utls/u_conn.go

// Copyright 2017 Google Inc. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package tls

import (
	"bufio"
	"bytes"
	"context"
	"crypto/cipher"
	"crypto/ecdh"
	"encoding/binary"
	"errors"
	"fmt"
	"hash"
	"net"
	"strconv"
)

type ClientHelloBuildStatus int

const NotBuilt ClientHelloBuildStatus = 0
const BuildByUtls ClientHelloBuildStatus = 1
const BuildByGoTLS ClientHelloBuildStatus = 2

type UConn struct {
	*Conn

	Extensions        []TLSExtension
	ClientHelloID     ClientHelloID
	sessionController *sessionController

	clientHelloBuildStatus ClientHelloBuildStatus

	HandshakeState PubClientHandshakeState

	greaseSeed [ssl_grease_last_index]uint16

	omitSNIExtension bool

	// skipResumptionOnNilExtension is copied from `Config.PreferSkipResumptionOnNilExtension`.
	//
	// By default, if ClientHelloSpec is predefined or utls-generated (as opposed to HelloCustom), this flag will be updated to true.
	skipResumptionOnNilExtension bool

	// certCompressionAlgs represents the set of advertised certificate compression
	// algorithms, as specified in the ClientHello. This is only relevant client-side, for the
	// server certificate. All other forms of certificate compression are unsupported.
	certCompressionAlgs []CertCompressionAlgo

	// ech extension is a shortcut to the ECH extension in the Extensions slice if there is one.
	ech ECHExtension
}

// UClient returns a new uTLS client, with behavior depending on clientHelloID.
// Config CAN be nil, but make sure to eventually specify ServerName.
func UClient(conn net.Conn, config *Config, clientHelloID ClientHelloID) *UConn {
	if config == nil {
		config = &Config{}
	}
	tlsConn := Conn{conn: conn, config: config, isClient: true}
	handshakeState := PubClientHandshakeState{C: &tlsConn, Hello: &PubClientHelloMsg{}}
	uconn := UConn{Conn: &tlsConn, ClientHelloID: clientHelloID, HandshakeState: handshakeState}
	uconn.HandshakeState.uconn = &uconn
	uconn.handshakeFn = uconn.clientHandshake
	uconn.sessionController = newSessionController(&uconn)
	uconn.utls.sessionController = uconn.sessionController
	uconn.skipResumptionOnNilExtension = config.PreferSkipResumptionOnNilExtension || clientHelloID.Client != helloCustom
	return &uconn
}

// BuildHandshakeState behavior varies based on ClientHelloID and
// whether it was already called before.
// If HelloGolang:
//
//	[only once] make default ClientHello and overwrite existing state
//
// If any other mimicking ClientHelloID is used:
//
//	[only once] make ClientHello based on ID and overwrite existing state
//	[each call] apply uconn.Extensions config to internal crypto/tls structures
//	[each call] marshal ClientHello.
//
// BuildHandshakeState is automatically called before uTLS performs handshake,
// amd should only be called explicitly to inspect/change fields of
// default/mimicked ClientHello.
func (uconn *UConn) BuildHandshakeState() error {
	if uconn.ClientHelloID == HelloGolang {
		if uconn.clientHelloBuildStatus == BuildByGoTLS {
			return nil
		}
		uAssert(uconn.clientHelloBuildStatus == NotBuilt, "BuildHandshakeState failed: invalid call, client hello has already been built by utls")

