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// Copyright 2024 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package reality
import (
"bytes"
"errors"
"fmt"
"slices"
"strings"
"golang.org/x/crypto/cryptobyte"
"github.com/xtls/reality/hpke"
)
// sortedSupportedAEADs is just a sorted version of hpke.SupportedAEADS.
// We need this so that when we insert them into ECHConfigs the ordering
// is stable.
var sortedSupportedAEADs []uint16
func init() {
for aeadID := range hpke.SupportedAEADs {
sortedSupportedAEADs = append(sortedSupportedAEADs, aeadID)
}
slices.Sort(sortedSupportedAEADs)
}
type EchCipher struct {
KDFID uint16
AEADID uint16
}
type echExtension struct {
Type uint16
Data []byte
}
type EchConfig struct {
raw []byte
Version uint16
Length uint16
ConfigID uint8
KemID uint16
PublicKey []byte
SymmetricCipherSuite []EchCipher
MaxNameLength uint8
PublicName []byte
Extensions []echExtension
}
var errMalformedECHConfigList = errors.New("tls: malformed ECHConfigList")
type echConfigErr struct {
field string
}
func (e *echConfigErr) Error() string {
if e.field == "" {
return "tls: malformed ECHConfig"
}
return fmt.Sprintf("tls: malformed ECHConfig, invalid %s field", e.field)
}
func parseECHConfig(enc []byte) (skip bool, ec EchConfig, err error) {
s := cryptobyte.String(enc)
ec.raw = []byte(enc)
if !s.ReadUint16(&ec.Version) {
return false, EchConfig{}, &echConfigErr{"version"}
}
if !s.ReadUint16(&ec.Length) {
return false, EchConfig{}, &echConfigErr{"length"}
}
if len(ec.raw) < int(ec.Length)+4 {
return false, EchConfig{}, &echConfigErr{"length"}
}
ec.raw = ec.raw[:ec.Length+4]
if ec.Version != extensionEncryptedClientHello {
s.Skip(int(ec.Length))
return true, EchConfig{}, nil
}
if !s.ReadUint8(&ec.ConfigID) {
return false, EchConfig{}, &echConfigErr{"config_id"}
}
if !s.ReadUint16(&ec.KemID) {
return false, EchConfig{}, &echConfigErr{"kem_id"}
}
if !readUint16LengthPrefixed(&s, &ec.PublicKey) {
return false, EchConfig{}, &echConfigErr{"public_key"}
}
var cipherSuites cryptobyte.String
if !s.ReadUint16LengthPrefixed(&cipherSuites) {
return false, EchConfig{}, &echConfigErr{"cipher_suites"}
}
for !cipherSuites.Empty() {
var c EchCipher
if !cipherSuites.ReadUint16(&c.KDFID) {
return false, EchConfig{}, &echConfigErr{"cipher_suites kdf_id"}
}
if !cipherSuites.ReadUint16(&c.AEADID) {
return false, EchConfig{}, &echConfigErr{"cipher_suites aead_id"}
}
ec.SymmetricCipherSuite = append(ec.SymmetricCipherSuite, c)
}
if !s.ReadUint8(&ec.MaxNameLength) {
return false, EchConfig{}, &echConfigErr{"maximum_name_length"}
}
var publicName cryptobyte.String
if !s.ReadUint8LengthPrefixed(&publicName) {
return false, EchConfig{}, &echConfigErr{"public_name"}
}
ec.PublicName = publicName
var extensions cryptobyte.String
if !s.ReadUint16LengthPrefixed(&extensions) {
return false, EchConfig{}, &echConfigErr{"extensions"}
}
for !extensions.Empty() {
var e echExtension
if !extensions.ReadUint16(&e.Type) {
return false, EchConfig{}, &echConfigErr{"extensions type"}
}
if !extensions.ReadUint16LengthPrefixed((*cryptobyte.String)(&e.Data)) {
return false, EchConfig{}, &echConfigErr{"extensions data"}
}
ec.Extensions = append(ec.Extensions, e)
}
return false, ec, nil
}
// parseECHConfigList parses a draft-ietf-tls-esni-18 ECHConfigList, returning a
// slice of parsed ECHConfigs, in the same order they were parsed, or an error
// if the list is malformed.
