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GNU AFFERO GENERAL PUBLIC LICENSE
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dev:
mkdir -p build && go build -o build/client-hello-mirror

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README.md Normal file
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# TLS Client Hello Mirror
This test:
* reflects the complete Client Hello message, preserving the order in which TLS parameters and extensions are sent;
* can be used to check for TLS privacy pitfalls ([session resumption](https://svs.informatik.uni-hamburg.de/publications/2018/2018-12-06-Sy-ACSAC-Tracking_Users_across_the_Web_via_TLS_Session_Resumption.pdf), [TLS fingerprinting](https://tlsfingerprint.io/), [system time exposure](https://datatracker.ietf.org/doc/html/rfc5246/#section-7.4.1.2));
* supports both HTTP and [Gemini](https://gemini.circumlunar.space/) on the same port;
* is [free as in freedom](https://www.gnu.org/philosophy/free-sw.en.html) and trivial to self-host.
The API is largely stable - fields may be added, but existing fields will not be modified or removed. IANA-assigned codes for TLS parameters and extensions are documented at:
* [TLS parameters](https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml)
* [TLS extensions](https://www.iana.org/assignments/tls-extensiontype-values/tls-extensiontype-values.xhtml)
Note that these lists do not include draft extensions and [GREASE](https://datatracker.ietf.org/doc/html/rfc8701) values. Missing values will be documented here as the project evolves.
## Installation
Instructions for Debian:
```
# install Go
apt install golang
# fetch and build the program
go get tildegit.org/nervuri/client-hello-mirror
# generate TLS certificate
openssl req -new -subj "/CN=localhost" -x509 -newkey ED25519 -days 36500 -nodes -out cert.pem -keyout privkey.pem
# run on port 4444
~/go/bin/client-hello-mirror -c cert.pem -k privkey.pem :4444
```
## Roadmap
* detect if client random begins with UNIX timestamp, as per [RFC 5246, section 7.4.1.2](https://datatracker.ietf.org/doc/html/rfc5246/#section-7.4.1.2)
* HTML & gemtext front-end
* documentation
* detect client vulnerability to session [prolongation attacks](https://svs.informatik.uni-hamburg.de/publications/2018/2018-12-06-Sy-ACSAC-Tracking_Users_across_the_Web_via_TLS_Session_Resumption.pdf#page=3)
* support sessionID-based resumption (Go's `crypto/tls` library currently does not)
* support early data / 0-RTT (Go's `crypto/tls` library currently does not)
## License
[AGPL v3.0 or later](https://www.gnu.org/licenses/agpl-3.0.en.html). If you host a modified version, you must provide users access to its source code under the same license.
## Contributing
This project is hosted [at tildegit.org](https://tildegit.org/nervuri/client-hello-mirror). If you don't want to make an account, just [shoot me an email](https://nervuri.net/contact) with your [patch](https://git-send-email.io/) / suggestion / bug report / whatever else.