		// use default Golang ClientHello.
		hello, keySharePrivate, err := uconn.makeClientHello()
		if err != nil {
			return err
		}

		uconn.HandshakeState.Hello = hello.getPublicPtr()
		if ecdheKey, ok := keySharePrivate.(*ecdh.PrivateKey); ok {
			uconn.HandshakeState.State13.EcdheKey = ecdheKey
		} else if kemKey, ok := keySharePrivate.(*kemPrivateKey); ok {
			uconn.HandshakeState.State13.KEMKey = kemKey.ToPublic()
		} else {
			return fmt.Errorf("uTLS: unknown keySharePrivate type: %T", keySharePrivate)
		}
		uconn.HandshakeState.C = uconn.Conn
		uconn.clientHelloBuildStatus = BuildByGoTLS
	} else {
		uAssert(uconn.clientHelloBuildStatus == BuildByUtls || uconn.clientHelloBuildStatus == NotBuilt, "BuildHandshakeState failed: invalid call, client hello has already been built by go-tls")
		if uconn.clientHelloBuildStatus == NotBuilt {
			err := uconn.applyPresetByID(uconn.ClientHelloID)
			if err != nil {
				return err
			}
			if uconn.omitSNIExtension {
				uconn.removeSNIExtension()
			}
		}

		err := uconn.ApplyConfig()
		if err != nil {
			return err
		}

		err = uconn.uLoadSession()
		if err != nil {
			return err
		}

		err = uconn.MarshalClientHello()
		if err != nil {
			return err
		}

		uconn.uApplyPatch()

		uconn.sessionController.finalCheck()
		uconn.clientHelloBuildStatus = BuildByUtls
	}
	return nil
}

func (uconn *UConn) uLoadSession() error {
	if cfg := uconn.config; cfg.SessionTicketsDisabled || cfg.ClientSessionCache == nil {
		return nil
	}
	switch uconn.sessionController.shouldLoadSession() {
	case shouldReturn:
	case shouldSetTicket:
		uconn.sessionController.setSessionTicketToUConn()
	case shouldSetPsk:
		uconn.sessionController.setPskToUConn()
	case shouldLoad:
		hello := uconn.HandshakeState.Hello.getPrivatePtr()
		uconn.sessionController.utlsAboutToLoadSession()
		session, earlySecret, binderKey, err := uconn.loadSession(hello)
		if session == nil || err != nil {
			return err
		}
		if session.version == VersionTLS12 {
			// We use the session ticket extension for tls 1.2 session resumption
			uconn.sessionController.initSessionTicketExt(session, hello.sessionTicket)
			uconn.sessionController.setSessionTicketToUConn()
		} else {
			uconn.sessionController.initPskExt(session, earlySecret, binderKey, hello.pskIdentities)
		}
	}

	return nil
}

func (uconn *UConn) uApplyPatch() {
	helloLen := len(uconn.HandshakeState.Hello.Raw)
	if uconn.sessionController.shouldUpdateBinders() {
		uconn.sessionController.updateBinders()
		uconn.sessionController.setPskToUConn()
	}
	uAssert(helloLen == len(uconn.HandshakeState.Hello.Raw), "tls: uApplyPatch Failed: the patch should never change the length of the marshaled clientHello")
}

func (uconn *UConn) DidTls12Resume() bool {
	return uconn.didResume
}

// SetSessionState sets the session ticket, which may be preshared or fake.
// If session is nil, the body of session ticket extension will be unset,
// but the extension itself still MAY be present for mimicking purposes.
// Session tickets to be reused - use same cache on following connections.
//
// Deprecated: This method is deprecated in favor of SetSessionTicketExtension,
// as it only handles session override of TLS 1.2
func (uconn *UConn) SetSessionState(session *ClientSessionState) error {
	sessionTicketExt := &SessionTicketExtension{Initialized: true}
	if session != nil {
		sessionTicketExt.Ticket = session.ticket
		sessionTicketExt.Session = session.session
	}
	return uconn.SetSessionTicketExtension(sessionTicketExt)
}

// SetSessionTicket sets the session ticket extension.
// If extension is nil, this will be a no-op.
func (uconn *UConn) SetSessionTicketExtension(sessionTicketExt ISessionTicketExtension) error {
	if uconn.config.SessionTicketsDisabled || uconn.config.ClientSessionCache == nil {
		return fmt.Errorf("tls: SetSessionTicketExtension failed: session is disabled")
	}
	if sessionTicketExt == nil {
		return nil
	}
	return uconn.sessionController.overrideSessionTicketExt(sessionTicketExt)
}

