u_tls_extensions.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 (
	"errors"
	"io"
	"strings"

	"golang.org/x/crypto/cryptobyte"
)

// ExtensionIDToExtension returns a TLSExtension for the given extension ID.
func ExtensionIDToExtension(id uint16) TLSExtensionWriter {
	// deep copy
	switch id {
	case extensionServerName:
		return &SNIExtension{}
	case extensionStatusRequest:
		return &StatusRequestExtension{}
	case extensionSupportedCurves:
		return &SupportedCurvesExtension{}
	case extensionSupportedPoints:
		return &SupportedPointsExtension{}
	case extensionSignatureAlgorithms:
		return &SignatureAlgorithmsExtension{}
	case extensionALPN:
		return &ALPNExtension{}
	case extensionStatusRequestV2:
		return &StatusRequestV2Extension{}
	case extensionSCT:
		return &SCTExtension{}
	case utlsExtensionPadding:
		return &UtlsPaddingExtension{}
	case utlsExtensionExtendedMasterSecret:
		return &UtlsExtendedMasterSecretExtension{}
	case fakeExtensionTokenBinding:
		return &FakeTokenBindingExtension{}
	case utlsExtensionCompressCertificate:
		return &UtlsCompressCertExtension{}
	case fakeExtensionDelegatedCredentials:
		return &FakeDelegatedCredentialsExtension{}
	case extensionSessionTicket:
		return &SessionTicketExtension{}
	case fakeExtensionPreSharedKey:
		return &FakePreSharedKeyExtension{}
	// case extensionEarlyData:
	// 	return &EarlyDataExtension{}
	case extensionSupportedVersions:
		return &SupportedVersionsExtension{}
	// case extensionCookie:
	// 	return &CookieExtension{}
	case extensionPSKModes:
		return &PSKKeyExchangeModesExtension{}
	// case extensionCertificateAuthorities:
	// 	return &CertificateAuthoritiesExtension{}
	case extensionSignatureAlgorithmsCert:
		return &SignatureAlgorithmsCertExtension{}
	case extensionKeyShare:
		return &KeyShareExtension{}
	case extensionNextProtoNeg:
		return &NPNExtension{}
	case utlsExtensionApplicationSettings:
		return &ApplicationSettingsExtension{}
	case fakeOldExtensionChannelID:
		return &FakeChannelIDExtension{true}
	case fakeExtensionChannelID:
		return &FakeChannelIDExtension{}
	case fakeRecordSizeLimit:
		return &FakeRecordSizeLimitExtension{}
	case extensionRenegotiationInfo:
		return &RenegotiationInfoExtension{}
	default:
		if isGREASEUint16(id) {
			return &UtlsGREASEExtension{}
		}
		return nil // not returning GenericExtension, it should be handled by caller
	}
}

type TLSExtension interface {
	writeToUConn(*UConn) error

	Len() int // includes header

	// Read reads up to len(p) bytes into p.
	// It returns the number of bytes read (0 <= n <= len(p)) and any error encountered.
	Read(p []byte) (n int, err error) // implements io.Reader
}

// TLSExtensionWriter is an interface allowing a TLS extension to be
// auto-constucted/recovered by reading in a byte stream.
type TLSExtensionWriter interface {
	TLSExtension

	// Write writes up to len(b) bytes from b.
	// It returns the number of bytes written (0 <= n <= len(b)) and any error encountered.
	Write(b []byte) (n int, err error)
}

type NPNExtension struct {
	NextProtos []string
}

func (e *NPNExtension) writeToUConn(uc *UConn) error {
	uc.config.NextProtos = e.NextProtos
	uc.HandshakeState.Hello.NextProtoNeg = true
	return nil
}

func (e *NPNExtension) Len() int {
	return 4
}

func (e *NPNExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}
	b[0] = byte(extensionNextProtoNeg >> 8)
	b[1] = byte(extensionNextProtoNeg & 0xff)
	// The length is always 0
	return e.Len(), io.EOF
}

// Write is a no-op for NPNExtension. NextProtos are not included in the
// ClientHello.
func (e *NPNExtension) Write(_ []byte) (int, error) {
	return 0, nil
}

type SNIExtension struct {
	ServerName string // not an array because go crypto/tls doesn't support multiple SNIs
}

func (e *SNIExtension) writeToUConn(uc *UConn) error {
	uc.config.ServerName = e.ServerName
	hostName := hostnameInSNI(e.ServerName)
	uc.HandshakeState.Hello.ServerName = hostName

	return nil
}

func (e *SNIExtension) Len() int {
	// Literal IP addresses, absolute FQDNs, and empty strings are not permitted as SNI values.
	// See RFC 6066, Section 3.
	hostName := hostnameInSNI(e.ServerName)
	if len(hostName) == 0 {
		return 0
	}
	return 4 + 2 + 1 + 2 + len(hostName)
}

func (e *SNIExtension) Read(b []byte) (int, error) {
	// Literal IP addresses, absolute FQDNs, and empty strings are not permitted as SNI values.
	// See RFC 6066, Section 3.
	hostName := hostnameInSNI(e.ServerName)
	if len(hostName) == 0 {
		return 0, io.EOF
	}
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}
	// RFC 3546, section 3.1
	b[0] = byte(extensionServerName >> 8)
	b[1] = byte(extensionServerName)
	b[2] = byte((len(hostName) + 5) >> 8)
	b[3] = byte(len(hostName) + 5)
	b[4] = byte((len(hostName) + 3) >> 8)
	b[5] = byte(len(hostName) + 3)
	// b[6] Server Name Type: host_name (0)
	b[7] = byte(len(hostName) >> 8)
	b[8] = byte(len(hostName))
	copy(b[9:], []byte(hostName))
	return e.Len(), io.EOF
}

// Write is a no-op for StatusRequestExtension.
// SNI should not be fingerprinted and is user controlled.
func (e *SNIExtension) Write(b []byte) (int, error) {
	fullLen := len(b)
	extData := cryptobyte.String(b)
	// RFC 6066, Section 3
	var nameList cryptobyte.String
	if !extData.ReadUint16LengthPrefixed(&nameList) || nameList.Empty() {
		return fullLen, errors.New("unable to read server name extension data")
	}
	var serverName string
	for !nameList.Empty() {
		var nameType uint8
		var serverNameBytes cryptobyte.String
		if !nameList.ReadUint8(&nameType) ||
			!nameList.ReadUint16LengthPrefixed(&serverNameBytes) ||
			serverNameBytes.Empty() {
			return fullLen, errors.New("unable to read server name extension data")
		}
		if nameType != 0 {
			continue
		}
		if len(serverName) != 0 {
			return fullLen, errors.New("multiple names of the same name_type in server name extension are prohibited")
		}
		serverName = string(serverNameBytes)
		if strings.HasSuffix(serverName, ".") {
			return fullLen, errors.New("SNI value may not include a trailing dot")
		}
	}
	// clientHelloSpec.Extensions = append(clientHelloSpec.Extensions, &SNIExtension{}) // gaukas moved this line out from the loop.

