52, 0x45, 0x53, 0x50, 0x4f, 0x4e, 0x53, 0x45, 0x10, 0x01, 0x42, 0x75, 0x0a,
+	0x24, 0x63, 0x6f, 0x6d, 0x2e, 0x74, 0x65, 0x73, 0x6c, 0x61, 0x2e, 0x67, 0x65, 0x6e, 0x65, 0x72,
+	0x61, 0x74, 0x65, 0x64, 0x2e, 0x75, 0x6e, 0x69, 0x76, 0x65, 0x72, 0x73, 0x61, 0x6c, 0x6d, 0x65,
+	0x73, 0x73, 0x61, 0x67, 0x65, 0x5a, 0x4d, 0x67, 0x69, 0x74, 0x68, 0x75, 0x62, 0x2e, 0x63, 0x6f,
+	0x6d, 0x2f, 0x74, 0x65, 0x73, 0x6c, 0x61, 0x6d, 0x6f, 0x74, 0x6f, 0x72, 0x73, 0x2f, 0x76, 0x65,
+	0x68, 0x69, 0x63, 0x6c, 0x65, 0x2d, 0x63, 0x6f, 0x6d, 0x6d, 0x61, 0x6e, 0x64, 0x2f, 0x70, 0x6b,
+	0x67, 0x2f, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x63, 0x6f, 0x6c, 0x2f, 0x70, 0x72, 0x6f, 0x74, 0x6f,
+	0x62, 0x75, 0x66, 0x2f, 0x75, 0x6e, 0x69, 0x76, 0x65, 0x72, 0x73, 0x61, 0x6c, 0x6d, 0x65, 0x73,
+	0x73, 0x61, 0x67, 0x65, 0x62, 0x06, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x33,
 }
 
 var (
diff --git a/pkg/protocol/protobuf/vcsec/vcsec.pb.go b/pkg/protocol/protobuf/vcsec/vcsec.pb.go
index 5585b30..a0d7abf 100644
--- a/pkg/protocol/protobuf/vcsec/vcsec.pb.go
+++ b/pkg/protocol/protobuf/vcsec/vcsec.pb.go
@@ -1143,7 +1143,6 @@ type InformationRequest struct {
 
 	InformationRequestType InformationRequestType `protobuf:"varint,1,opt,name=informationRequestType,proto3,enum=VCSEC.InformationRequestType" json:"informationRequestType,omitempty"`
 	// Types that are assignable to Key:
-	//
 	//	*InformationRequest_KeyId
 	//	*InformationRequest_PublicKey
 	//	*InformationRequest_Slot
@@ -1411,7 +1410,6 @@ type ReplaceKey struct {
 	unknownFields protoimpl.UnknownFields
 
 	// Types that are assignable to KeyToReplace:
-	//
 	//	*ReplaceKey_PublicKeyToReplace
 	//	*ReplaceKey_SlotToReplace
 	KeyToReplace isReplaceKey_KeyToReplace `protobuf_oneof:"keyToReplace"`
@@ -1516,7 +1514,6 @@ type WhitelistOperation struct {
 	unknownFields protoimpl.UnknownFields
 
 	// Types that are assignable to SubMessage:
-	//
 	//	*WhitelistOperation_AddPublicKeyToWhitelist
 	//	*WhitelistOperation_RemovePublicKeyFromWhitelist
 	//	*WhitelistOperation_AddPermissionsToPublicKey
@@ -1836,7 +1833,6 @@ type CommandStatus struct {
 
 	OperationStatus OperationStatus_E `protobuf:"varint,1,opt,name=operationStatus,proto3,enum=VCSEC.OperationStatus_E" json:"operationStatus,omitempty"`
 	// Types that are assignable to SubMessage:
-	//
 	//	*CommandStatus_SignedMessageStatus
 	//	*CommandStatus_WhitelistOperationStatus
 	SubMessage isCommandStatus_SubMessage `protobuf_oneof:"sub_message"`
@@ -1924,7 +1920,6 @@ type UnsignedMessage struct {
 	unknownFields protoimpl.UnknownFields
 