// SetPskExtension sets the psk extension for tls 1.3 resumption. This is a no-op if the psk is nil.
func (uconn *UConn) SetPskExtension(pskExt PreSharedKeyExtension) error {
	if uconn.config.SessionTicketsDisabled || uconn.config.ClientSessionCache == nil {
		return fmt.Errorf("tls: SetPskExtension failed: session is disabled")
	}
	if pskExt == nil {
		return nil
	}

	uconn.HandshakeState.Hello.TicketSupported = true
	return uconn.sessionController.overridePskExt(pskExt)
}

// If you want session tickets to be reused - use same cache on following connections
func (uconn *UConn) SetSessionCache(cache ClientSessionCache) {
	uconn.config.ClientSessionCache = cache
	uconn.HandshakeState.Hello.TicketSupported = true
}

// SetClientRandom sets client random explicitly.
// BuildHandshakeFirst() must be called before SetClientRandom.
// r must to be 32 bytes long.
func (uconn *UConn) SetClientRandom(r []byte) error {
	if len(r) != 32 {
		return errors.New("Incorrect client random length! Expected: 32, got: " + strconv.Itoa(len(r)))
	} else {
		uconn.HandshakeState.Hello.Random = make([]byte, 32)
		copy(uconn.HandshakeState.Hello.Random, r)
		return nil
	}
}

func (uconn *UConn) SetSNI(sni string) {
	hname := hostnameInSNI(sni)
	uconn.config.ServerName = hname
	for _, ext := range uconn.Extensions {
		sniExt, ok := ext.(*SNIExtension)
		if ok {
			sniExt.ServerName = hname
		}
	}
}

// RemoveSNIExtension removes SNI from the list of extensions sent in ClientHello
// It returns an error when used with HelloGolang ClientHelloID
func (uconn *UConn) RemoveSNIExtension() error {
	if uconn.ClientHelloID == HelloGolang {
		return fmt.Errorf("cannot call RemoveSNIExtension on a UConn with a HelloGolang ClientHelloID")
	}
	uconn.omitSNIExtension = true
	return nil
}

func (uconn *UConn) removeSNIExtension() {
	filteredExts := make([]TLSExtension, 0, len(uconn.Extensions))
	for _, e := range uconn.Extensions {
		if _, ok := e.(*SNIExtension); !ok {
			filteredExts = append(filteredExts, e)
		}
	}
	uconn.Extensions = filteredExts
}

// Handshake runs the client handshake using given clientHandshakeState
// Requires hs.hello, and, optionally, hs.session to be set.
func (c *UConn) Handshake() error {
	return c.HandshakeContext(context.Background())
}

// HandshakeContext runs the client or server handshake
// protocol if it has not yet been run.
//
// The provided Context must be non-nil. If the context is canceled before
// the handshake is complete, the handshake is interrupted and an error is returned.
// Once the handshake has completed, cancellation of the context will not affect the
// connection.
func (c *UConn) HandshakeContext(ctx context.Context) error {
	// Delegate to unexported method for named return
	// without confusing documented signature.
	return c.handshakeContext(ctx)
}

func (c *UConn) handshakeContext(ctx context.Context) (ret error) {
	// Fast sync/atomic-based exit if there is no handshake in flight and the
	// last one succeeded without an error. Avoids the expensive context setup
	// and mutex for most Read and Write calls.
	if c.isHandshakeComplete.Load() {
		return nil
	}

	handshakeCtx, cancel := context.WithCancel(ctx)
	// Note: defer this before starting the "interrupter" goroutine
	// so that we can tell the difference between the input being canceled and
	// this cancellation. In the former case, we need to close the connection.
	defer cancel()