func parseECHConfigList(data []byte) ([]EchConfig, error) {
s := cryptobyte.String(data)
var length uint16
if !s.ReadUint16(&length) {
return nil, errMalformedECHConfigList
}
if length != uint16(len(data)-2) {
return nil, errMalformedECHConfigList
}
var configs []EchConfig
for len(s) > 0 {
if len(s) < 4 {
return nil, errors.New("tls: malformed ECHConfig")
}
configLen := uint16(s[2])<<8 | uint16(s[3])
skip, ec, err := parseECHConfig(s)
if err != nil {
return nil, err
}
s = s[configLen+4:]
if !skip {
configs = append(configs, ec)
}
}
return configs, nil
}
func pickECHConfig(list []EchConfig) *EchConfig {
for _, ec := range list {
if _, ok := hpke.SupportedKEMs[ec.KemID]; !ok {
continue
}
var validSCS bool
for _, cs := range ec.SymmetricCipherSuite {
if _, ok := hpke.SupportedAEADs[cs.AEADID]; !ok {
continue
}
if _, ok := hpke.SupportedKDFs[cs.KDFID]; !ok {
continue
}
validSCS = true
break
}
if !validSCS {
continue
}
if !validDNSName(string(ec.PublicName)) {
continue
}
var unsupportedExt bool
for _, ext := range ec.Extensions {
// If high order bit is set to 1 the extension is mandatory.
// Since we don't support any extensions, if we see a mandatory
// bit, we skip the config.
if ext.Type&uint16(1<<15) != 0 {
unsupportedExt = true
}
}
if unsupportedExt {
continue
}
return &ec
}
return nil
}
func pickECHCipherSuite(suites []EchCipher) (EchCipher, error) {
for _, s := range suites {
// NOTE: all of the supported AEADs and KDFs are fine, rather than
// imposing some sort of preference here, we just pick the first valid
// suite.
if _, ok := hpke.SupportedAEADs[s.AEADID]; !ok {
continue
}
if _, ok := hpke.SupportedKDFs[s.KDFID]; !ok {
continue
}
return s, nil
}
return EchCipher{}, errors.New("tls: no supported symmetric ciphersuites for ECH")
}
func encodeInnerClientHello(inner *clientHelloMsg, maxNameLength int) ([]byte, error) {
h, err := inner.marshalMsg(true)
if err != nil {
return nil, err
}
h = h[4:] // strip four byte prefix
var paddingLen int
if inner.serverName != "" {
paddingLen = max(0, maxNameLength-len(inner.serverName))
} else {
paddingLen = maxNameLength + 9
}
paddingLen = 31 - ((len(h) + paddingLen - 1) % 32)
return append(h, make([]byte, paddingLen)...), nil
}
func skipUint8LengthPrefixed(s *cryptobyte.String) bool {
var skip uint8
if !s.ReadUint8(&skip) {
return false
}
return s.Skip(int(skip))
}
func skipUint16LengthPrefixed(s *cryptobyte.String) bool {
var skip uint16
if !s.ReadUint16(&skip) {
return false
}
return s.Skip(int(skip))
}
type rawExtension struct {
extType uint16
data []byte
}
func extractRawExtensions(hello *clientHelloMsg) ([]rawExtension, error) {
s := cryptobyte.String(hello.original)
if !s.Skip(4+2+32) || // header, version, random
!skipUint8LengthPrefixed(&s) || // session ID
!skipUint16LengthPrefixed(&s) || // cipher suites
!skipUint8LengthPrefixed(&s) { // compression methods
return nil, errors.New("tls: malformed outer client hello")
}
var rawExtensions []rawExtension
var extensions cryptobyte.String
if !s.ReadUint16LengthPrefixed(&extensions) {
return nil, errors.New("tls: malformed outer client hello")
}
for !extensions.Empty() {
var extension uint16
var extData cryptobyte.String
if !extensions.ReadUint16(&extension) ||
!extensions.ReadUint16LengthPrefixed(&extData) {
return nil, errors.New("tls: invalid inner client hello")
}
rawExtensions = append(rawExtensions, rawExtension{extension, extData})
}
return rawExtensions, nil
}
func decodeInnerClientHello(outer *clientHelloMsg, encoded []byte) (*clientHelloMsg, error) {
// Reconstructing the inner client hello from its encoded form is somewhat
// complicated. It is missing its header (message type and length), session
// ID, and the extensions may be compressed. Since we need to put the
// extensions back in the same order as they were in the raw outer hello,
// and since we don't store the raw extensions, or the order we parsed them
// in, we need to reparse the raw extensions from the outer hello in order
// to properly insert them into the inner hello. This _should_ result in raw
// bytes which match the hello as it was generated by the client.