	// don't copy SNI from ClientHello to ClientHelloSpec!
	return fullLen, nil
}

type StatusRequestExtension struct {
}

func (e *StatusRequestExtension) writeToUConn(uc *UConn) error {
	uc.HandshakeState.Hello.OcspStapling = true
	return nil
}

func (e *StatusRequestExtension) Len() int {
	return 9
}

func (e *StatusRequestExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}
	// RFC 4366, section 3.6
	b[0] = byte(extensionStatusRequest >> 8)
	b[1] = byte(extensionStatusRequest)
	b[2] = 0
	b[3] = 5
	b[4] = 1 // OCSP type
	// Two zero valued uint16s for the two lengths.
	return e.Len(), io.EOF
}

// Write is a no-op for StatusRequestExtension. No data for this extension.
func (e *StatusRequestExtension) Write(b []byte) (int, error) {
	fullLen := len(b)
	extData := cryptobyte.String(b)
	// RFC 4366, Section 3.6
	var statusType uint8
	var ignored cryptobyte.String
	if !extData.ReadUint8(&statusType) ||
		!extData.ReadUint16LengthPrefixed(&ignored) ||
		!extData.ReadUint16LengthPrefixed(&ignored) {
		return fullLen, errors.New("unable to read status request extension data")
	}

	if statusType != statusTypeOCSP {
		return fullLen, errors.New("status request extension statusType is not statusTypeOCSP(1)")
	}

	return fullLen, nil
}

type StatusRequestV2Extension struct {
}

func (e *StatusRequestV2Extension) writeToUConn(uc *UConn) error {
	uc.HandshakeState.Hello.OcspStapling = true
	return nil
}

func (e *StatusRequestV2Extension) Len() int {
	return 13
}

func (e *StatusRequestV2Extension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}
	// RFC 4366, section 3.6
	b[0] = byte(extensionStatusRequestV2 >> 8)
	b[1] = byte(extensionStatusRequestV2)
	b[2] = 0
	b[3] = 9
	b[4] = 0
	b[5] = 7
	b[6] = 2 // OCSP type
	b[7] = 0
	b[8] = 4
	// Two zero valued uint16s for the two lengths.
	return e.Len(), io.EOF
}

// Write is a no-op for StatusRequestV2Extension. No data for this extension.
func (e *StatusRequestV2Extension) Write(b []byte) (int, error) {
	fullLen := len(b)
	extData := cryptobyte.String(b)
	// RFC 4366, Section 3.6
	var statusType uint8
	var ignored cryptobyte.String
	if !extData.ReadUint16LengthPrefixed(&ignored) ||
		!extData.ReadUint8(&statusType) ||
		!extData.ReadUint16LengthPrefixed(&ignored) ||
		!extData.ReadUint16LengthPrefixed(&ignored) ||
		!extData.ReadUint16LengthPrefixed(&ignored) {
		return fullLen, errors.New("unable to read status request v2 extension data")
	}

	if statusType != statusV2TypeOCSP {
		return fullLen, errors.New("status request v2 extension statusType is not statusV2TypeOCSP(2)")
	}

	return fullLen, nil
}

type SupportedCurvesExtension struct {
	Curves []CurveID
}

func (e *SupportedCurvesExtension) writeToUConn(uc *UConn) error {
	uc.config.CurvePreferences = e.Curves
	uc.HandshakeState.Hello.SupportedCurves = e.Curves
	return nil
}

func (e *SupportedCurvesExtension) Len() int {
	return 6 + 2*len(e.Curves)
}

func (e *SupportedCurvesExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}
	// http://tools.ietf.org/html/rfc4492#section-5.5.1
	b[0] = byte(extensionSupportedCurves >> 8)
	b[1] = byte(extensionSupportedCurves)
	b[2] = byte((2 + 2*len(e.Curves)) >> 8)
	b[3] = byte(2 + 2*len(e.Curves))
	b[4] = byte((2 * len(e.Curves)) >> 8)
	b[5] = byte(2 * len(e.Curves))
	for i, curve := range e.Curves {
		b[6+2*i] = byte(curve >> 8)
		b[7+2*i] = byte(curve)
	}
	return e.Len(), io.EOF
}

func (e *SupportedCurvesExtension) Write(b []byte) (int, error) {
	fullLen := len(b)
	extData := cryptobyte.String(b)
	// RFC 4492, sections 5.1.1 and RFC 8446, Section 4.2.7
	var curvesBytes cryptobyte.String
	if !extData.ReadUint16LengthPrefixed(&curvesBytes) || curvesBytes.Empty() {
		return 0, errors.New("unable to read supported curves extension data")
	}
	curves := []CurveID{}
	for !curvesBytes.Empty() {
		var curve uint16
		if !curvesBytes.ReadUint16(&curve) {
			return 0, errors.New("unable to read supported curves extension data")
		}
		curves = append(curves, CurveID(unGREASEUint16(curve)))
	}
	e.Curves = curves
	return fullLen, nil
}

type SupportedPointsExtension struct {
	SupportedPoints []uint8
}

func (e *SupportedPointsExtension) writeToUConn(uc *UConn) error {
	uc.HandshakeState.Hello.SupportedPoints = e.SupportedPoints
	return nil
}

func (e *SupportedPointsExtension) Len() int {
	return 5 + len(e.SupportedPoints)
}

func (e *SupportedPointsExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}
	// http://tools.ietf.org/html/rfc4492#section-5.5.2
	b[0] = byte(extensionSupportedPoints >> 8)
	b[1] = byte(extensionSupportedPoints)
	b[2] = byte((1 + len(e.SupportedPoints)) >> 8)
	b[3] = byte(1 + len(e.SupportedPoints))
	b[4] = byte(len(e.SupportedPoints))
	for i, pointFormat := range e.SupportedPoints {
		b[5+i] = pointFormat
	}
	return e.Len(), io.EOF
}