 	// Types that are assignable to SubMessage:
-	//
 	//	*UnsignedMessage_InformationRequest
 	//	*UnsignedMessage_RKEAction
 	//	*UnsignedMessage_ClosureMoveRequest
@@ -2262,7 +2257,6 @@ type FromVCSECMessage struct {
 	unknownFields protoimpl.UnknownFields
 
 	// Types that are assignable to SubMessage:
-	//
 	//	*FromVCSECMessage_VehicleStatus
 	//	*FromVCSECMessage_CommandStatus
 	//	*FromVCSECMessage_WhitelistInfo
diff --git a/pkg/protocol/protocol.md b/pkg/protocol/protocol.md
index 5009d85..a3c0069 100644
--- a/pkg/protocol/protocol.md
+++ b/pkg/protocol/protocol.md
@@ -59,10 +59,10 @@ above are for printability.
  * The vehicle sets `signedMessageStatus` to indicate protocol-layer errors.
    [Application-layer errors](#Response-handling) appear in
    `protobuf_message_as_bytes`.
- * The `flags` field is a bit mask of `universal_message.Flags` values. There
-   is currently no need for clients to use this field; future versions of the
-   protocol may use this field to add authenticated data that is discarded by
-   older vehicles.
+ * The `flags` field is a bit mask of `universal_message.Flags` values.
+   Vehicles authenticate this value, but ignore unrecognized bits. Clients
+   should always set the `FLAG_ENCRYPT_RESPONSE` bit, which instructs vehicles
+   with compatible firmware (2024.38+) to encrypt the response.
 
 ## Decoding messages
 
@@ -275,7 +275,7 @@ in the remainder of the document.
 
 *Do not use this key except to debug implementations using the test vectors
 included in these documents. Since the private key is published, enrolling the
-public key on a vehicle may result in unauthorized access.
+public key on a vehicle may result in unauthorized access.*
 
 In `vehicle.key`:
 
@@ -464,7 +464,7 @@ fceb679ee7bca756fcd441bf238bf2f338629b41d9eb9c67be1b32c9672ce300
 
 ---
 
-Next, the client [serializes the following metadata](#Metadata serialization):
+Next, the client [serializes the following metadata](#Metadata-serialization):
 
  * `TAG_SIGNATURE_TYPE`: `Signatures.SIGNATURE_TYPE_HMAC`
  * `TAG_PERSONALIZATION`: `VIN`
@@ -534,6 +534,7 @@ The client serializes the following metadata values into a string `M`:
 | Epoch          | `Signatures.TAG_EPOCH`          | Copied from `session_info.epoch`           |
 | Expiration time| `Signatures.TAG_EXPIRES_AT`     | Time in seconds according to domain's clock |
 | Counter        | `Signatures.TAG_COUNTER`        | Monotonic counter, initially `session_info.counter` |
+| Flags          | `Signatures.TAG_FLAGS`          | Request flag bit mask. Only included if non-zero. |
 
 Setting the expiration time requires the client to track the difference between
 the domain's clock and the local clock. Note that each domain has its own clock.
@@ -731,6 +732,65 @@ Error codes and their remediation are summarized in
 [universal_message.proto](protobuf/universal_message.proto).
 See comments in the `MessageFault_E` definition.
 