	// Start the "interrupter" goroutine, if this context might be canceled.
	// (The background context cannot).
	//
	// The interrupter goroutine waits for the input context to be done and
	// closes the connection if this happens before the function returns.
	if c.quic != nil {
		c.quic.cancelc = handshakeCtx.Done()
		c.quic.cancel = cancel
	} else if ctx.Done() != nil {
		done := make(chan struct{})
		interruptRes := make(chan error, 1)
		defer func() {
			close(done)
			if ctxErr := <-interruptRes; ctxErr != nil {
				// Return context error to user.
				ret = ctxErr
			}
		}()
		go func() {
			select {
			case <-handshakeCtx.Done():
				// Close the connection, discarding the error
				_ = c.conn.Close()
				interruptRes <- handshakeCtx.Err()
			case <-done:
				interruptRes <- nil
			}
		}()
	}

	c.handshakeMutex.Lock()
	defer c.handshakeMutex.Unlock()

	if err := c.handshakeErr; err != nil {
		return err
	}
	if c.isHandshakeComplete.Load() {
		return nil
	}

	c.in.Lock()
	defer c.in.Unlock()

	// [uTLS section begins]
	if c.isClient {
		err := c.BuildHandshakeState()
		if err != nil {
			return err
		}
	}
	// [uTLS section ends]
	c.handshakeErr = c.handshakeFn(handshakeCtx)
	if c.handshakeErr == nil {
		c.handshakes++
	} else {
		// If an error occurred during the hadshake try to flush the
		// alert that might be left in the buffer.
		c.flush()
	}

	if c.handshakeErr == nil && !c.isHandshakeComplete.Load() {
		c.handshakeErr = errors.New("tls: internal error: handshake should have had a result")
	}
	if c.handshakeErr != nil && c.isHandshakeComplete.Load() {
		panic("tls: internal error: handshake returned an error but is marked successful")
	}

	if c.quic != nil {
		if c.handshakeErr == nil {
			c.quicHandshakeComplete()
			// Provide the 1-RTT read secret now that the handshake is complete.
			// The QUIC layer MUST NOT decrypt 1-RTT packets prior to completing
			// the handshake (RFC 9001, Section 5.7).
			c.quicSetReadSecret(QUICEncryptionLevelApplication, c.cipherSuite, c.in.trafficSecret)
		} else {
			var a alert
			c.out.Lock()
			if !errors.As(c.out.err, &a) {
				a = alertInternalError
			}
			c.out.Unlock()
			// Return an error which wraps both the handshake error and
			// any alert error we may have sent, or alertInternalError
			// if we didn't send an alert.
			// Truncate the text of the alert to 0 characters.
			c.handshakeErr = fmt.Errorf("%w%.0w", c.handshakeErr, AlertError(a))
		}
		close(c.quic.blockedc)
		close(c.quic.signalc)
	}

	return c.handshakeErr
}

// Copy-pasted from tls.Conn in its entirety. But c.Handshake() is now utls' one, not tls.
// Write writes data to the connection.
func (c *UConn) Write(b []byte) (int, error) {
	// interlock with Close below
	for {
		x := c.activeCall.Load()
		if x&1 != 0 {
			return 0, net.ErrClosed
		}
		if c.activeCall.CompareAndSwap(x, x+2) {
			defer c.activeCall.Add(-2)
			break
		}
	}

	if err := c.Handshake(); err != nil {
		return 0, err
	}

	c.out.Lock()
	defer c.out.Unlock()

	if err := c.out.err; err != nil {
		return 0, err
	}

	if !c.isHandshakeComplete.Load() {
		return 0, alertInternalError
	}

	if c.closeNotifySent {
		return 0, errShutdown
	}

	// SSL 3.0 and TLS 1.0 are susceptible to a chosen-plaintext
	// attack when using block mode ciphers due to predictable IVs.
	// This can be prevented by splitting each Application Data
	// record into two records, effectively randomizing the IV.
	//
	// https://www.openssl.org/~bodo/tls-cbc.txt
	// https://bugzilla.mozilla.org/show_bug.cgi?id=665814
	// https://www.imperialviolet.org/2012/01/15/beastfollowup.html

	var m int
	if len(b) > 1 && c.vers <= VersionTLS10 {
		if _, ok := c.out.cipher.(cipher.BlockMode); ok {
			n, err := c.writeRecordLocked(recordTypeApplicationData, b[:1])
			if err != nil {
				return n, c.out.setErrorLocked(err)
			}
			m, b = 1, b[1:]
		}
	}

	n, err := c.writeRecordLocked(recordTypeApplicationData, b)
	return n + m, c.out.setErrorLocked(err)
}