func (e *SupportedPointsExtension) Write(b []byte) (int, error) {
	fullLen := len(b)
	extData := cryptobyte.String(b)
	// RFC 4492, Section 5.1.2
	supportedPoints := []uint8{}
	if !readUint8LengthPrefixed(&extData, &supportedPoints) ||
		len(supportedPoints) == 0 {
		return 0, errors.New("unable to read supported points extension data")
	}
	e.SupportedPoints = supportedPoints
	return fullLen, nil
}

type SignatureAlgorithmsExtension struct {
	SupportedSignatureAlgorithms []SignatureScheme
}

func (e *SignatureAlgorithmsExtension) writeToUConn(uc *UConn) error {
	uc.HandshakeState.Hello.SupportedSignatureAlgorithms = e.SupportedSignatureAlgorithms
	return nil
}

func (e *SignatureAlgorithmsExtension) Len() int {
	return 6 + 2*len(e.SupportedSignatureAlgorithms)
}

func (e *SignatureAlgorithmsExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}
	// https://tools.ietf.org/html/rfc5246#section-7.4.1.4.1
	b[0] = byte(extensionSignatureAlgorithms >> 8)
	b[1] = byte(extensionSignatureAlgorithms)
	b[2] = byte((2 + 2*len(e.SupportedSignatureAlgorithms)) >> 8)
	b[3] = byte(2 + 2*len(e.SupportedSignatureAlgorithms))
	b[4] = byte((2 * len(e.SupportedSignatureAlgorithms)) >> 8)
	b[5] = byte(2 * len(e.SupportedSignatureAlgorithms))
	for i, sigAndHash := range e.SupportedSignatureAlgorithms {
		b[6+2*i] = byte(sigAndHash >> 8)
		b[7+2*i] = byte(sigAndHash)
	}
	return e.Len(), io.EOF
}

func (e *SignatureAlgorithmsExtension) Write(b []byte) (int, error) {
	fullLen := len(b)
	extData := cryptobyte.String(b)
	// RFC 5246, Section 7.4.1.4.1
	var sigAndAlgs cryptobyte.String
	if !extData.ReadUint16LengthPrefixed(&sigAndAlgs) || sigAndAlgs.Empty() {
		return 0, errors.New("unable to read signature algorithms extension data")
	}
	supportedSignatureAlgorithms := []SignatureScheme{}
	for !sigAndAlgs.Empty() {
		var sigAndAlg uint16
		if !sigAndAlgs.ReadUint16(&sigAndAlg) {
			return 0, errors.New("unable to read signature algorithms extension data")
		}
		supportedSignatureAlgorithms = append(
			supportedSignatureAlgorithms, SignatureScheme(sigAndAlg))
	}
	e.SupportedSignatureAlgorithms = supportedSignatureAlgorithms
	return fullLen, nil
}

type SignatureAlgorithmsCertExtension struct {
	SupportedSignatureAlgorithms []SignatureScheme
}

func (e *SignatureAlgorithmsCertExtension) writeToUConn(uc *UConn) error {
	uc.HandshakeState.Hello.SupportedSignatureAlgorithms = e.SupportedSignatureAlgorithms
	return nil
}

func (e *SignatureAlgorithmsCertExtension) Len() int {
	return 6 + 2*len(e.SupportedSignatureAlgorithms)
}

func (e *SignatureAlgorithmsCertExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}
	// https://tools.ietf.org/html/rfc5246#section-7.4.1.4.1
	b[0] = byte(extensionSignatureAlgorithmsCert >> 8)
	b[1] = byte(extensionSignatureAlgorithmsCert)
	b[2] = byte((2 + 2*len(e.SupportedSignatureAlgorithms)) >> 8)
	b[3] = byte(2 + 2*len(e.SupportedSignatureAlgorithms))
	b[4] = byte((2 * len(e.SupportedSignatureAlgorithms)) >> 8)
	b[5] = byte(2 * len(e.SupportedSignatureAlgorithms))
	for i, sigAndHash := range e.SupportedSignatureAlgorithms {
		b[6+2*i] = byte(sigAndHash >> 8)
		b[7+2*i] = byte(sigAndHash)
	}
	return e.Len(), io.EOF
}

// Write implementation copied from SignatureAlgorithmsExtension.Write
//
// Warning: not tested.
func (e *SignatureAlgorithmsCertExtension) Write(b []byte) (int, error) {
	fullLen := len(b)
	extData := cryptobyte.String(b)
	// RFC 8446, Section 4.2.3
	var sigAndAlgs cryptobyte.String
	if !extData.ReadUint16LengthPrefixed(&sigAndAlgs) || sigAndAlgs.Empty() {
		return 0, errors.New("unable to read signature algorithms extension data")
	}
	supportedSignatureAlgorithms := []SignatureScheme{}
	for !sigAndAlgs.Empty() {
		var sigAndAlg uint16
		if !sigAndAlgs.ReadUint16(&sigAndAlg) {
			return 0, errors.New("unable to read signature algorithms extension data")
		}
		supportedSignatureAlgorithms = append(
			supportedSignatureAlgorithms, SignatureScheme(sigAndAlg))
	}
	e.SupportedSignatureAlgorithms = supportedSignatureAlgorithms
	return fullLen, nil
}

type RenegotiationInfoExtension struct {
	// Renegotiation field limits how many times client will perform renegotiation: no limit, once, or never.
	// The extension still will be sent, even if Renegotiation is set to RenegotiateNever.
	Renegotiation RenegotiationSupport
}

func (e *RenegotiationInfoExtension) writeToUConn(uc *UConn) error {
	uc.config.Renegotiation = e.Renegotiation
	switch e.Renegotiation {
	case RenegotiateOnceAsClient:
		fallthrough
	case RenegotiateFreelyAsClient:
		uc.HandshakeState.Hello.SecureRenegotiationSupported = true
	case RenegotiateNever:
	default:
	}
	return nil
}

func (e *RenegotiationInfoExtension) Len() int {
	return 5
}

func (e *RenegotiationInfoExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}

	var extInnerBody []byte // inner body is empty
	innerBodyLen := len(extInnerBody)
	extBodyLen := innerBodyLen + 1

	b[0] = byte(extensionRenegotiationInfo >> 8)
	b[1] = byte(extensionRenegotiationInfo & 0xff)
	b[2] = byte(extBodyLen >> 8)
	b[3] = byte(extBodyLen)
	b[4] = byte(innerBodyLen)
	copy(b[5:], extInnerBody)

	return e.Len(), io.EOF
}

func (e *RenegotiationInfoExtension) Write(_ []byte) (int, error) {
	e.Renegotiation = RenegotiateOnceAsClient
	return 0, nil
}