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// SPDX-License-Identifier: BSD-3-Clause
// Based on Go's crypto/tls library:
// https://github.com/golang/go/blob/master/src/crypto/tls/handshake_messages.go#L69
// https://cs.opensource.google/go/go/+/refs/tags/go1.18:src/crypto/tls/handshake_messages.go;l=69
package main
import (
"crypto/md5"
"encoding/hex"
"encoding/json"
"golang.org/x/crypto/cryptobyte"
"log"
"strconv"
"strings"
)
// TLS versions
const (
versionSSL30 = 0x0300 // 768
versionTLS10 = 0x0301 // 769
versionTLS11 = 0x0302 // 770
versionTLS12 = 0x0303 // 771
versionTLS13 = 0x0304 // 772
)
// TLS compression types
const (
compressionNone uint8 = 0
)
// TLS extension numbers
const (
extensionServerName uint16 = 0
extensionStatusRequest uint16 = 5
extensionSupportedGroups uint16 = 10 // elliptic_curves in TLS versions prior to 1.3, see RFC 8446, Section 4.2.7
extensionSupportedPointFormats uint16 = 11
extensionSignatureAlgorithms uint16 = 13
extensionALPN uint16 = 16
extensionSCT uint16 = 18
extensionPadding uint16 = 21
extensionEncryptThenMac uint16 = 22
extensionExtendedMasterSecret uint16 = 23
extensionCompressCertificate uint16 = 27
extensionSessionTicket uint16 = 35
extensionPreSharedKey uint16 = 41
extensionEarlyData uint16 = 42
extensionSupportedVersions uint16 = 43
extensionCookie uint16 = 44
extensionPSKModes uint16 = 45
extensionCertificateAuthorities uint16 = 47
extensionPostHandshakeAuth uint16 = 49
extensionSignatureAlgorithmsCert uint16 = 50
extensionKeyShare uint16 = 51
extensionNextProtoNeg uint16 = 13172 // not IANA assigned
extensionApplicationSettings uint16 = 17513 // not IANA assigned
extensionRenegotiationInfo uint16 = 0xff01
)
// TLS signaling cipher suite values
const (
scsvRenegotiation uint16 = 0x00ff
)
// TLS CertificateStatusType (RFC 3546)
const (
statusTypeOCSP uint8 = 1
)
// curveID is the type of a TLS identifier for an elliptic curve. See
// https://www.iana.org/assignments/tls-parameters/tls-parameters.xml#tls-parameters-8.
//
// In TLS 1.3, this type is called NamedGroup, but at this time this library
// only supports Elliptic Curve based groups. See RFC 8446, Section 4.2.7.
type curveID uint16
const (
curveP256 curveID = 23
curveP384 curveID = 24
curveP521 curveID = 25
x25519 curveID = 29
)
// TLS 1.3 Key Share. See RFC 8446, Section 4.2.8.
type keyShare struct {
Group curveID `json:"group"`
Data byteSlice `json:"data"`
}
// TLS 1.3 PSK Identity. Can be a Session Ticket, or a reference to a saved
// session. See RFC 8446, Section 4.2.11.
type pskIdentity struct {
Identity byteSlice `json:"identity"`
ObfuscatedTicketAge uint32 `json:"obfuscated_ticket_age"`
}
// signatureScheme identifies a signature algorithm supported by TLS. See
// RFC 8446, Section 4.2.3.
type signatureScheme uint16
const (
// RSASSA-PKCS1-v1_5 algorithms.
PKCS1WithSHA256 signatureScheme = 0x0401
PKCS1WithSHA384 signatureScheme = 0x0501
PKCS1WithSHA512 signatureScheme = 0x0601
// RSASSA-PSS algorithms with public key OID rsaEncryption.
PSSWithSHA256 signatureScheme = 0x0804
PSSWithSHA384 signatureScheme = 0x0805
PSSWithSHA512 signatureScheme = 0x0806
// ECDSA algorithms. Only constrained to a specific curve in TLS 1.3.
ECDSAWithP256AndSHA256 signatureScheme = 0x0403
ECDSAWithP384AndSHA384 signatureScheme = 0x0503
ECDSAWithP521AndSHA512 signatureScheme = 0x0603
// Legacy signature and hash algorithms for TLS 1.2.
PKCS1WithSHA1 signatureScheme = 0x0201
ECDSAWithSHA1 signatureScheme = 0x0203
)
// Check if this is a GREASE value (RFC 8701)
func isGREASE(value uint16) bool {
// Values for: cipher suites, ALPN,
// extensions, named groups, signature algorithms and TLS versions:
greaseValues := [16]uint16{
0x0a0a, 0x1a1a, 0x2a2a, 0x3a3a, 0x4a4a, 0x5a5a, 0x6a6a, 0x7a7a,
0x8a8a, 0x9a9a, 0xaaaa, 0xbaba, 0xcaca, 0xdada, 0xeaea, 0xfafa,
}
// Values for PSK key exchange modes:
//greasePSKModes := [8]uint8{
// 0x0b, 0x2a, 0x49, 0x68, 0x87, 0xa6, 0xc5, 0xe4,
//}
// Run the check
for _, greaseValue := range greaseValues {
if value == greaseValue {
return true
}
}
return false
}
// readUint8LengthPrefixed acts like s.ReadUint8LengthPrefixed, but targets a
// []byte instead of a cryptobyte.String.