innerReader := cryptobyte.String(encoded)
var versionAndRandom, sessionID, cipherSuites, compressionMethods []byte
var extensions cryptobyte.String
if !innerReader.ReadBytes(&versionAndRandom, 2+32) ||
!readUint8LengthPrefixed(&innerReader, &sessionID) ||
len(sessionID) != 0 ||
!readUint16LengthPrefixed(&innerReader, &cipherSuites) ||
!readUint8LengthPrefixed(&innerReader, &compressionMethods) ||
!innerReader.ReadUint16LengthPrefixed(&extensions) {
return nil, errors.New("tls: invalid inner client hello")
}
// The specification says we must verify that the trailing padding is all
// zeros. This is kind of weird for TLS messages, where we generally just
// throw away any trailing garbage.
for _, p := range innerReader {
if p != 0 {
return nil, errors.New("tls: invalid inner client hello")
}
}
rawOuterExts, err := extractRawExtensions(outer)
if err != nil {
return nil, err
}
recon := cryptobyte.NewBuilder(nil)
recon.AddUint8(typeClientHello)
recon.AddUint24LengthPrefixed(func(recon *cryptobyte.Builder) {
recon.AddBytes(versionAndRandom)
recon.AddUint8LengthPrefixed(func(recon *cryptobyte.Builder) {
recon.AddBytes(outer.sessionId)
})
recon.AddUint16LengthPrefixed(func(recon *cryptobyte.Builder) {
recon.AddBytes(cipherSuites)
})
recon.AddUint8LengthPrefixed(func(recon *cryptobyte.Builder) {
recon.AddBytes(compressionMethods)
})
recon.AddUint16LengthPrefixed(func(recon *cryptobyte.Builder) {
for !extensions.Empty() {
var extension uint16
var extData cryptobyte.String
if !extensions.ReadUint16(&extension) ||
!extensions.ReadUint16LengthPrefixed(&extData) {
recon.SetError(errors.New("tls: invalid inner client hello"))
return
}
if extension == extensionECHOuterExtensions {
if !extData.ReadUint8LengthPrefixed(&extData) {
recon.SetError(errors.New("tls: invalid inner client hello"))
return
}
var i int
for !extData.Empty() {
var extType uint16
if !extData.ReadUint16(&extType) {
recon.SetError(errors.New("tls: invalid inner client hello"))
return
}
if extType == extensionEncryptedClientHello {
recon.SetError(errors.New("tls: invalid outer extensions"))
return
}
for ; i <= len(rawOuterExts); i++ {
if i == len(rawOuterExts) {
recon.SetError(errors.New("tls: invalid outer extensions"))
return
}
if rawOuterExts[i].extType == extType {
break
}
}
recon.AddUint16(rawOuterExts[i].extType)
recon.AddUint16LengthPrefixed(func(recon *cryptobyte.Builder) {
recon.AddBytes(rawOuterExts[i].data)
})
}
} else {
recon.AddUint16(extension)
recon.AddUint16LengthPrefixed(func(recon *cryptobyte.Builder) {
recon.AddBytes(extData)
})
}
}
})
})
reconBytes, err := recon.Bytes()
if err != nil {
return nil, err
}
inner := &clientHelloMsg{}
if !inner.unmarshal(reconBytes) {
return nil, errors.New("tls: invalid reconstructed inner client hello")
}
if !bytes.Equal(inner.encryptedClientHello, []byte{uint8(innerECHExt)}) {
return nil, errInvalidECHExt
}
hasTLS13 := false
for _, v := range inner.supportedVersions {
// Skip GREASE values (values of the form 0x?A0A).
// GREASE (Generate Random Extensions And Sustain Extensibility) is a mechanism used by
// browsers like Chrome to ensure TLS implementations correctly ignore unknown values.
// GREASE values follow a specific pattern: 0x?A0A, where ? can be any hex digit.
