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,

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")
+	}
+}

internal/server/forward_test.go

@@ -0,0 +1,118 @@
+package server
+
+import (
+	"bytes"
+	"encoding/binary"
+	"io"
+	"net"
+	"testing"
+	"time"
+
+	"teleport/pkg/config"
+	"teleport/pkg/encryption"
+)
+
+func testServer(t *testing.T) *TeleportServer {
+	t.Helper()
+	return NewTeleportServer(&config.Config{
+		EncryptionKey: "a0e3dd20a761b118ca234160dd8b87230a001e332a97c9cfe3b8b9c99efaae03",
+	})
+}
+
+func TestForwardDataHidesHTTPPlaintextOnTunnel(t *testing.T) {
+	ts := testServer(t)
+	defer ts.Stop()
+
+	targetA, targetB := net.Pipe()
+	tunnelA, tunnelB := net.Pipe()
+	defer targetA.Close()
+	defer targetB.Close()
+	defer tunnelA.Close()
+	defer tunnelB.Close()
+
+	// target -> tunnel (encrypt)
+	go ts.forwardData(targetA, tunnelA, true)
+
+	resp := []byte("HTTP/1.1 200 OK\r\nContent-Type: text/plain\r\n\r\nhello")
+	writeErr := make(chan error, 1)
+	go func() {
+		_, err := targetB.Write(resp)
+		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)
+	}
+	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("HTTP/1.1")) ||
+		bytes.Contains(ciphertext, []byte("Content-Type")) ||
+		bytes.Contains(ciphertext, []byte("hello")) {
+		t.Fatal("HTTP plaintext visible on server->client 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, ts.derivedKey)
+	if err != nil {
+		t.Fatalf("decrypt of tunneled frame failed: %v", err)
+	}
+	if !bytes.Equal(plain, resp) {
+		t.Errorf("decrypted payload mismatch: got %q want %q", plain, resp)
+	}
+
+	select {
+	case err := <-writeErr:
+		if err != nil {
+			t.Fatalf("write to target conn failed: %v", err)
+		}
+	case <-time.After(5 * time.Second):
+		t.Fatal("timed out waiting for target write")
+	}
+}
+
+func TestForwardDataDecryptsTunnelToTarget(t *testing.T) {
+	ts := testServer(t)
+	defer ts.Stop()
+
+	tunnelA, tunnelB := net.Pipe()
+	targetA, targetB := net.Pipe()
+	defer tunnelA.Close()
+	defer tunnelB.Close()
+	defer targetA.Close()
+	defer targetB.Close()
+
+	go ts.forwardData(tunnelA, targetA, false)
+
+	req := []byte("GET / HTTP/1.1\r\nHost: example.com\r\n\r\n")
+	writeErr := make(chan error, 1)
+	go func() {
+		writeErr <- encryption.WriteEncryptedFrame(tunnelB, req, ts.derivedKey)
+	}()
+
+	targetB.SetReadDeadline(time.Now().Add(5 * time.Second))
+	got := make([]byte, len(req))
+	if _, err := io.ReadFull(targetB, got); err != nil {
+		t.Fatalf("failed to read decrypted payload: %v", err)
+	}
+	if !bytes.Equal(got, req) {
+		t.Errorf("got %q want %q", got, req)
+	}
+
+	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")
+	}
+}

internal/server/server.go

@@ -35,6 +35,7 @@ type TeleportServer struct {
 	goroutineSem      chan struct{} // Semaphore for limiting concurrent goroutines
 	maxGoroutines     int           // Maximum concurrent goroutines
 	metrics           *metrics.Metrics
+	derivedKey        []byte
 }
 
 // NewTeleportServer creates a new teleport server
@@ -76,6 +77,7 @@ func NewTeleportServer(config *config.Config) *TeleportServer {
 		goroutineSem:     make(chan struct{}, maxGoroutines),
 		maxGoroutines:    maxGoroutines,
 		metrics:          metricsInstance,
+		derivedKey:       encryption.DeriveKey(config.EncryptionKey),
 	}
 }
 
@@ -403,12 +405,12 @@ func (ts *TeleportServer) handleTCPForward(clientConn net.Conn, rule *config.Por
 
 	go func() {
 		defer wg.Done()
-		ts.forwardData(clientConn, targetConn)
+		ts.forwardData(clientConn, targetConn, false) // decrypt from tunnel
 	}()
 
 	go func() {
 		defer wg.Done()
-		ts.forwardData(targetConn, clientConn)
+		ts.forwardData(targetConn, clientConn, true) // encrypt onto tunnel
 	}()
 
 	wg.Wait()
@@ -704,8 +706,19 @@ func (ts *TeleportServer) deserializeTaggedUDPPacket(data []byte) (types.TaggedU
 	}, nil
 }
 