type ALPNExtension struct {
	AlpnProtocols []string
}

func (e *ALPNExtension) writeToUConn(uc *UConn) error {
	uc.config.NextProtos = e.AlpnProtocols
	uc.HandshakeState.Hello.AlpnProtocols = e.AlpnProtocols
	return nil
}

func (e *ALPNExtension) Len() int {
	bLen := 2 + 2 + 2
	for _, s := range e.AlpnProtocols {
		bLen += 1 + len(s)
	}
	return bLen
}

func (e *ALPNExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}

	b[0] = byte(extensionALPN >> 8)
	b[1] = byte(extensionALPN & 0xff)
	lengths := b[2:]
	b = b[6:]

	stringsLength := 0
	for _, s := range e.AlpnProtocols {
		l := len(s)
		b[0] = byte(l)
		copy(b[1:], s)
		b = b[1+l:]
		stringsLength += 1 + l
	}

	lengths[2] = byte(stringsLength >> 8)
	lengths[3] = byte(stringsLength)
	stringsLength += 2
	lengths[0] = byte(stringsLength >> 8)
	lengths[1] = byte(stringsLength)

	return e.Len(), io.EOF
}

func (e *ALPNExtension) Write(b []byte) (int, error) {
	fullLen := len(b)
	extData := cryptobyte.String(b)
	// RFC 7301, Section 3.1
	var protoList cryptobyte.String
	if !extData.ReadUint16LengthPrefixed(&protoList) || protoList.Empty() {
		return 0, errors.New("unable to read ALPN extension data")
	}
	alpnProtocols := []string{}
	for !protoList.Empty() {
		var proto cryptobyte.String
		if !protoList.ReadUint8LengthPrefixed(&proto) || proto.Empty() {
			return 0, errors.New("unable to read ALPN extension data")
		}
		alpnProtocols = append(alpnProtocols, string(proto))

	}
	e.AlpnProtocols = alpnProtocols
	return fullLen, nil
}

// ApplicationSettingsExtension represents the TLS ALPS extension.
// At the time of this writing, this extension is currently a draft:
// https://datatracker.ietf.org/doc/html/draft-vvv-tls-alps-01
type ApplicationSettingsExtension struct {
	SupportedProtocols []string
}

func (e *ApplicationSettingsExtension) writeToUConn(uc *UConn) error {
	return nil
}

func (e *ApplicationSettingsExtension) Len() int {
	bLen := 2 + 2 + 2 // Type + Length + ALPS Extension length
	for _, s := range e.SupportedProtocols {
		bLen += 1 + len(s) // Supported ALPN Length + actual length of protocol
	}
	return bLen
}

func (e *ApplicationSettingsExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}

	// Read Type.
	b[0] = byte(utlsExtensionApplicationSettings >> 8)   // hex: 44 dec: 68
	b[1] = byte(utlsExtensionApplicationSettings & 0xff) // hex: 69 dec: 105

	lengths := b[2:] // get the remaining buffer without Type
	b = b[6:]        // set the buffer to the buffer without Type, Length and ALPS Extension Length (so only the Supported ALPN list remains)

	stringsLength := 0
	for _, s := range e.SupportedProtocols {
		l := len(s)            // Supported ALPN Length
		b[0] = byte(l)         // Supported ALPN Length in bytes hex: 02 dec: 2
		copy(b[1:], s)         // copy the Supported ALPN as bytes to the buffer
		b = b[1+l:]            // set the buffer to the buffer without the Supported ALPN Length and Supported ALPN (so we can continue to the next protocol in this loop)
		stringsLength += 1 + l // Supported ALPN Length (the field itself) + Supported ALPN Length (the value)
	}

	lengths[2] = byte(stringsLength >> 8) // ALPS Extension Length hex: 00 dec: 0
	lengths[3] = byte(stringsLength)      // ALPS Extension Length hex: 03 dec: 3
	stringsLength += 2                    // plus ALPS Extension Length field length
	lengths[0] = byte(stringsLength >> 8) // Length hex:00 dec: 0
	lengths[1] = byte(stringsLength)      // Length hex: 05 dec: 5

	return e.Len(), io.EOF
}

// Write implementation copied from ALPNExtension.Write
func (e *ApplicationSettingsExtension) Write(b []byte) (int, error) {
	fullLen := len(b)
	extData := cryptobyte.String(b)
	// https://datatracker.ietf.org/doc/html/draft-vvv-tls-alps-01
	var protoList cryptobyte.String
	if !extData.ReadUint16LengthPrefixed(&protoList) || protoList.Empty() {
		return 0, errors.New("unable to read ALPN extension data")
	}
	alpnProtocols := []string{}
	for !protoList.Empty() {
		var proto cryptobyte.String
		if !protoList.ReadUint8LengthPrefixed(&proto) || proto.Empty() {
			return 0, errors.New("unable to read ALPN extension data")
		}
		alpnProtocols = append(alpnProtocols, string(proto))

	}
	e.SupportedProtocols = alpnProtocols
	return fullLen, nil
}

type SCTExtension struct {
}

func (e *SCTExtension) writeToUConn(uc *UConn) error {
	uc.HandshakeState.Hello.Scts = true
	return nil
}

func (e *SCTExtension) Len() int {
	return 4
}

func (e *SCTExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}
	// https://tools.ietf.org/html/rfc6962#section-3.3.1
	b[0] = byte(extensionSCT >> 8)
	b[1] = byte(extensionSCT)
	// zero uint16 for the zero-length extension_data
	return e.Len(), io.EOF
}

func (e *SCTExtension) Write(_ []byte) (int, error) {
	return 0, nil
}

type SessionTicketExtension struct {
	Session *ClientSessionState
}

func (e *SessionTicketExtension) writeToUConn(uc *UConn) error {
	if e.Session != nil {
		uc.HandshakeState.Session = e.Session
		uc.HandshakeState.Hello.SessionTicket = e.Session.sessionTicket
	}
	return nil
}

func (e *SessionTicketExtension) Len() int {
	if e.Session != nil {
		return 4 + len(e.Session.sessionTicket)
	}
	return 4
}

func (e *SessionTicketExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}

	extBodyLen := e.Len() - 4

	b[0] = byte(extensionSessionTicket >> 8)
	b[1] = byte(extensionSessionTicket)
	b[2] = byte(extBodyLen >> 8)
	b[3] = byte(extBodyLen)
	if extBodyLen > 0 {
		copy(b[4:], e.Session.sessionTicket)
	}
	return e.Len(), io.EOF
}

func (e *SessionTicketExtension) Write(_ []byte) (int, error) {
	// RFC 5077, Section 3.2
	return 0, nil
}