// clientHandshakeWithOneState checks that exactly one expected state is set (1.2 or 1.3)
// and performs client TLS handshake with that state
func (c *UConn) clientHandshake(ctx context.Context) (err error) {
	// [uTLS section begins]
	hello := c.HandshakeState.Hello.getPrivatePtr()
	defer func() { c.HandshakeState.Hello = hello.getPublicPtr() }()

	sessionIsLocked := c.utls.sessionController.isSessionLocked()

	// after this point exactly 1 out of 2 HandshakeState pointers is non-nil,
	// useTLS13 variable tells which pointer
	// [uTLS section ends]

	if c.config == nil {
		c.config = defaultConfig()
	}

	// This may be a renegotiation handshake, in which case some fields
	// need to be reset.
	c.didResume = false

	// [uTLS section begins]
	// don't make new ClientHello, use hs.hello
	// preserve the checks from beginning and end of makeClientHello()
	if len(c.config.ServerName) == 0 && !c.config.InsecureSkipVerify && len(c.config.InsecureServerNameToVerify) == 0 {
		return errors.New("tls: at least one of ServerName, InsecureSkipVerify or InsecureServerNameToVerify must be specified in the tls.Config")
	}

	nextProtosLength := 0
	for _, proto := range c.config.NextProtos {
		if l := len(proto); l == 0 || l > 255 {
			return errors.New("tls: invalid NextProtos value")
		} else {
			nextProtosLength += 1 + l
		}
	}

	if nextProtosLength > 0xffff {
		return errors.New("tls: NextProtos values too large")
	}

	if c.handshakes > 0 {
		hello.secureRenegotiation = c.clientFinished[:]
	}

	var (
		session     *SessionState
		earlySecret []byte
		binderKey   []byte
	)
	if !sessionIsLocked {
		// [uTLS section ends]

		session, earlySecret, binderKey, err = c.loadSession(hello)

		// [uTLS section start]
	} else {
		session = c.HandshakeState.Session
		earlySecret = c.HandshakeState.State13.EarlySecret
		binderKey = c.HandshakeState.State13.BinderKey
	}
	// [uTLS section ends]
	if err != nil {
		return err
	}
	if session != nil {
		defer func() {
			// If we got a handshake failure when resuming a session, throw away
			// the session ticket. See RFC 5077, Section 3.2.
			//
			// RFC 8446 makes no mention of dropping tickets on failure, but it
			// does require servers to abort on invalid binders, so we need to
			// delete tickets to recover from a corrupted PSK.
			if err != nil {
				if cacheKey := c.clientSessionCacheKey(); cacheKey != "" {
					c.config.ClientSessionCache.Put(cacheKey, nil)
				}
			}
		}()
	}

	if _, err := c.writeHandshakeRecord(hello, nil); err != nil {
		return err
	}

	if hello.earlyData {
		suite := cipherSuiteTLS13ByID(session.cipherSuite)
		transcript := suite.hash.New()
		if err := transcriptMsg(hello, transcript); err != nil {
			return err
		}
		earlyTrafficSecret := suite.deriveSecret(earlySecret, clientEarlyTrafficLabel, transcript)
		c.quicSetWriteSecret(QUICEncryptionLevelEarly, suite.id, earlyTrafficSecret)
	}

	msg, err := c.readHandshake(nil)
	if err != nil {
		return err
	}

	serverHello, ok := msg.(*serverHelloMsg)
	if !ok {
		c.sendAlert(alertUnexpectedMessage)
		return unexpectedMessageError(serverHello, msg)
	}