// These values should be ignored when processing supported TLS versions.
if v&0x0F0F == 0x0A0A && v&0xff == v>>8 {
continue
}
// Ensure at least TLS 1.3 is offered.
if v == VersionTLS13 {
hasTLS13 = true
} else if v < VersionTLS13 {
// Reject if any non-GREASE value is below TLS 1.3, as ECH requires TLS 1.3+.
return nil, errors.New("tls: client sent encrypted_client_hello extension with unsupported versions")
}
}
if !hasTLS13 {
return nil, errors.New("tls: client sent encrypted_client_hello extension but did not offer TLS 1.3")
}
return inner, nil
}
func decryptECHPayload(context *hpke.Recipient, hello, payload []byte) ([]byte, error) {
outerAAD := bytes.Replace(hello[4:], payload, make([]byte, len(payload)), 1)
return context.Open(outerAAD, payload)
}
func generateOuterECHExt(id uint8, kdfID, aeadID uint16, encodedKey []byte, payload []byte) ([]byte, error) {
var b cryptobyte.Builder
b.AddUint8(0) // outer
b.AddUint16(kdfID)
b.AddUint16(aeadID)
b.AddUint8(id)
b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) { b.AddBytes(encodedKey) })
b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) { b.AddBytes(payload) })
return b.Bytes()
}
func computeAndUpdateOuterECHExtension(outer, inner *clientHelloMsg, ech *echClientContext, useKey bool) error {
var encapKey []byte
if useKey {
encapKey = ech.encapsulatedKey
}
encodedInner, err := encodeInnerClientHello(inner, int(ech.config.MaxNameLength))
if err != nil {
return err
}
// NOTE: the tag lengths for all of the supported AEADs are the same (16
// bytes), so we have hardcoded it here. If we add support for another AEAD
// with a different tag length, we will need to change this.
encryptedLen := len(encodedInner) + 16 // AEAD tag length
outer.encryptedClientHello, err = generateOuterECHExt(ech.config.ConfigID, ech.kdfID, ech.aeadID, encapKey, make([]byte, encryptedLen))
if err != nil {
return err
}
serializedOuter, err := outer.marshal()
if err != nil {
return err
}
serializedOuter = serializedOuter[4:] // strip the four byte prefix
encryptedInner, err := ech.hpkeContext.Seal(serializedOuter, encodedInner)
if err != nil {
return err
}
outer.encryptedClientHello, err = generateOuterECHExt(ech.config.ConfigID, ech.kdfID, ech.aeadID, encapKey, encryptedInner)
if err != nil {
return err
}
return nil
}
// validDNSName is a rather rudimentary check for the validity of a DNS name.
// This is used to check if the public_name in a ECHConfig is valid when we are
// picking a config. This can be somewhat lax because even if we pick a
// valid-looking name, the DNS layer will later reject it anyway.
func validDNSName(name string) bool {
if len(name) > 253 {
return false
}
labels := strings.Split(name, ".")
if len(labels) <= 1 {
return false
}
for _, l := range labels {
labelLen := len(l)
if labelLen == 0 {
return false
}
for i, r := range l {
if r == '-' && (i == 0 || i == labelLen-1) {
return false
}
if (r < '0' || r > '9') && (r < 'a' || r > 'z') && (r < 'A' || r > 'Z') && r != '-' {
return false
}
}
}
return true
}
// ECHRejectionError is the error type returned when ECH is rejected by a remote
// server. If the server offered a ECHConfigList to use for retries, the
// RetryConfigList field will contain this list.
//
// The client may treat an ECHRejectionError with an empty set of RetryConfigs
// as a secure signal from the server.