-// forwardData forwards data between two connections
-func (ts *TeleportServer) forwardData(src, dst net.Conn) {
+// forwardData copies between a local/target 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 (ts *TeleportServer) forwardData(src, dst net.Conn, toTunnel bool) {
+	if toTunnel {
+		ts.forwardPlainToTunnel(src, dst)
+		return
+	}
+	ts.forwardTunnelToPlain(src, dst)
+}
+
+func (ts *TeleportServer) forwardPlainToTunnel(src, dst net.Conn) {
 	buffer := make([]byte, 4096)
 	for {
 		select {
@@ -714,14 +727,32 @@ func (ts *TeleportServer) 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], ts.derivedKey); err != nil {
+				src.Close()
+				return
+			}
+		}
+	}
+}
 
-			_, err = dst.Write(buffer[:n])
+func (ts *TeleportServer) forwardTunnelToPlain(src, dst net.Conn) {
+	for {
+		select {
+		case <-ts.ctx.Done():
+			return
+		default:
+			plain, err := encryption.ReadEncryptedFrame(src, ts.derivedKey)
 			if err != nil {
-				// Close the source connection when destination closes
+				dst.Close()
+				return
+			}
+			if _, err := dst.Write(plain); err != nil {
 				src.Close()
 				return
 			}
@@ -757,9 +788,8 @@ func (ts *TeleportServer) readRequest(conn net.Conn, request *types.PortForwardR
 		bytesRead += n
 	}
 
-	// Decrypt the data
-	key := encryption.DeriveKey(ts.config.EncryptionKey)
-	decryptedData, err := encryption.DecryptData(encryptedData, key)
+	// Decrypt the data (key derived once at process start)
+	decryptedData, err := encryption.DecryptData(encryptedData, ts.derivedKey)
 	if err != nil {
 		logger.WithFields(map[string]interface{}{
 			"error":                 err,

pkg/encryption/frame.go

@@ -0,0 +1,68 @@
+package encryption
+
+import (
+	"encoding/binary"
+	"fmt"
+	"io"
+)
+
+// MaxFrameSize is the maximum AES-GCM ciphertext length accepted on a framed
+// TCP tunnel (uint32 length prefix + ciphertext).
+const MaxFrameSize = 1024 * 1024 // 1 MiB
+
+// WriteEncryptedFrame encrypts plaintext with AES-GCM (EncryptData) and writes
+// a uint32 big-endian length prefix followed by the ciphertext.
+func WriteEncryptedFrame(w io.Writer, plaintext, key []byte) error {
+	ciphertext, err := EncryptData(plaintext, key)
+	if err != nil {
+		return err
+	}
+	if len(ciphertext) == 0 {
+		return fmt.Errorf("encrypted data cannot be empty")
+	}
+	if len(ciphertext) > MaxFrameSize {
+		return fmt.Errorf("frame too large: %d bytes (max %d)", len(ciphertext), MaxFrameSize)
+	}
+
+	var length [4]byte
+	binary.BigEndian.PutUint32(length[:], uint32(len(ciphertext)))
+	if err := writeFull(w, length[:]); err != nil {
+		return err
+	}
+	return writeFull(w, ciphertext)
+}
+
+// ReadEncryptedFrame reads a uint32 big-endian length, caps it, reads the
+// ciphertext, and decrypts it with AES-GCM (DecryptData).
+func ReadEncryptedFrame(r io.Reader, key []byte) ([]byte, error) {
+	var length uint32
+	if err := binary.Read(r, binary.BigEndian, &length); err != nil {
+		return nil, err
+	}
+	if length == 0 {
+		return nil, fmt.Errorf("frame length cannot be zero")
+	}
+	if length > MaxFrameSize {
+		return nil, fmt.Errorf("frame too large: %d bytes (max %d)", length, MaxFrameSize)
+	}
+
+	ciphertext := make([]byte, length)
+	if _, err := io.ReadFull(r, ciphertext); err != nil {
+		return nil, fmt.Errorf("failed to read frame data: %v", err)
+	}
+	return DecryptData(ciphertext, key)
+}
+
+func writeFull(w io.Writer, data []byte) error {
+	for len(data) > 0 {
+		n, err := w.Write(data)
+		if err != nil {
+			return err
+		}
+		if n == 0 {
+			return io.ErrShortWrite
+		}
+		data = data[n:]
+	}
+	return nil
+}