	if err := c.pickTLSVersion(serverHello); err != nil {
		return err
	}

	// uTLS: do not create new handshakeState, use existing one
	if c.vers == VersionTLS13 {
		hs13 := c.HandshakeState.toPrivate13()
		hs13.serverHello = serverHello
		hs13.hello = hello
		if hs13.keySharesParams == nil {
			hs13.keySharesParams = NewKeySharesParameters()
		}
		if !sessionIsLocked {
			hs13.earlySecret = earlySecret
			hs13.binderKey = binderKey
			hs13.session = session
		}
		hs13.ctx = ctx
		// In TLS 1.3, session tickets are delivered after the handshake.
		err = hs13.handshake()
		if handshakeState := hs13.toPublic13(); handshakeState != nil {
			c.HandshakeState = *handshakeState
		}
		return err
	}

	hs12 := c.HandshakeState.toPrivate12()
	hs12.serverHello = serverHello
	hs12.hello = hello
	hs12.ctx = ctx
	hs12.session = session
	err = hs12.handshake()
	if handshakeState := hs12.toPublic12(); handshakeState != nil {
		c.HandshakeState = *handshakeState
	}
	if err != nil {
		return err
	}
	return nil
}

func (uconn *UConn) ApplyConfig() error {
	for _, ext := range uconn.Extensions {
		err := ext.writeToUConn(uconn)
		if err != nil {
			return err
		}
	}
	return nil
}

func (uconn *UConn) MarshalClientHello() error {
	if uconn.ech != nil {
		if err := uconn.ech.Configure(uconn.config.ECHConfigs); err != nil {
			return err
		}
		return uconn.ech.MarshalClientHello(uconn)
	}
	hello := uconn.HandshakeState.Hello
	headerLength := 2 + 32 + 1 + len(hello.SessionId) +
		2 + len(hello.CipherSuites)*2 +
		1 + len(hello.CompressionMethods)

	extensionsLen := 0
	var paddingExt *UtlsPaddingExtension // reference to padding extension, if present
	for _, ext := range uconn.Extensions {
		if pe, ok := ext.(*UtlsPaddingExtension); !ok {
			// If not padding - just add length of extension to total length
			extensionsLen += ext.Len()
		} else {
			// If padding - process it later
			if paddingExt == nil {
				paddingExt = pe
			} else {
				return errors.New("multiple padding extensions")
			}
		}
	}

	if paddingExt != nil {
		// determine padding extension presence and length
		paddingExt.Update(headerLength + 4 + extensionsLen + 2)
		extensionsLen += paddingExt.Len()
	}

	helloLen := headerLength
	if len(uconn.Extensions) > 0 {
		helloLen += 2 + extensionsLen // 2 bytes for extensions' length
	}

	helloBuffer := bytes.Buffer{}
	bufferedWriter := bufio.NewWriterSize(&helloBuffer, helloLen+4) // 1 byte for tls record type, 3 for length
	// We use buffered Writer to avoid checking write errors after every Write(): whenever first error happens
	// Write() will become noop, and error will be accessible via Flush(), which is called once in the end

	binary.Write(bufferedWriter, binary.BigEndian, typeClientHello)
	helloLenBytes := []byte{byte(helloLen >> 16), byte(helloLen >> 8), byte(helloLen)} // poor man's uint24
	binary.Write(bufferedWriter, binary.BigEndian, helloLenBytes)
	binary.Write(bufferedWriter, binary.BigEndian, hello.Vers)

	binary.Write(bufferedWriter, binary.BigEndian, hello.Random)

	binary.Write(bufferedWriter, binary.BigEndian, uint8(len(hello.SessionId)))
	binary.Write(bufferedWriter, binary.BigEndian, hello.SessionId)

	binary.Write(bufferedWriter, binary.BigEndian, uint16(len(hello.CipherSuites)<<1))
	for _, suite := range hello.CipherSuites {
		binary.Write(bufferedWriter, binary.BigEndian, suite)
	}