type ECHRejectionError struct {
RetryConfigList []byte
}
func (e *ECHRejectionError) Error() string {
return "tls: server rejected ECH"
}
var errMalformedECHExt = errors.New("tls: malformed encrypted_client_hello extension")
var errInvalidECHExt = errors.New("tls: client sent invalid encrypted_client_hello extension")
type echExtType uint8
const (
innerECHExt echExtType = 1
outerECHExt echExtType = 0
)
func parseECHExt(ext []byte) (echType echExtType, cs EchCipher, configID uint8, encap []byte, payload []byte, err error) {
data := make([]byte, len(ext))
copy(data, ext)
s := cryptobyte.String(data)
var echInt uint8
if !s.ReadUint8(&echInt) {
err = errMalformedECHExt
return
}
echType = echExtType(echInt)
if echType == innerECHExt {
if !s.Empty() {
err = errMalformedECHExt
return
}
return echType, cs, 0, nil, nil, nil
}
if echType != outerECHExt {
err = errInvalidECHExt
return
}
if !s.ReadUint16(&cs.KDFID) {
err = errMalformedECHExt
return
}
if !s.ReadUint16(&cs.AEADID) {
err = errMalformedECHExt
return
}
if !s.ReadUint8(&configID) {
err = errMalformedECHExt
return
}
if !readUint16LengthPrefixed(&s, &encap) {
err = errMalformedECHExt
return
}
if !readUint16LengthPrefixed(&s, &payload) {
err = errMalformedECHExt
return
}
// NOTE: clone encap and payload so that mutating them does not mutate the
// raw extension bytes.
return echType, cs, configID, bytes.Clone(encap), bytes.Clone(payload), nil
}
func marshalEncryptedClientHelloConfigList(configs []EncryptedClientHelloKey) ([]byte, error) {
builder := cryptobyte.NewBuilder(nil)
builder.AddUint16LengthPrefixed(func(builder *cryptobyte.Builder) {
for _, c := range configs {
builder.AddBytes(c.Config)
}
})
return builder.Bytes()
}
func (c *Conn) processECHClientHello(outer *clientHelloMsg, echKeys []EncryptedClientHelloKey) (*clientHelloMsg, *echServerContext, error) {
echType, echCiphersuite, configID, encap, payload, err := parseECHExt(outer.encryptedClientHello)
if err != nil {
if errors.Is(err, errInvalidECHExt) {
c.sendAlert(alertIllegalParameter)
} else {
c.sendAlert(alertDecodeError)
}
return nil, nil, errInvalidECHExt
}
if echType == innerECHExt {
return outer, &echServerContext{inner: true}, nil
}
if len(echKeys) == 0 {
return outer, nil, nil
}
for _, echKey := range echKeys {
skip, config, err := parseECHConfig(echKey.Config)
if err != nil || skip {
c.sendAlert(alertInternalError)
return nil, nil, fmt.Errorf("tls: invalid EncryptedClientHelloKeys Config: %s", err)
}
if skip {
continue
}
echPriv, err := hpke.ParseHPKEPrivateKey(config.KemID, echKey.PrivateKey)
if err != nil {
c.sendAlert(alertInternalError)
return nil, nil, fmt.Errorf("tls: invalid EncryptedClientHelloKeys PrivateKey: %s", err)
}
info := append([]byte("tls ech\x00"), echKey.Config...)
hpkeContext, err := hpke.SetupRecipient(hpke.DHKEM_X25519_HKDF_SHA256, echCiphersuite.KDFID, echCiphersuite.AEADID, echPriv, info, encap)
if err != nil {
// attempt next trial decryption
continue
}
encodedInner, err := decryptECHPayload(hpkeContext, outer.original, payload)
if err != nil {
// attempt next trial decryption
continue
}
// NOTE: we do not enforce that the sent server_name matches the ECH
// configs PublicName, since this is not particularly important, and
// the client already had to know what it was in order to properly
// encrypt the payload. This is only a MAY in the spec, so we're not
// doing anything revolutionary.
echInner, err := decodeInnerClientHello(outer, encodedInner)
if err != nil {
c.sendAlert(alertIllegalParameter)
return nil, nil, errInvalidECHExt
}
c.echAccepted = true
return echInner, &echServerContext{
hpkeContext: hpkeContext,
configID: configID,
ciphersuite: echCiphersuite,
}, nil
}
return outer, nil, nil
}
func buildRetryConfigList(keys []EncryptedClientHelloKey) ([]byte, error) {
var atLeastOneRetryConfig bool
var retryBuilder cryptobyte.Builder
retryBuilder.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
for _, c := range keys {
if !c.SendAsRetry {
continue
}
atLeastOneRetryConfig = true
b.AddBytes(c.Config)
}
})
if !atLeastOneRetryConfig {
return nil, nil
}
return retryBuilder.Bytes()
}
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