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"
)

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
}

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
}

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
}

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
}

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
}

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
}

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
}

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
}

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
}

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
}

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
}

// 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(ExtensionALPS >> 8)   // hex: 44 dec: 68
	b[1] = byte(ExtensionALPS & 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
}

type ALPSExtension struct {
	SupportedProtocols []string
}

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

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

	// Read Type.
	b[0] = byte(ExtensionALPS >> 8)   // hex: 44 dec: 68
	b[1] = byte(ExtensionALPS & 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
}

type SCTExtension struct {
}

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

// why 6 ? https://github.com/refraction-networking/utls/compare/master...Mathtin:utls:master
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
}

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
}

// 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(ExtensionExtendedMasterSecret >> 8)
	b[1] = byte(ExtensionExtendedMasterSecret)
	// The length is 0
	return e.Len(), io.EOF
}

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
}

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(ExtensionPadding >> 8)
	b[1] = byte(ExtensionPadding)
	b[2] = byte(e.PaddingLen >> 8)
	b[3] = byte(e.PaddingLen)
	return e.Len(), io.EOF
}

// 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(ExtensionCompressCertificate >> 8)
	b[1] = byte(ExtensionCompressCertificate & 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
}

// 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
}

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

// @see https://github.com/refraction-networking/utls/commit/5f9c546e4d228e91d0f9dfae05b4df73e1b6a6b9
type PreSharedKeyExtension struct{}

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

func (e *PreSharedKeyExtension) Len() int {
	return 51
}

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

	extBodyLen := e.Len() - 4

	b[0] = byte(ExtensionPreSharedKey >> 8)
	b[1] = byte(ExtensionPreSharedKey)
	b[2] = byte(extBodyLen >> 8)
	b[3] = byte(extBodyLen)

	// identities.length = 4+6 = b[4...6]
	b[4] = byte(0)
	b[5] = byte(10)
	// identity.length = 4 = b[4...6]
	b[6] = byte(0)
	b[7] = byte(4)
	// identity.body = b[6...10]
	b[8] = byte(45)
	b[9] = byte(80)
	b[10] = byte(23)
	b[11] = byte(168)
	// obfuscated_ticket_age = b[10...14]
	b[12] = byte(0)
	b[13] = byte(0)
	b[14] = byte(0)
	b[15] = byte(0)
	// PskBinderEntry.length = 4 = b[14...16]
	b[16] = byte(0)
	b[17] = byte(33)
	// PskBinderEntry.body = b[16...20]
	for i := 18; i < 51; i++ {
		b[i] = byte(5)
	}
	return e.Len(), io.EOF
}

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) 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
}

// 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
}

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
}

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
}