Encrypt TCP tunnel payload with AES-GCM.

TCP forwardData copied plaintext after the handshake. Frame each chunk as uint32 length plus AES-GCM ciphertext in both client and server, both directions.

internal/client/client.go

@@ -28,6 +28,7 @@ type TeleportClient struct {
 	cancel           context.CancelFunc
 	connectionPool   chan net.Conn
 	maxPoolSize      int
+	derivedKey       []byte
 }
 
 // NewTeleportClient creates a new teleport client
@@ -49,6 +50,7 @@ func NewTeleportClient(config *config.Config) *TeleportClient {
 		cancel:           cancel,
 		connectionPool:   make(chan net.Conn, maxPoolSize),
 		maxPoolSize:      maxPoolSize,
+		derivedKey:       encryption.DeriveKey(config.EncryptionKey),
 	}
 }
 
@@ -650,23 +652,34 @@ func (tc *TeleportClient) handleTCPConnection(clientConn net.Conn, rule config.P
 	var wg sync.WaitGroup
 	wg.Add(2)
 
-	// Forward data from client to server
+	// Encrypt local plaintext onto the tunnel
 	go func() {
 		defer wg.Done()
-		tc.forwardData(clientConn, serverConn)
+		tc.forwardData(clientConn, serverConn, true)
 	}()
 
-	// Forward data from server to client
+	// Decrypt tunnel frames to the local connection
 	go func() {
 		defer wg.Done()
-		tc.forwardData(serverConn, clientConn)
+		tc.forwardData(serverConn, clientConn, false)
 	}()
 
 	wg.Wait()
 }
 
-// forwardData forwards data from src to dst
-func (tc *TeleportClient) forwardData(src, dst net.Conn) {
+// forwardData copies between a local/plaintext conn and the tunnel.
+// When toTunnel is true, plaintext is AES-GCM encrypted and written as
+// uint32 length + ciphertext. When false, framed ciphertext is decrypted
+// and written as plaintext.
+func (tc *TeleportClient) forwardData(src, dst net.Conn, toTunnel bool) {
+	if toTunnel {
+		tc.forwardPlainToTunnel(src, dst)
+		return
+	}
+	tc.forwardTunnelToPlain(src, dst)
+}
+
+func (tc *TeleportClient) forwardPlainToTunnel(src, dst net.Conn) {
 	buffer := make([]byte, 4096)
 	for {
 		select {
@@ -675,14 +688,32 @@ func (tc *TeleportClient) forwardData(src, dst net.Conn) {
 		default:
 			n, err := src.Read(buffer)
 			if err != nil {
-				// Close the destination connection when source closes
 				dst.Close()
 				return
 			}
+			if n == 0 {
+				continue
+			}
+			if err := encryption.WriteEncryptedFrame(dst, buffer[:n], tc.derivedKey); err != nil {
+				src.Close()
+				return
+			}
+		}
+	}
+}
 
-			_, err = dst.Write(buffer[:n])
+func (tc *TeleportClient) forwardTunnelToPlain(src, dst net.Conn) {
+	for {
+		select {
+		case <-tc.ctx.Done():
+			return
+		default:
+			plain, err := encryption.ReadEncryptedFrame(src, tc.derivedKey)
 			if err != nil {
-				// Close the source connection when destination closes
+				dst.Close()
+				return
+			}
+			if _, err := dst.Write(plain); err != nil {
 				src.Close()
 				return
 			}
@@ -703,9 +734,8 @@ func (tc *TeleportClient) sendRequestToConnection(conn net.Conn, request types.P
 		return err
 	}
 
-	// Encrypt the data
-	key := encryption.DeriveKey(tc.config.EncryptionKey)
-	encryptedData, err := encryption.EncryptData(data, key)
+	// Encrypt the data (key derived once at process start)
+	encryptedData, err := encryption.EncryptData(data, tc.derivedKey)
 	if err != nil {
 		logger.WithFields(map[string]interface{}{
 			"error":       err,