// GenericExtension allows to include in ClientHello arbitrary unsupported extensions.
type GenericExtension struct {
	Id   uint16
	Data []byte
}

func (e *GenericExtension) writeToUConn(uc *UConn) error {
	return nil
}

func (e *GenericExtension) Len() int {
	return 4 + len(e.Data)
}

func (e *GenericExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}

	b[0] = byte(e.Id >> 8)
	b[1] = byte(e.Id)
	b[2] = byte(len(e.Data) >> 8)
	b[3] = byte(len(e.Data))
	if len(e.Data) > 0 {
		copy(b[4:], e.Data)
	}
	return e.Len(), io.EOF
}

type UtlsExtendedMasterSecretExtension struct {
}

// TODO: update when this extension is implemented in crypto/tls
// but we probably won't have to enable it in Config
func (e *UtlsExtendedMasterSecretExtension) writeToUConn(uc *UConn) error {
	uc.HandshakeState.Hello.Ems = true
	return nil
}

func (e *UtlsExtendedMasterSecretExtension) Len() int {
	return 4
}

func (e *UtlsExtendedMasterSecretExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}
	// https://tools.ietf.org/html/rfc7627
	b[0] = byte(utlsExtensionExtendedMasterSecret >> 8)
	b[1] = byte(utlsExtensionExtendedMasterSecret)
	// The length is 0
	return e.Len(), io.EOF
}

func (e *UtlsExtendedMasterSecretExtension) Write(_ []byte) (int, error) {
	// https://tools.ietf.org/html/rfc7627
	return 0, nil
}

var extendedMasterSecretLabel = []byte("extended master secret")

// extendedMasterFromPreMasterSecret generates the master secret from the pre-master
// secret and session hash. See https://tools.ietf.org/html/rfc7627#section-4
func extendedMasterFromPreMasterSecret(version uint16, suite *cipherSuite, preMasterSecret []byte, sessionHash []byte) []byte {
	masterSecret := make([]byte, masterSecretLength)
	prfForVersion(version, suite)(masterSecret, preMasterSecret, extendedMasterSecretLabel, sessionHash)
	return masterSecret
}

// GREASE stinks with dead parrots, have to be super careful, and, if possible, not include GREASE
// https://github.com/google/boringssl/blob/1c68fa2350936ca5897a66b430ebaf333a0e43f5/ssl/internal.h
const (
	ssl_grease_cipher = iota
	ssl_grease_group
	ssl_grease_extension1
	ssl_grease_extension2
	ssl_grease_version
	ssl_grease_ticket_extension
	ssl_grease_last_index = ssl_grease_ticket_extension
)

// it is responsibility of user not to generate multiple grease extensions with same value
type UtlsGREASEExtension struct {
	Value uint16
	Body  []byte // in Chrome first grease has empty body, second grease has a single zero byte
}

func (e *UtlsGREASEExtension) writeToUConn(uc *UConn) error {
	return nil
}

// will panic if ssl_grease_last_index[index] is out of bounds.
func GetBoringGREASEValue(greaseSeed [ssl_grease_last_index]uint16, index int) uint16 {
	// GREASE value is back from deterministic to random.
	// https://github.com/google/boringssl/blob/a365138ac60f38b64bfc608b493e0f879845cb88/ssl/handshake_client.c#L530
	ret := uint16(greaseSeed[index])
	/* This generates a random value of the form 0xωaωa, for all 0 ≤ ω < 16. */
	ret = (ret & 0xf0) | 0x0a
	ret |= ret << 8
	return ret
}

func (e *UtlsGREASEExtension) Len() int {
	return 4 + len(e.Body)
}

func (e *UtlsGREASEExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}

	b[0] = byte(e.Value >> 8)
	b[1] = byte(e.Value)
	b[2] = byte(len(e.Body) >> 8)
	b[3] = byte(len(e.Body))
	if len(e.Body) > 0 {
		copy(b[4:], e.Body)
	}
	return e.Len(), io.EOF
}

func (e *UtlsGREASEExtension) Write(b []byte) (int, error) {
	e.Value = GREASE_PLACEHOLDER
	e.Body = make([]byte, len(b))
	n := copy(e.Body, b)
	return n, nil
}

type UtlsPaddingExtension struct {
	PaddingLen int
	WillPad    bool // set to false to disable extension

	// Functor for deciding on padding length based on unpadded ClientHello length.
	// If willPad is false, then this extension should not be included.
	GetPaddingLen func(clientHelloUnpaddedLen int) (paddingLen int, willPad bool)
}

func (e *UtlsPaddingExtension) writeToUConn(uc *UConn) error {
	return nil
}

func (e *UtlsPaddingExtension) Len() int {
	if e.WillPad {
		return 4 + e.PaddingLen
	} else {
		return 0
	}
}

func (e *UtlsPaddingExtension) Update(clientHelloUnpaddedLen int) {
	if e.GetPaddingLen != nil {
		e.PaddingLen, e.WillPad = e.GetPaddingLen(clientHelloUnpaddedLen)
	}
}

func (e *UtlsPaddingExtension) Read(b []byte) (int, error) {
	if !e.WillPad {
		return 0, io.EOF
	}
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}
	// https://tools.ietf.org/html/rfc7627
	b[0] = byte(utlsExtensionPadding >> 8)
	b[1] = byte(utlsExtensionPadding)
	b[2] = byte(e.PaddingLen >> 8)
	b[3] = byte(e.PaddingLen)
	return e.Len(), io.EOF
}

func (e *UtlsPaddingExtension) Write(_ []byte) (int, error) {
	e.GetPaddingLen = BoringPaddingStyle
	return 0, nil
}

// https://github.com/google/boringssl/blob/7d7554b6b3c79e707e25521e61e066ce2b996e4c/ssl/t1_lib.c#L2803
func BoringPaddingStyle(unpaddedLen int) (int, bool) {
	if unpaddedLen > 0xff && unpaddedLen < 0x200 {
		paddingLen := 0x200 - unpaddedLen
		if paddingLen >= 4+1 {
			paddingLen -= 4
		} else {
			paddingLen = 1
		}
		return paddingLen, true
	}
	return 0, false
}

