split SNI and ECH extensions in the ClientHello (#5107)

* create a new type for crypto stream used for Initial data

This currently the exact same implementation as the other
streams, thus no functional change is expected.

* handshake: implement a function to find the SNI and the ECH extension

* move the SNI parsing logic to the quic package

* implement splitting logic

* generalize cutting logic

* introduce QUIC_GO_DISABLE_CLIENTHELLO_SCRAMBLING

* improve testing

sni_test.go

@@ -0,0 +1,81 @@
+package quic
+
+import (
+	"context"
+	"crypto/rand"
+	"crypto/tls"
+	"io"
+	mrand "math/rand/v2"
+	"testing"
+
+	"github.com/quic-go/quic-go/internal/testdata"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func checkClientHello(t testing.TB, clientHello []byte) {
+	t.Helper()
+
+	conn := tls.QUICServer(&tls.QUICConfig{
+		TLSConfig: testdata.GetTLSConfig(),
+	})
+	require.NoError(t, conn.Start(context.Background()))
+	defer conn.Close()
+	require.NoError(t, conn.HandleData(tls.QUICEncryptionLevelInitial, clientHello))
+}
+
+func getClientHello(t testing.TB, serverName string) []byte {
+	t.Helper()
+
+	c := tls.QUICClient(&tls.QUICConfig{
+		TLSConfig: &tls.Config{
+			ServerName:         serverName,
+			MinVersion:         tls.VersionTLS13,
+			InsecureSkipVerify: serverName == "",
+			// disable post-quantum curves
+			CurvePreferences: []tls.CurveID{tls.CurveP256},
+		},
+	})
+	b := make([]byte, mrand.IntN(200))
+	rand.Read(b)
+	c.SetTransportParameters(b)
+	require.NoError(t, c.Start(context.Background()))
+
+	ev := c.NextEvent()
+	require.Equal(t, tls.QUICWriteData, ev.Kind)
+	checkClientHello(t, ev.Data)
+	return ev.Data
+}
+
+func TestFindSNI(t *testing.T) {
+	t.Run("without SNI", func(t *testing.T) {
+		testFindSNI(t, "")
+	})
+	t.Run("without subdomain", func(t *testing.T) {
+		testFindSNI(t, "quic-go.net")
+	})
+	t.Run("with subdomain", func(t *testing.T) {
+		testFindSNI(t, "sub.do.ma.in.quic-go.net")
+	})
+}
+
+func testFindSNI(t *testing.T, serverName string) {
+	clientHello := getClientHello(t, serverName)
+	sniPos, sniLen, echPos, err := findSNIAndECH(clientHello)
+	require.NoError(t, err)
+	assert.Equal(t, -1, echPos)
+	if serverName == "" {
+		require.Equal(t, -1, sniPos)
+		return
+	}
+	assert.Equal(t, len(serverName), sniLen)
+	require.NotEqual(t, -1, sniPos)
+	require.Equal(t, serverName, string(clientHello[sniPos:sniPos+sniLen]))
+
+	// incomplete ClientHellos result in an io.ErrUnexpectedEOF
+	for i := range clientHello {
+		_, _, _, err := findSNIAndECH(clientHello[:i])
+		require.ErrorIs(t, err, io.ErrUnexpectedEOF)
+	}
+}