func readUint8LengthPrefixed(s *cryptobyte.String, out *[]byte) bool {
return s.ReadUint8LengthPrefixed((*cryptobyte.String)(out))
}
// readUint16LengthPrefixed acts like s.ReadUint16LengthPrefixed, but targets a
// []byte instead of a cryptobyte.String.
func readUint16LengthPrefixed(s *cryptobyte.String, out *[]byte) bool {
return s.ReadUint16LengthPrefixed((*cryptobyte.String)(out))
}
type byteSlice []byte
func (bs byteSlice) MarshalText() ([]byte, error) {
// Used in json.Marshal() to convert []byte to hex string.
return []byte(hex.EncodeToString(bs)), nil
}
type uint8Slice []uint8
func (numbers uint8Slice) MarshalJSON() ([]byte, error) {
// Used in json.Marshal() to convert []uint8 to array.
newSlice := make([]uint, len(numbers))
for i, n := range numbers {
newSlice[i] = uint(n)
}
return json.Marshal(newSlice)
}
type extensionData struct {
Raw byteSlice `json:"raw"`
ServerName string `json:"server_name,omitempty"`
StatusType uint8 `json:"status_type,omitempty"`
SupportedGroups []curveID `json:"supported_groups,omitempty"`
SupportedPointFormats uint8Slice `json:"supported_point_formats,omitempty"`
SupportedSignatureAlgorithms []signatureScheme `json:"supported_signature_algorithms,omitempty"`
RenegotiationInfo []byte `json:"renegotiation_info,omitempty"`
AlpnProtocols []string `json:"alpn_protocols,omitempty"`
SupportedVersions []uint16 `json:"supported_tls_versions,omitempty"`
Cookie byteSlice `json:"cookie,omitempty"`
KeyShares []keyShare `json:"key_shares,omitempty"`
PskModes uint8Slice `json:"psk_modes,omitempty"`
PskIdentities []pskIdentity `json:"psk_identities,omitempty"`
PskBinders []byteSlice `json:"psk_binders,omitempty"`
Length uint16 `json:"length,omitempty"` // padding
}
type extension struct {
Code uint16 `json:"code"`
Name string `json:"name"`
Data extensionData `json:"data"`
}
type highlights struct {
//SupportedTLSVersions []uint16
SecureRenegotiationSupport bool `json:"secure_renegotiation_support"`
OcspStaplingSupport bool `json:"ocsp_stapling_support"`
SctSupport bool `json:"sct_support"`
// Go's crypto/tls server does not support early data.
EarlyData bool `json:"-"` // don't include in JSON
JA3 string `json:"ja3"`
JA3MD5 byteSlice `json:"ja3_md5"`
}
type clientHelloMsg struct {
Raw byteSlice `json:"raw"`
RecordHeaderTLSVersion uint16 `json:"record_header_tls_version"` // TLSv1.0 (769)
TLSVersion uint16 `json:"client_tls_version"` // TLSv1.2 (771)
Random byteSlice `json:"random"`
SessionID byteSlice `json:"session_id"`
CipherSuites []uint16 `json:"cipher_suites"`
CompressionMethods uint8Slice `json:"compression_methods"`
Extensions []extension `json:"extensions"`
Highlights highlights `json:"highlights"`
}
func (m *clientHelloMsg) unmarshal(data []byte) bool {
*m = clientHelloMsg{Raw: data}
s := cryptobyte.String(data)
var random []byte
var sessionID []byte
if !s.Skip(1) || !s.ReadUint16(&m.RecordHeaderTLSVersion) || !s.Skip(2) ||
!s.Skip(4) || // message type and uint24 length field
!s.ReadUint16(&m.TLSVersion) || !s.ReadBytes(&random, 32) ||
!readUint8LengthPrefixed(&s, &sessionID) {
return false
}
m.Random = random
m.SessionID = sessionID
var cipherSuites cryptobyte.String
if !s.ReadUint16LengthPrefixed(&cipherSuites) {
return false
}
m.CipherSuites = []uint16{}
m.Highlights.SecureRenegotiationSupport = false
for !cipherSuites.Empty() {
var suite uint16