// UtlsCompressCertExtension is only implemented client-side, for server certificates. Alternate
// certificate message formats (https://datatracker.ietf.org/doc/html/rfc7250) are not supported.
//
// See https://datatracker.ietf.org/doc/html/rfc8879#section-3
type UtlsCompressCertExtension struct {
	Algorithms []CertCompressionAlgo
}

func (e *UtlsCompressCertExtension) writeToUConn(uc *UConn) error {
	uc.certCompressionAlgs = e.Algorithms
	return nil
}

func (e *UtlsCompressCertExtension) Len() int {
	return 4 + 1 + (2 * len(e.Algorithms))
}

func (e *UtlsCompressCertExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}
	b[0] = byte(utlsExtensionCompressCertificate >> 8)
	b[1] = byte(utlsExtensionCompressCertificate & 0xff)

	extLen := 2 * len(e.Algorithms)
	if extLen > 255 {
		return 0, errors.New("too many certificate compression methods")
	}

	// Extension data length.
	b[2] = byte((extLen + 1) >> 8)
	b[3] = byte((extLen + 1) & 0xff)

	// Methods length.
	b[4] = byte(extLen)

	i := 5
	for _, compMethod := range e.Algorithms {
		b[i] = byte(compMethod >> 8)
		b[i+1] = byte(compMethod)
		i += 2
	}
	return e.Len(), io.EOF
}

func (e *UtlsCompressCertExtension) Write(b []byte) (int, error) {
	fullLen := len(b)
	extData := cryptobyte.String(b)
	methods := []CertCompressionAlgo{}
	methodsRaw := new(cryptobyte.String)
	if !extData.ReadUint8LengthPrefixed(methodsRaw) {
		return 0, errors.New("unable to read cert compression algorithms extension data")
	}
	for !methodsRaw.Empty() {
		var method uint16
		if !methodsRaw.ReadUint16(&method) {
			return 0, errors.New("unable to read cert compression algorithms extension data")
		}
		methods = append(methods, CertCompressionAlgo(method))
	}

	e.Algorithms = methods
	return fullLen, nil
}

/* TLS 1.3 */
type KeyShareExtension struct {
	KeyShares []KeyShare
}

func (e *KeyShareExtension) Len() int {
	return 4 + 2 + e.keySharesLen()
}

func (e *KeyShareExtension) keySharesLen() int {
	extLen := 0
	for _, ks := range e.KeyShares {
		extLen += 4 + len(ks.Data)
	}
	return extLen
}

func (e *KeyShareExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}

	b[0] = byte(extensionKeyShare >> 8)
	b[1] = byte(extensionKeyShare)
	keySharesLen := e.keySharesLen()
	b[2] = byte((keySharesLen + 2) >> 8)
	b[3] = byte(keySharesLen + 2)
	b[4] = byte((keySharesLen) >> 8)
	b[5] = byte(keySharesLen)

	i := 6
	for _, ks := range e.KeyShares {
		b[i] = byte(ks.Group >> 8)
		b[i+1] = byte(ks.Group)
		b[i+2] = byte(len(ks.Data) >> 8)
		b[i+3] = byte(len(ks.Data))
		copy(b[i+4:], ks.Data)
		i += 4 + len(ks.Data)
	}

	return e.Len(), io.EOF
}

func (e *KeyShareExtension) Write(b []byte) (int, error) {
	fullLen := len(b)
	extData := cryptobyte.String(b)
	// RFC 8446, Section 4.2.8
	var clientShares cryptobyte.String
	if !extData.ReadUint16LengthPrefixed(&clientShares) {
		return 0, errors.New("unable to read key share extension data")
	}
	keyShares := []KeyShare{}
	for !clientShares.Empty() {
		var ks KeyShare
		var group uint16
		if !clientShares.ReadUint16(&group) ||
			!readUint16LengthPrefixed(&clientShares, &ks.Data) ||
			len(ks.Data) == 0 {
			return 0, errors.New("unable to read key share extension data")
		}
		ks.Group = CurveID(unGREASEUint16(group))
		// if not GREASE, key share data will be discarded as it should
		// be generated per connection
		if ks.Group != GREASE_PLACEHOLDER {
			ks.Data = nil
		}
		keyShares = append(keyShares, ks)
	}
	e.KeyShares = keyShares
	return fullLen, nil
}

func (e *KeyShareExtension) writeToUConn(uc *UConn) error {
	uc.HandshakeState.Hello.KeyShares = e.KeyShares
	return nil
}

type PSKKeyExchangeModesExtension struct {
	Modes []uint8
}

func (e *PSKKeyExchangeModesExtension) Len() int {
	return 4 + 1 + len(e.Modes)
}

func (e *PSKKeyExchangeModesExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}

	if len(e.Modes) > 255 {
		return 0, errors.New("too many PSK Key Exchange modes")
	}

	b[0] = byte(extensionPSKModes >> 8)
	b[1] = byte(extensionPSKModes)

	modesLen := len(e.Modes)
	b[2] = byte((modesLen + 1) >> 8)
	b[3] = byte(modesLen + 1)
	b[4] = byte(modesLen)

	if len(e.Modes) > 0 {
		copy(b[5:], e.Modes)
	}

	return e.Len(), io.EOF
}

func (e *PSKKeyExchangeModesExtension) Write(b []byte) (int, error) {
	fullLen := len(b)
	extData := cryptobyte.String(b)
	// RFC 8446, Section 4.2.9
	// TODO: PSK Modes have their own form of GREASE-ing which is not currently implemented
	// the current functionality will NOT re-GREASE/re-randomize these values when using a fingerprinted spec
	// https://github.com/refraction-networking/utls/pull/58#discussion_r522354105
	// https://tools.ietf.org/html/draft-ietf-tls-grease-01#section-2
	pskModes := []uint8{}
	if !readUint8LengthPrefixed(&extData, &pskModes) {
		return 0, errors.New("unable to read PSK extension data")
	}
	e.Modes = pskModes
	return fullLen, nil
}

func (e *PSKKeyExchangeModesExtension) writeToUConn(uc *UConn) error {
	uc.HandshakeState.Hello.PskModes = e.Modes
	return nil
}

type SupportedVersionsExtension struct {
	Versions []uint16
}

func (e *SupportedVersionsExtension) writeToUConn(uc *UConn) error {
	uc.HandshakeState.Hello.SupportedVersions = e.Versions
	return nil
}

func (e *SupportedVersionsExtension) Len() int {
	return 4 + 1 + (2 * len(e.Versions))
}

func (e *SupportedVersionsExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}
	extLen := 2 * len(e.Versions)
	if extLen > 255 {
		return 0, errors.New("too many supported versions")
	}

	b[0] = byte(extensionSupportedVersions >> 8)
	b[1] = byte(extensionSupportedVersions)
	b[2] = byte((extLen + 1) >> 8)
	b[3] = byte(extLen + 1)
	b[4] = byte(extLen)

	i := 5
	for _, sv := range e.Versions {
		b[i] = byte(sv >> 8)
		b[i+1] = byte(sv)
		i += 2
	}
	return e.Len(), io.EOF
}

func (e *SupportedVersionsExtension) Write(b []byte) (int, error) {
	fullLen := len(b)
	extData := cryptobyte.String(b)
	// RFC 8446, Section 4.2.1
	var versList cryptobyte.String
	if !extData.ReadUint8LengthPrefixed(&versList) || versList.Empty() {
		return 0, errors.New("unable to read supported versions extension data")
	}
	supportedVersions := []uint16{}
	for !versList.Empty() {
		var vers uint16
		if !versList.ReadUint16(&vers) {
			return 0, errors.New("unable to read supported versions extension data")
		}
		supportedVersions = append(supportedVersions, unGREASEUint16(vers))
	}
	e.Versions = supportedVersions
	return fullLen, nil
}