	binary.Write(bufferedWriter, binary.BigEndian, uint8(len(hello.CompressionMethods)))
	binary.Write(bufferedWriter, binary.BigEndian, hello.CompressionMethods)

	if len(uconn.Extensions) > 0 {
		binary.Write(bufferedWriter, binary.BigEndian, uint16(extensionsLen))
		for _, ext := range uconn.Extensions {
			if _, err := bufferedWriter.ReadFrom(ext); err != nil {
				return err
			}
		}
	}

	err := bufferedWriter.Flush()
	if err != nil {
		return err
	}

	if helloBuffer.Len() != 4+helloLen {
		return errors.New("utls: unexpected ClientHello length. Expected: " + strconv.Itoa(4+helloLen) +
			". Got: " + strconv.Itoa(helloBuffer.Len()))
	}

	hello.Raw = helloBuffer.Bytes()
	return nil
}

// get current state of cipher and encrypt zeros to get keystream
func (uconn *UConn) GetOutKeystream(length int) ([]byte, error) {
	zeros := make([]byte, length)

	if outCipher, ok := uconn.out.cipher.(cipher.AEAD); ok {
		// AEAD.Seal() does not mutate internal state, other ciphers might
		return outCipher.Seal(nil, uconn.out.seq[:], zeros, nil), nil
	}
	return nil, errors.New("could not convert OutCipher to cipher.AEAD")
}

// SetTLSVers sets min and max TLS version in all appropriate places.
// Function will use first non-zero version parsed in following order:
//  1. Provided minTLSVers, maxTLSVers
//  2. specExtensions may have SupportedVersionsExtension
//  3. [default] min = TLS 1.0, max = TLS 1.2
//
// Error is only returned if things are in clearly undesirable state
// to help user fix them.
func (uconn *UConn) SetTLSVers(minTLSVers, maxTLSVers uint16, specExtensions []TLSExtension) error {
	if minTLSVers == 0 && maxTLSVers == 0 {
		// if version is not set explicitly in the ClientHelloSpec, check the SupportedVersions extension
		supportedVersionsExtensionsPresent := 0
		for _, e := range specExtensions {
			switch ext := e.(type) {
			case *SupportedVersionsExtension:
				findVersionsInSupportedVersionsExtensions := func(versions []uint16) (uint16, uint16) {
					// returns (minVers, maxVers)
					minVers := uint16(0)
					maxVers := uint16(0)
					for _, vers := range versions {
						if isGREASEUint16(vers) {
							continue
						}
						if maxVers < vers || maxVers == 0 {
							maxVers = vers
						}
						if minVers > vers || minVers == 0 {
							minVers = vers
						}
					}
					return minVers, maxVers
				}

				supportedVersionsExtensionsPresent += 1
				minTLSVers, maxTLSVers = findVersionsInSupportedVersionsExtensions(ext.Versions)
				if minTLSVers == 0 && maxTLSVers == 0 {
					return fmt.Errorf("SupportedVersions extension has invalid Versions field")
				} // else: proceed
			}
		}
		switch supportedVersionsExtensionsPresent {
		case 0:
			// if mandatory for TLS 1.3 extension is not present, just default to 1.2
			minTLSVers = VersionTLS10
			maxTLSVers = VersionTLS12
		case 1:
		default:
			return fmt.Errorf("uconn.Extensions contains %v separate SupportedVersions extensions",
				supportedVersionsExtensionsPresent)
		}
	}

	if minTLSVers < VersionTLS10 || minTLSVers > VersionTLS13 {
		return fmt.Errorf("uTLS does not support 0x%X as min version", minTLSVers)
	}

	if maxTLSVers < VersionTLS10 || maxTLSVers > VersionTLS13 {
		return fmt.Errorf("uTLS does not support 0x%X as max version", maxTLSVers)
	}

	uconn.HandshakeState.Hello.SupportedVersions = makeSupportedVersions(minTLSVers, maxTLSVers)
	uconn.config.MinVersion = minTLSVers
	uconn.config.MaxVersion = maxTLSVers