if !cipherSuites.ReadUint16(&suite) {
return false
}
if suite == scsvRenegotiation {
m.Highlights.SecureRenegotiationSupport = true
}
m.CipherSuites = append(m.CipherSuites, suite)
}
var compressionMethods []uint8
if !readUint8LengthPrefixed(&s, &compressionMethods) {
return false
}
m.CompressionMethods = compressionMethods
if s.Empty() {
// ClientHello is optionally followed by extension data
return true
}
var extensions cryptobyte.String
if !s.ReadUint16LengthPrefixed(&extensions) || !s.Empty() {
return false
}
for !extensions.Empty() {
var extension extension
var extData cryptobyte.String
if !extensions.ReadUint16(&extension.Code) ||
!extensions.ReadUint16LengthPrefixed(&extData) {
return false
}
extension.Data.Raw = []byte(extData)
switch extension.Code {
case extensionServerName:
// RFC 6066, Section 3
extension.Name = "server_name" // Server Name Indication
var nameList cryptobyte.String
if !extData.ReadUint16LengthPrefixed(&nameList) || nameList.Empty() {
return false
}
for !nameList.Empty() {
var nameType uint8
var serverName cryptobyte.String
if !nameList.ReadUint8(&nameType) ||
!nameList.ReadUint16LengthPrefixed(&serverName) ||
serverName.Empty() {
return false
}
if nameType != 0 {
continue
}
if len(extension.Data.ServerName) != 0 {
// Multiple names of the same name_type are prohibited.
return false
}
sn := string(serverName)
extension.Data.ServerName = sn
// An SNI value may not include a trailing dot.
if strings.HasSuffix(sn, ".") {
return false
}
}
case extensionNextProtoNeg:
// draft-agl-tls-nextprotoneg-04
extension.Name = "next_protocol_negotiation" // Next Protocol Negotiation
case extensionStatusRequest:
// RFC 4366, Section 3.6
extension.Name = "status_request" // Certificate Status Request
var ignored cryptobyte.String
if !extData.ReadUint8(&extension.Data.StatusType) ||
!extData.ReadUint16LengthPrefixed(&ignored) || // responder_id_list
!extData.ReadUint16LengthPrefixed(&ignored) { // request_extensions
return false
}
m.Highlights.OcspStaplingSupport = extension.Data.StatusType == statusTypeOCSP
case extensionSupportedGroups:
// RFC 4492, sections 5.1.1 and RFC 8446, Section 4.2.7
extension.Name = "supported_groups" // Supported Groups
//extension.Name = "elliptic_curves" // old name
var curves cryptobyte.String
if !extData.ReadUint16LengthPrefixed(&curves) || curves.Empty() {
return false
}
extension.Data.SupportedGroups = make([]curveID, 0)
for !curves.Empty() {
var curve uint16
if !curves.ReadUint16(&curve) {
return false
}
extension.Data.SupportedGroups = append(
extension.Data.SupportedGroups, curveID(curve))
}
case extensionSupportedPointFormats:
// RFC 4492, Section 5.1.2
extension.Name = "ec_point_formats" // Supported Point Formats
var supportedPointFormats []uint8
if !readUint8LengthPrefixed(&extData, &supportedPointFormats) ||
len(supportedPointFormats) == 0 {
return false
}
extension.Data.SupportedPointFormats = supportedPointFormats
case extensionSessionTicket:
// RFC 5077, Section 3.2
extension.Name = "session_ticket" // Session Ticket
case extensionSignatureAlgorithms:
// RFC 5246, Section 7.4.1.4.1
extension.Name = "signature_algorithms" // Signature Algorithms
var sigAndAlgs cryptobyte.String
if !extData.ReadUint16LengthPrefixed(&sigAndAlgs) || sigAndAlgs.Empty() {
return false
}
for !sigAndAlgs.Empty() {
var sigAndAlg uint16
if !sigAndAlgs.ReadUint16(&sigAndAlg) {
