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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 (

"errors"

"strings"

"golang.org/x/crypto/cryptobyte"

"github.com/xtls/reality/hpke"

)

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 errMalformedECHConfig = errors.New("tls: malformed ECHConfigList")

// 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)

// Skip the length prefix

var length uint16

if !s.ReadUint16(&length) {

return nil, errMalformedECHConfig

}

if length != uint16(len(data)-2) {

return nil, errMalformedECHConfig

}

var configs []echConfig

for len(s) > 0 {

var ec echConfig

ec.raw = []byte(s)

if !s.ReadUint16(&ec.Version) {

return nil, errMalformedECHConfig

}

if !s.ReadUint16(&ec.Length) {

return nil, errMalformedECHConfig

}

if len(ec.raw) < int(ec.Length)+4 {

return nil, errMalformedECHConfig

}

ec.raw = ec.raw[:ec.Length+4]

if ec.Version != extensionEncryptedClientHello {

s.Skip(int(ec.Length))

continue

}

if !s.ReadUint8(&ec.ConfigID) {

return nil, errMalformedECHConfig

}

if !s.ReadUint16(&ec.KemID) {

return nil, errMalformedECHConfig

}

if !s.ReadUint16LengthPrefixed((*cryptobyte.String)(&ec.PublicKey)) {

return nil, errMalformedECHConfig

}

var cipherSuites cryptobyte.String

if !s.ReadUint16LengthPrefixed(&cipherSuites) {

return nil, errMalformedECHConfig

}

for !cipherSuites.Empty() {

var c echCipher

if !cipherSuites.ReadUint16(&c.KDFID) {

return nil, errMalformedECHConfig

}

if !cipherSuites.ReadUint16(&c.AEADID) {

return nil, errMalformedECHConfig

}

ec.SymmetricCipherSuite = append(ec.SymmetricCipherSuite, c)

}

if !s.ReadUint8(&ec.MaxNameLength) {

return nil, errMalformedECHConfig

}

var publicName cryptobyte.String

if !s.ReadUint8LengthPrefixed(&publicName) {

return nil, errMalformedECHConfig

}

ec.PublicName = publicName

var extensions cryptobyte.String

if !s.ReadUint16LengthPrefixed(&extensions) {

return nil, errMalformedECHConfig

}

for !extensions.Empty() {

var e echExtension

if !extensions.ReadUint16(&e.Type) {

return nil, errMalformedECHConfig

}

if !extensions.ReadUint16LengthPrefixed((*cryptobyte.String)(&e.Data)) {

return nil, errMalformedECHConfig

}

ec.Extensions = append(ec.Extensions, e)

}

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 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 *echContext, 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"

}