// MUST NOT be part of initial ClientHello
type CookieExtension struct {
	Cookie []byte
}

func (e *CookieExtension) writeToUConn(uc *UConn) error {
	return nil
}

func (e *CookieExtension) Len() int {
	return 4 + len(e.Cookie)
}

func (e *CookieExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}

	b[0] = byte(extensionCookie >> 8)
	b[1] = byte(extensionCookie)
	b[2] = byte(len(e.Cookie) >> 8)
	b[3] = byte(len(e.Cookie))
	if len(e.Cookie) > 0 {
		copy(b[4:], e.Cookie)
	}
	return e.Len(), io.EOF
}

/*
FAKE EXTENSIONS
*/

type FakeChannelIDExtension struct {
	// The extension ID changed from 30031 to 30032. Set to true to use the old extension ID.
	OldExtensionID bool
}

func (e *FakeChannelIDExtension) writeToUConn(uc *UConn) error {
	return nil
}

func (e *FakeChannelIDExtension) Len() int {
	return 4
}

func (e *FakeChannelIDExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}
	extensionID := fakeExtensionChannelID
	if e.OldExtensionID {
		extensionID = fakeOldExtensionChannelID
	}
	// https://tools.ietf.org/html/draft-balfanz-tls-channelid-00
	b[0] = byte(extensionID >> 8)
	b[1] = byte(extensionID & 0xff)
	// The length is 0
	return e.Len(), io.EOF
}

func (e *FakeChannelIDExtension) Write(_ []byte) (int, error) {
	return 0, nil
}

type FakeRecordSizeLimitExtension struct {
	Limit uint16
}

func (e *FakeRecordSizeLimitExtension) writeToUConn(uc *UConn) error {
	return nil
}

func (e *FakeRecordSizeLimitExtension) Len() int {
	return 6
}

func (e *FakeRecordSizeLimitExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}
	// https://tools.ietf.org/html/draft-balfanz-tls-channelid-00
	b[0] = byte(fakeRecordSizeLimit >> 8)
	b[1] = byte(fakeRecordSizeLimit & 0xff)

	b[2] = byte(0)
	b[3] = byte(2)

	b[4] = byte(e.Limit >> 8)
	b[5] = byte(e.Limit & 0xff)
	return e.Len(), io.EOF
}

func (e *FakeRecordSizeLimitExtension) Write(b []byte) (int, error) {
	fullLen := len(b)
	extData := cryptobyte.String(b)
	if !extData.ReadUint16(&e.Limit) {
		return 0, errors.New("unable to read record size limit extension data")
	}
	return fullLen, nil
}

type DelegatedCredentialsExtension struct {
	AlgorithmsSignature []SignatureScheme
}

func (e *DelegatedCredentialsExtension) writeToUConn(uc *UConn) error {
	return nil
}

func (e *DelegatedCredentialsExtension) Len() int {
	return 6 + 2*len(e.AlgorithmsSignature)
}

func (e *DelegatedCredentialsExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}
	b[0] = byte(extensionDelegatedCredentials >> 8)
	b[1] = byte(extensionDelegatedCredentials)
	b[2] = byte((2 + 2*len(e.AlgorithmsSignature)) >> 8)
	b[3] = byte(2 + 2*len(e.AlgorithmsSignature))
	b[4] = byte((2 * len(e.AlgorithmsSignature)) >> 8)
	b[5] = byte(2 * len(e.AlgorithmsSignature))
	for i, sigAndHash := range e.AlgorithmsSignature {
		b[6+2*i] = byte(sigAndHash >> 8)
		b[7+2*i] = byte(sigAndHash)
	}
	return e.Len(), io.EOF
}

// https://tools.ietf.org/html/rfc8472#section-2
type FakeTokenBindingExtension struct {
	MajorVersion, MinorVersion uint8
	KeyParameters              []uint8
}

func (e *FakeTokenBindingExtension) writeToUConn(uc *UConn) error {
	return nil
}

func (e *FakeTokenBindingExtension) Len() int {
	// extension ID + data length + versions + key parameters length + key parameters
	return 2 + 2 + 2 + 1 + len(e.KeyParameters)
}

func (e *FakeTokenBindingExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}
	dataLen := e.Len() - 4
	b[0] = byte(fakeExtensionTokenBinding >> 8)
	b[1] = byte(fakeExtensionTokenBinding & 0xff)
	b[2] = byte(dataLen >> 8)
	b[3] = byte(dataLen & 0xff)
	b[4] = e.MajorVersion
	b[5] = e.MinorVersion
	b[6] = byte(len(e.KeyParameters))
	if len(e.KeyParameters) > 0 {
		copy(b[7:], e.KeyParameters)
	}
	return e.Len(), io.EOF
}

func (e *FakeTokenBindingExtension) Write(b []byte) (int, error) {
	fullLen := len(b)
	extData := cryptobyte.String(b)
	var keyParameters cryptobyte.String
	if !extData.ReadUint8(&e.MajorVersion) ||
		!extData.ReadUint8(&e.MinorVersion) ||
		!extData.ReadUint8LengthPrefixed(&keyParameters) {
		return 0, errors.New("unable to read token binding extension data")
	}
	e.KeyParameters = keyParameters
	return fullLen, nil
}