pkg/encryption/frame_test.go

@@ -0,0 +1,136 @@
+package encryption
+
+import (
+	"bytes"
+	"encoding/binary"
+	"io"
+	"strings"
+	"testing"
+)
+
+func testKey(t *testing.T) []byte {
+	t.Helper()
+	return DeriveKey("test-encryption-key-for-tcp-frames")
+}
+
+func TestEncryptedFrameRoundTrip(t *testing.T) {
+	key := testKey(t)
+	original := []byte("Hello, framed TCP tunnel!")
+
+	var buf bytes.Buffer
+	if err := WriteEncryptedFrame(&buf, original, key); err != nil {
+		t.Fatalf("WriteEncryptedFrame failed: %v", err)
+	}
+
+	got, err := ReadEncryptedFrame(&buf, key)
+	if err != nil {
+		t.Fatalf("ReadEncryptedFrame failed: %v", err)
+	}
+	if !bytes.Equal(got, original) {
+		t.Errorf("round trip mismatch: got %q want %q", got, original)
+	}
+}
+
+func TestEncryptedFrameMultipleChunks(t *testing.T) {
+	key := testKey(t)
+	chunks := [][]byte{
+		[]byte("chunk-one"),
+		[]byte("chunk-two-is-longer"),
+		[]byte{0x00, 0x01, 0xff},
+	}
+
+	var buf bytes.Buffer
+	for _, c := range chunks {
+		if err := WriteEncryptedFrame(&buf, c, key); err != nil {
+			t.Fatalf("WriteEncryptedFrame failed: %v", err)
+		}
+	}
+
+	for i, want := range chunks {
+		got, err := ReadEncryptedFrame(&buf, key)
+		if err != nil {
+			t.Fatalf("ReadEncryptedFrame chunk %d failed: %v", i, err)
+		}
+		if !bytes.Equal(got, want) {
+			t.Errorf("chunk %d mismatch: got %q want %q", i, got, want)
+		}
+	}
+}
+
+func TestEncryptedFrameRejectsZeroLength(t *testing.T) {
+	key := testKey(t)
+	var buf bytes.Buffer
+	if err := binary.Write(&buf, binary.BigEndian, uint32(0)); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := ReadEncryptedFrame(&buf, key); err == nil {
+		t.Fatal("expected error for zero-length frame")
+	}
+}
+
+func TestEncryptedFrameRejectsOversizeLength(t *testing.T) {
+	key := testKey(t)
+	var buf bytes.Buffer
+	if err := binary.Write(&buf, binary.BigEndian, uint32(MaxFrameSize+1)); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := ReadEncryptedFrame(&buf, key); err == nil {
+		t.Fatal("expected error for oversize frame length")
+	} else if !strings.Contains(err.Error(), "too large") {
+		t.Errorf("expected too-large error, got %v", err)
+	}
+}
+
+func TestEncryptedFrameWrongKey(t *testing.T) {
+	key1 := DeriveKey("tcp-frame-key-1")
+	key2 := DeriveKey("tcp-frame-key-2")
+
+	var buf bytes.Buffer
+	if err := WriteEncryptedFrame(&buf, []byte("secret"), key1); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := ReadEncryptedFrame(&buf, key2); err == nil {
+		t.Fatal("decrypting with the wrong key should fail")
+	}
+}
+
+func TestHTTPPlaintextNotVisibleOnFramedTunnel(t *testing.T) {
+	key := testKey(t)
+	req := []byte("GET / HTTP/1.1\r\nHost: example.com\r\n\r\n")
+
+	var wire bytes.Buffer
+	if err := WriteEncryptedFrame(&wire, req, key); err != nil {
+		t.Fatalf("WriteEncryptedFrame failed: %v", err)
+	}
+	sniffed := wire.Bytes()
+
+	if bytes.Contains(sniffed, []byte("GET /")) {
+		t.Fatal("sniffer saw HTTP request line on the tunnel")
+	}
+	if bytes.Contains(sniffed, []byte("Host:")) {
+		t.Fatal("sniffer saw HTTP Host header on the tunnel")
+	}
+	if bytes.Contains(sniffed, []byte("example.com")) {
+		t.Fatal("sniffer saw HTTP hostname on the tunnel")
+	}
+
+	// Length prefix is 4 bytes; ciphertext must be longer than plaintext.
+	if len(sniffed) < 4+len(req) {
+		t.Fatalf("framed ciphertext too short: %d", len(sniffed))
+	}
+
+	got, err := ReadEncryptedFrame(bytes.NewReader(sniffed), key)
+	if err != nil {
+		t.Fatalf("decrypt failed: %v", err)
+	}
+	if !bytes.Equal(got, req) {
+		t.Errorf("decrypted HTTP mismatch: got %q", got)
+	}
+}
+
+func TestReadEncryptedFrameEOF(t *testing.T) {
+	key := testKey(t)
+	if _, err := ReadEncryptedFrame(bytes.NewReader(nil), key); err != io.EOF {
+		t.Errorf("expected io.EOF, got %v", err)
+	}
+}