return false
}
extension.Data.SupportedSignatureAlgorithms = append(
extension.Data.SupportedSignatureAlgorithms,
signatureScheme(sigAndAlg))
}
case extensionSignatureAlgorithmsCert:
// RFC 8446, Section 4.2.3
extension.Name = "signature_algorithms_cert" // Signature Algorithms Cert
var sigAndAlgs cryptobyte.String
if !extData.ReadUint16LengthPrefixed(&sigAndAlgs) || sigAndAlgs.Empty() {
return false
}
for !sigAndAlgs.Empty() {
var sigAndAlg uint16
if !sigAndAlgs.ReadUint16(&sigAndAlg) {
return false
}
extension.Data.SupportedSignatureAlgorithms = append(
extension.Data.SupportedSignatureAlgorithms,
signatureScheme(sigAndAlg))
}
case extensionRenegotiationInfo:
// RFC 5746, Section 3.2
extension.Name = "renegotiation_info" // Renegotiation Indication
if !readUint8LengthPrefixed(
&extData,
&extension.Data.RenegotiationInfo) {
return false
}
m.Highlights.SecureRenegotiationSupport = true
case extensionALPN:
// RFC 7301, Section 3.1
extension.Name = "application_layer_protocol_negotiation" // Application-Layer Protocol Negotiation
var protoList cryptobyte.String
if !extData.ReadUint16LengthPrefixed(&protoList) || protoList.Empty() {
return false
}
for !protoList.Empty() {
var proto cryptobyte.String
if !protoList.ReadUint8LengthPrefixed(&proto) || proto.Empty() {
return false
}
extension.Data.AlpnProtocols = append(extension.Data.AlpnProtocols, string(proto))
}
case extensionSCT:
// RFC 6962, Section 3.3.1
extension.Name = "signed_certificate_timestamp" // Signed Certificate Timestamp
m.Highlights.SctSupport = true
case extensionSupportedVersions:
// RFC 8446, Section 4.2.1
extension.Name = "supported_versions" // Supported Versions
var versList cryptobyte.String
if !extData.ReadUint8LengthPrefixed(&versList) || versList.Empty() {
return false
}
for !versList.Empty() {
var vers uint16
if !versList.ReadUint16(&vers) {
return false
}
extension.Data.SupportedVersions = append(extension.Data.SupportedVersions, vers)
}
case extensionCookie:
// RFC 8446, Section 4.2.2
extension.Name = "cookie" // Cookie
var cookie []byte
if !readUint16LengthPrefixed(&extData, &cookie) ||
len(extension.Data.Cookie) == 0 {
return false
}
extension.Data.Cookie = cookie
case extensionKeyShare:
// RFC 8446, Section 4.2.8
extension.Name = "key_share" // Key Share
var clientShares cryptobyte.String
if !extData.ReadUint16LengthPrefixed(&clientShares) {
return false
}
for !clientShares.Empty() {
var ks keyShare
var ksData []byte
if !clientShares.ReadUint16((*uint16)(&ks.Group)) ||
!readUint16LengthPrefixed(&clientShares, &ksData) ||
len(ksData) == 0 {
return false
}
ks.Data = ksData
extension.Data.KeyShares = append(extension.Data.KeyShares, ks)
}
case extensionEarlyData:
// RFC 8446, Section 4.2.10
extension.Name = "early_data" // Early Data Indication
m.Highlights.EarlyData = true
case extensionPSKModes:
// RFC 8446, Section 4.2.9
extension.Name = "psk_key_exchange_modes" // Pre-Shared Key Exchange Modes
var pskModes []uint8
if !readUint8LengthPrefixed(&extData, &pskModes) {
return false
}
extension.Data.PskModes = pskModes
case extensionPreSharedKey:
// RFC 8446, Section 4.2.11
extension.Name = "pre_shared_key" // Pre-Shared Key
if !extensions.Empty() {
return false // pre_shared_key must be the last extension
}
var identities cryptobyte.String
if !extData.ReadUint16LengthPrefixed(&identities) || identities.Empty() {