// https://datatracker.ietf.org/doc/html/draft-ietf-tls-subcerts-15#section-4.1.1

type FakeDelegatedCredentialsExtension struct {
	SupportedSignatureAlgorithms []SignatureScheme
}

func (e *FakeDelegatedCredentialsExtension) writeToUConn(uc *UConn) error {
	return nil
}

func (e *FakeDelegatedCredentialsExtension) Len() int {
	return 6 + 2*len(e.SupportedSignatureAlgorithms)
}

func (e *FakeDelegatedCredentialsExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}
	// https://datatracker.ietf.org/doc/html/draft-ietf-tls-subcerts-15#section-4.1.1
	b[0] = byte(fakeExtensionDelegatedCredentials >> 8)
	b[1] = byte(fakeExtensionDelegatedCredentials)
	b[2] = byte((2 + 2*len(e.SupportedSignatureAlgorithms)) >> 8)
	b[3] = byte((2 + 2*len(e.SupportedSignatureAlgorithms)))
	b[4] = byte((2 * len(e.SupportedSignatureAlgorithms)) >> 8)
	b[5] = byte((2 * len(e.SupportedSignatureAlgorithms)))
	for i, sigAndHash := range e.SupportedSignatureAlgorithms {
		b[6+2*i] = byte(sigAndHash >> 8)
		b[7+2*i] = byte(sigAndHash)
	}
	return e.Len(), io.EOF
}

func (e *FakeDelegatedCredentialsExtension) Write(b []byte) (int, error) {
	fullLen := len(b)
	extData := cryptobyte.String(b)
	//https://datatracker.ietf.org/doc/html/draft-ietf-tls-subcerts-15#section-4.1.1
	var supportedAlgs cryptobyte.String
	if !extData.ReadUint16LengthPrefixed(&supportedAlgs) || supportedAlgs.Empty() {
		return 0, errors.New("unable to read signature algorithms extension data")
	}
	supportedSignatureAlgorithms := []SignatureScheme{}
	for !supportedAlgs.Empty() {
		var sigAndAlg uint16
		if !supportedAlgs.ReadUint16(&sigAndAlg) {
			return 0, errors.New("unable to read signature algorithms extension data")
		}
		supportedSignatureAlgorithms = append(
			supportedSignatureAlgorithms, SignatureScheme(sigAndAlg))
	}
	e.SupportedSignatureAlgorithms = supportedSignatureAlgorithms
	return fullLen, nil
}

// FakePreSharedKeyExtension is an extension used to set the PSK extension in the
// ClientHello.
//
// Unfortunately, even when the PSK extension is set, there will be no PSK-based
// resumption since crypto/tls does not implement PSK.
type FakePreSharedKeyExtension struct {
	PskIdentities []PskIdentity
	PskBinders    [][]byte
}

func (e *FakePreSharedKeyExtension) writeToUConn(uc *UConn) error {
	uc.HandshakeState.Hello.PskIdentities = e.PskIdentities
	uc.HandshakeState.Hello.PskBinders = e.PskBinders
	return nil
}

func (e *FakePreSharedKeyExtension) Len() int {
	length := 4 // extension type + extension length
	length += 2 // identities length
	for _, identity := range e.PskIdentities {
		length += 2 + len(identity.Label) + 4 // identity length + identity + obfuscated ticket age
	}
	length += 2 // binders length
	for _, binder := range e.PskBinders {
		length += len(binder)
	}
	return length
}

func (e *FakePreSharedKeyExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}

	b[0] = byte(extensionPreSharedKey >> 8)
	b[1] = byte(extensionPreSharedKey)
	b[2] = byte((e.Len() - 4) >> 8)
	b[3] = byte(e.Len() - 4)

	// identities length
	identitiesLength := 0
	for _, identity := range e.PskIdentities {
		identitiesLength += 2 + len(identity.Label) + 4 // identity length + identity + obfuscated ticket age
	}
	b[4] = byte(identitiesLength >> 8)
	b[5] = byte(identitiesLength)

	// identities
	offset := 6
	for _, identity := range e.PskIdentities {
		b[offset] = byte(len(identity.Label) >> 8)
		b[offset+1] = byte(len(identity.Label))
		offset += 2
		copy(b[offset:], identity.Label)
		offset += len(identity.Label)
		b[offset] = byte(identity.ObfuscatedTicketAge >> 24)
		b[offset+1] = byte(identity.ObfuscatedTicketAge >> 16)
		b[offset+2] = byte(identity.ObfuscatedTicketAge >> 8)
		b[offset+3] = byte(identity.ObfuscatedTicketAge)
		offset += 4
	}

	// binders length
	bindersLength := 0
	for _, binder := range e.PskBinders {
		bindersLength += len(binder)
	}
	b[offset] = byte(bindersLength >> 8)
	b[offset+1] = byte(bindersLength)
	offset += 2

	// binders
	for _, binder := range e.PskBinders {
		copy(b[offset:], binder)
		offset += len(binder)
	}

	return e.Len(), io.EOF
}

func (e *FakePreSharedKeyExtension) Write(b []byte) (n int, err error) {
	fullLen := len(b)
	s := cryptobyte.String(b)

	var identitiesLength uint16
	if !s.ReadUint16(&identitiesLength) {
		return 0, errors.New("tls: invalid PSK extension")
	}

	// identities
	for identitiesLength > 0 {
		var identityLength uint16
		if !s.ReadUint16(&identityLength) {
			return 0, errors.New("tls: invalid PSK extension")
		}
		identitiesLength -= 2

		if identityLength > identitiesLength {
			return 0, errors.New("tls: invalid PSK extension")
		}

		var identity []byte
		if !s.ReadBytes(&identity, int(identityLength)) {
			return 0, errors.New("tls: invalid PSK extension")
		}

		identitiesLength -= identityLength // identity

		var obfuscatedTicketAge uint32
		if !s.ReadUint32(&obfuscatedTicketAge) {
			return 0, errors.New("tls: invalid PSK extension")
		}

		e.PskIdentities = append(e.PskIdentities, PskIdentity{
			Label:               identity,
			ObfuscatedTicketAge: obfuscatedTicketAge,
		})

		identitiesLength -= 4 // obfuscated ticket age
	}

	var bindersLength uint16
	if !s.ReadUint16(&bindersLength) {
		return 0, errors.New("tls: invalid PSK extension")
	}

	// binders
	for bindersLength > 0 {
		var binderLength uint8
		if !s.ReadUint8(&binderLength) {
			return 0, errors.New("tls: invalid PSK extension")
		}
		bindersLength -= 1

		if uint16(binderLength) > bindersLength {
			return 0, errors.New("tls: invalid PSK extension")
		}

		var binder []byte
		if !s.ReadBytes(&binder, int(binderLength)) {
			return 0, errors.New("tls: invalid PSK extension")
		}

		e.PskBinders = append(e.PskBinders, binder)

		bindersLength -= uint16(binderLength)
	}

	return fullLen, nil
}