	return nil
}

func (uconn *UConn) SetUnderlyingConn(c net.Conn) {
	uconn.Conn.conn = c
}

func (uconn *UConn) GetUnderlyingConn() net.Conn {
	return uconn.Conn.conn
}

// MakeConnWithCompleteHandshake allows to forge both server and client side TLS connections.
// Major Hack Alert.
func MakeConnWithCompleteHandshake(tcpConn net.Conn, version uint16, cipherSuite uint16, masterSecret []byte, clientRandom []byte, serverRandom []byte, isClient bool) *Conn {
	tlsConn := &Conn{conn: tcpConn, config: &Config{}, isClient: isClient}
	cs := cipherSuiteByID(cipherSuite)
	if cs != nil {
		// This is mostly borrowed from establishKeys()
		clientMAC, serverMAC, clientKey, serverKey, clientIV, serverIV :=
			keysFromMasterSecret(version, cs, masterSecret, clientRandom, serverRandom,
				cs.macLen, cs.keyLen, cs.ivLen)

		var clientCipher, serverCipher interface{}
		var clientHash, serverHash hash.Hash
		if cs.cipher != nil {
			clientCipher = cs.cipher(clientKey, clientIV, true /* for reading */)
			clientHash = cs.mac(clientMAC)
			serverCipher = cs.cipher(serverKey, serverIV, false /* not for reading */)
			serverHash = cs.mac(serverMAC)
		} else {
			clientCipher = cs.aead(clientKey, clientIV)
			serverCipher = cs.aead(serverKey, serverIV)
		}

		if isClient {
			tlsConn.in.prepareCipherSpec(version, serverCipher, serverHash)
			tlsConn.out.prepareCipherSpec(version, clientCipher, clientHash)
		} else {
			tlsConn.in.prepareCipherSpec(version, clientCipher, clientHash)
			tlsConn.out.prepareCipherSpec(version, serverCipher, serverHash)
		}

		// skip the handshake states
		tlsConn.isHandshakeComplete.Store(true)
		tlsConn.cipherSuite = cipherSuite
		tlsConn.haveVers = true
		tlsConn.vers = version

		// Update to the new cipher specs
		// and consume the finished messages
		tlsConn.in.changeCipherSpec()
		tlsConn.out.changeCipherSpec()

		tlsConn.in.incSeq()
		tlsConn.out.incSeq()

		return tlsConn
	} else {
		// TODO: Support TLS 1.3 Cipher Suites
		return nil
	}
}

func makeSupportedVersions(minVers, maxVers uint16) []uint16 {
	a := make([]uint16, maxVers-minVers+1)
	for i := range a {
		a[i] = maxVers - uint16(i)
	}
	return a
}

// Extending (*Conn).readHandshake() to support more customized handshake messages.
func (c *Conn) utlsHandshakeMessageType(msgType byte) (handshakeMessage, error) {
	switch msgType {
	case utlsTypeCompressedCertificate:
		return new(utlsCompressedCertificateMsg), nil
	case utlsTypeEncryptedExtensions:
		if c.isClient {
			return new(encryptedExtensionsMsg), nil
		} else {
			return new(utlsClientEncryptedExtensionsMsg), nil
		}
	default:
		return nil, c.in.setErrorLocked(c.sendAlert(alertUnexpectedMessage))
	}
}

// Extending (*Conn).connectionStateLocked()
func (c *Conn) utlsConnectionStateLocked(state *ConnectionState) {
	state.PeerApplicationSettings = c.utls.peerApplicationSettings
	state.ECHRetryConfigs = c.utls.echRetryConfigs
}

type utlsConnExtraFields struct {
	// Application Settings (ALPS)
	hasApplicationSettings   bool
	peerApplicationSettings  []byte
	localApplicationSettings []byte

	// Encrypted Client Hello (ECH)
	echRetryConfigs []ECHConfig

	sessionController *sessionController
}