return false
}
for !identities.Empty() {
var psk pskIdentity
var identity []byte
if !readUint16LengthPrefixed(&identities, &identity) ||
!identities.ReadUint32(&psk.ObfuscatedTicketAge) ||
len(identity) == 0 {
return false
}
psk.Identity = identity
extension.Data.PskIdentities = append(extension.Data.PskIdentities, psk)
}
var binders cryptobyte.String
if !extData.ReadUint16LengthPrefixed(&binders) || binders.Empty() {
return false
}
for !binders.Empty() {
var binder []byte
if !readUint8LengthPrefixed(&binders, &binder) ||
len(binder) == 0 {
return false
}
extension.Data.PskBinders = append(extension.Data.PskBinders, binder)
}
case extensionPadding:
// RFC 7685
extension.Name = "padding" // Padding
extension.Data.Length = uint16(len(extData))
case extensionEncryptThenMac:
// RFC 7366
extension.Name = "encrypt_then_mac" // Encrypt-then-MAC"
case extensionExtendedMasterSecret:
// RFC 7627
extension.Name = "extended_master_secret" // Extended Master Secret
case extensionPostHandshakeAuth:
// RFC 8446, Section 4.2.6
extension.Name = "post_handshake_auth" // Post-Handshake Client Authentication
case extensionCompressCertificate:
// RFC 8879
extension.Name = "compress_certificate" // Certificate Compression
// TODO: parse - https://www.rfc-editor.org/rfc/rfc8879.html
case extensionApplicationSettings:
// draft-vvv-tls-alps
// Application-Layer Protocol Settings
extension.Name = " application_settings"
// TODO: parse - https://datatracker.ietf.org/doc/html/draft-vvv-tls-alps
default:
// Check if this is a GREASE extension (RFC 8701)
if isGREASE(extension.Code) {
extension.Name = "GREASE"
} else {
log.Println("Unknown extension:", extension.Code)
}
}
m.Extensions = append(m.Extensions, extension)
if !extData.Empty() &&
extension.Name != "GREASE" &&
extension.Code != extensionPadding {
log.Printf("Extension %d data not read.\n", extension.Code)
}
}
// JA3
var ja3 strings.Builder
var supportedGroups []curveID
var supportedPointFormats uint8Slice
ja3.WriteString(strconv.FormatUint(uint64(m.TLSVersion), 10) + ",")
for i, cs := range m.CipherSuites {
if !isGREASE(cs) { // ignore GREASE values
ja3.WriteString(strconv.FormatUint(uint64(cs), 10))
if i + 1 != len(m.CipherSuites) {
ja3.WriteString("-")
}
}
}
ja3.WriteString(",")
for i, e := range m.Extensions {
if !isGREASE(e.Code) { // ignore GREASE values
ja3.WriteString(strconv.FormatUint(uint64(e.Code), 10))
if i + 1 != len(m.Extensions) {
ja3.WriteString("-")
}
if e.Code == extensionSupportedGroups {
supportedGroups = e.Data.SupportedGroups
} else if e.Code == extensionSupportedPointFormats {
supportedPointFormats = e.Data.SupportedPointFormats
}
}
}
ja3.WriteString(",")
for i, g := range supportedGroups {
if !isGREASE(uint16(g)) { // ignore GREASE values
ja3.WriteString(strconv.FormatUint(uint64(g), 10))
if i + 1 != len(supportedGroups) {
ja3.WriteString("-")
}
}
}
ja3.WriteString(",")
for i, pf := range supportedPointFormats {
if !isGREASE(uint16(pf)) { // ignore GREASE values
ja3.WriteString(strconv.FormatUint(uint64(pf), 10))
if i + 1 != len(supportedPointFormats) {
ja3.WriteString("-")
}
}
}
m.Highlights.JA3 = ja3.String()
hash := md5.Sum([]byte(m.Highlights.JA3))
m.Highlights.JA3MD5 = hash[:]
return true
}

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// SPDX-License-Identifier: AGPL-3.0-or-later
package main
import (
"bufio"
