Encrypt TCP tunnel payload with AES-GCM (#6)

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,

internal/client/forward_test.go

@@ -0,0 +1,120 @@
+package client
+
+import (
+	"bytes"
+	"encoding/binary"
+	"io"
+	"net"
+	"testing"
+	"time"
+
+	"teleport/pkg/config"
+	"teleport/pkg/encryption"
+)
+
+func testClient(t *testing.T) *TeleportClient {
+	t.Helper()
+	return NewTeleportClient(&config.Config{
+		EncryptionKey: "a0e3dd20a761b118ca234160dd8b87230a001e332a97c9cfe3b8b9c99efaae03",
+	})
+}
+
+func TestForwardDataHidesHTTPPlaintextOnTunnel(t *testing.T) {
+	tc := testClient(t)
+	defer tc.Stop()
+
+	localA, localB := net.Pipe()
+	tunnelA, tunnelB := net.Pipe()
+	defer localA.Close()
+	defer localB.Close()
+	defer tunnelA.Close()
+	defer tunnelB.Close()
+
+	go tc.forwardData(localA, tunnelA, true)
+
+	req := []byte("GET /secret HTTP/1.1\r\nHost: example.com\r\n\r\n")
+	writeErr := make(chan error, 1)
+	go func() {
+		_, err := localB.Write(req)
+		writeErr <- err
+	}()
+
+	tunnelB.SetReadDeadline(time.Now().Add(5 * time.Second))
+	var length uint32
+	if err := binary.Read(tunnelB, binary.BigEndian, &length); err != nil {
+		t.Fatalf("failed to read frame length from tunnel: %v", err)
+	}
+	if length == 0 || length > encryption.MaxFrameSize {
+		t.Fatalf("unexpected frame length %d", length)
+	}
+	ciphertext := make([]byte, length)
+	if _, err := io.ReadFull(tunnelB, ciphertext); err != nil {
+		t.Fatalf("failed to read ciphertext from tunnel: %v", err)
+	}
+
+	if bytes.Contains(ciphertext, []byte("GET /")) ||
+		bytes.Contains(ciphertext, []byte("Host:")) ||
+		bytes.Contains(ciphertext, []byte("example.com")) {
+		t.Fatal("HTTP plaintext visible on client->server tunnel")
+	}
+
+	var frame bytes.Buffer
+	if err := binary.Write(&frame, binary.BigEndian, length); err != nil {
+		t.Fatal(err)
+	}
+	frame.Write(ciphertext)
+	plain, err := encryption.ReadEncryptedFrame(&frame, tc.derivedKey)
+	if err != nil {
+		t.Fatalf("decrypt of tunneled frame failed: %v", err)
+	}
+	if !bytes.Equal(plain, req) {
+		t.Errorf("decrypted payload mismatch: got %q want %q", plain, req)
+	}
+
+	select {
+	case err := <-writeErr:
+		if err != nil {
+			t.Fatalf("write to local conn failed: %v", err)
+		}
+	case <-time.After(5 * time.Second):
+		t.Fatal("timed out waiting for local write")
+	}
+}
+
+func TestForwardDataDecryptsTunnelToLocal(t *testing.T) {
+	tc := testClient(t)
+	defer tc.Stop()
+
+	tunnelA, tunnelB := net.Pipe()
+	localA, localB := net.Pipe()
+	defer tunnelA.Close()
+	defer tunnelB.Close()
+	defer localA.Close()
+	defer localB.Close()
+
+	go tc.forwardData(tunnelA, localA, false)
+
+	payload := []byte("HTTP/1.1 200 OK\r\nContent-Length: 5\r\n\r\nhello")
+	writeErr := make(chan error, 1)
+	go func() {
+		writeErr <- encryption.WriteEncryptedFrame(tunnelB, payload, tc.derivedKey)
+	}()
+
+	localB.SetReadDeadline(time.Now().Add(5 * time.Second))
+	got := make([]byte, len(payload))
+	if _, err := io.ReadFull(localB, got); err != nil {
+		t.Fatalf("failed to read decrypted payload: %v", err)
+	}
+	if !bytes.Equal(got, payload) {
+		t.Errorf("got %q want %q", got, payload)
+	}
+
+	select {
+	case err := <-writeErr:
+		if err != nil {
+			t.Fatalf("WriteEncryptedFrame failed: %v", err)
+		}
+	case <-time.After(5 * time.Second):
+		t.Fatal("timed out waiting for tunnel write")
+	}
+}