+### Response decryption
+
+If the client set the `FLAG_ENCRYPT_RESPONSE`, then vehicles running supported
+firmware (2024.38+) will encrypt the payload. Vehicles running previous
+firmware versions will ignore this flag, so clients do not need to check the
+vehicle's firmware version before setting it.
+
+If the response includes a `signature_data.AES_GCM_Response_data` field, then
+the `protobuf_message_as_bytes` payload is encrypted. Otherwise, the payload is
+plaintext.
+
+#### Request hash
+
+The client must compute the "request hash" of each request it sends. This value
+will be used when decrypting the response. The request hash is a single byte
+that encodes the authentication method used by the request (either
+`Signatures.SIGNATURE_TYPE_HMAC_PERSONALIZED` or
+`Signatures.SIGNATURE_TYPE_AES_GCM_PERSONALIZED`) followed by the request's
+authentication tag.
+
+ * If the request uses AES-GCM encryption, then the tag is the request's
+   `signature_data.AES_GCM_Personalized_Signature_Data.tag` field.
+ * If the request uses HMAC-SHA256 authentication, then the tag is the
+   request's `signature_data.HMAC_PersonalizedData.tag` field.
+
+If the request is sent to the Vehicle Security domain, then the request hash is
+truncated to 17 bytes (i.e., a single byte encoding the authentication method
+and the first 16 bytes of the MAC). Truncation only affects HMAC-SHA256
+requests, since AES-GCM tags are only 16 bytes.
+
+#### Counter verification
+
+The client must verify that the counter value contained in the response has
+not been previously used in a response for the same request. Note that if a
+request does trigger multiple responses, responses may be received out of
+order.
+
+Clients that fail to implement this check are vulnerable to replay attacks.
+
+#### Response metadata
+
+To decrypt the response, use AES-GCM with the shared key `K` defined above, the
+nonce and tag supplied in the response, and the associated authenticated data
+field obtained by [serializing](#Metadata serialization) the following metadata:
+
+| Value | Tag | Description |
+| ----- | --- | ----------- |
+| Signature type | `Signatures.TAG_SIGNATURE_TYPE` | `Signatures.SIGNATURE_TYPE_AES_GCM_RESPONSE` |
+| Domain         | `Signatures.TAG_DOMAIN`         | Typically `UniversalMesasge.DOMAIN_VEHICLE_SECURITY` or `UniversalMessage.DOMAIN_INFOTAINMENT` |
+| VIN            | `Signatures.TAG_PERSONALIZATION`| 17-character vehicle identification number |
+| Counter        | `Signatures.TAG_COUNTER`        | Monotonic counter, initially `session_info.counter` |
+| Flags          | `Signatures.TAG_FLAGS`          | Flags field of the response, not the request |
+| Request Hash   | `Signatures.TAG_REQUEST_HASH`   | See [Request hash](#request-hash) |
+| Fault          | `Signatures.TAG_FAULT`          | Message fault of the response |
+
+Note that the Flags field is always included in the response metadata, whereas
+for outgoing requests the flags are only included if non-zero. Encode the fault
+as a 32-bit big-endian integer.
+
 ### Infotainment application-layer responses
 
 If a reply comes from the Infotainment domain, the client should parse the
diff --git a/pkg/vehicle/vehicle.go b/pkg/vehicle/vehicle.go
index dc23c80..49c051f 100644
--- a/pkg/vehicle/vehicle.go
+++ b/pkg/vehicle/vehicle.go
@@ -18,6 +18,9 @@ import (
 	universal "github.com/teslamotors/vehicle-command/pkg/protocol/protobuf/universalmessage"
 )
 
+// DefaultFlags is a bitmask that controls what flags are set on requests.
+var DefaultFlags = uint32(1 << universal.Flags_FLAG_ENCRYPT_RESPONSE)
+
 var (
 	// ErrNoFleetAPIConnection indicates the client attempted to send a command that terminates on
 	// Tesla's backend (rather than a vehicle), but the Vehicle Connection does not use connector/inet.
@@ -59,8 +62,9 @@ type sender interface {
 
 // A Vehicle represents a Tesla vehicle.
 type Vehicle struct {
+	Flags uint32
+
 	dispatcher sender
-	Flags      uint32
 	vin        string
 
 	conn       connector.Connector
@@ -77,6 +81,7 @@ func NewVehicle(conn connector.Connector, privateKey authentication.ECDHPrivateK
 	}
 	vin := conn.VIN()
 	vehicle := &Vehicle{
+		Flags:        DefaultFlags,
 		dispatcher:   dispatch,
 		vin:          vin,
 		conn:         conn,