"bytes"
"crypto/tls"
"encoding/binary"
"encoding/json"
"flag"
"fmt"
"io"
"net"
"net/url"
"log"
"os"
"strings"
"time"
)
type tlsConnectionInfo struct {
TlsVersion uint16 `json:"tls_version"`
CipherSuite uint16 `json:"cipher_suite"`
SessionResumed bool `json:"session_resumed"`
NegotiatedProtocol string `json:"alpn_negotiated_protocol"` // ALPN
}
type prefixConn struct {
net.Conn
io.Reader
}
func (c prefixConn) Read(p []byte) (int, error) {
return c.Reader.Read(p)
}
const html = `<!DOCTYPE html>
<html lang="en">
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta name="theme-color" content="#000">
<meta name="referrer" content="no-referrer">
<title>TLS Client Hello Mirror</title>
<style>
:root {
color-scheme: dark;
}
::selection {
color: #FFF;
background-color: #070;
}
body {
color: #DDD;
background-color: #000;
margin: 1em auto;
max-width: 38em;
padding: 0 .62em;
font: 1.1em/1.62 sans-serif;
}
@media print{
body{
max-width: none;
}
}
a:link {color:#EEE;}
a:visited {color:#EEE;}
a:hover {color:#FFF;}
a:active {color:#FFF;}
</style>
<main>
<center>
<h1>TLS Client Hello Mirror</h1>
</center>
<h3><a href="/json/v1">Your browser's TLS Client Hello, reflected as JSON</a></h3>
<p>This test:
<ul>
<li>reflects the complete Client Hello message, preserving the order in which TLS parameters and extensions are sent;</li>
<li>can be used to check for TLS privacy pitfalls (<a href="https://svs.informatik.uni-hamburg.de/publications/2018/2018-12-06-Sy-ACSAC-Tracking_Users_across_the_Web_via_TLS_Session_Resumption.pdf">session resumption</a>, <a href="https://tlsfingerprint.io/">TLS fingerprinting</a>, <a href="https://datatracker.ietf.org/doc/html/rfc5246/#section-7.4.1.2">system time exposure</a>);</li>
<li>supports both HTTP and <a href="https://gemini.circumlunar.space/">Gemini</a>;</li>
<li>is <a href="https://www.gnu.org/philosophy/free-sw.en.html">free as in freedom</a> and trivial to self-host.</li>
</ul>
</p>
<p>JSON only, for now, but a UI is on <a href="https://tildegit.org/nervuri/client-hello-mirror#roadmap">the roadmap</a>.</p>
<p>The API is largely stable - fields may be added, but existing fields will not be modified or removed. IANA-assigned codes for TLS parameters and extensions are documented at:
<ul>
<li><a href="https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml">TLS parameters</a></li>
<li><a href="https://www.iana.org/assignments/tls-extensiontype-values/tls-extensiontype-values.xhtml">TLS extensions</a></li>
</ul>
Note that these lists do not include draft extensions and <a href="https://datatracker.ietf.org/doc/html/rfc8701">GREASE</a> values. Missing values will be documented here as the project evolves.
</p>
</main>
<hr>
<footer>
<a href="https://tildegit.org/nervuri/client-hello-mirror">Source</a> (contributions welcome)<br>
License: <a href="https://www.gnu.org/licenses/agpl-3.0.en.html">AGPL-3.0-or-later</a>
</footer>
</html>`
const gemtext = `# TLS Client Hello Mirror
=> /json/v1 Your browser's TLS Client Hello, reflected as JSON
This test:
* reflects the complete Client Hello message, preserving the order in which TLS parameters and extensions are sent;
* can be used to check for TLS privacy pitfalls (session resumption, TLS fingerprinting, system time exposure);
* supports multiple protocols (currently HTTP and Gemini);
* is free as in freedom and trivial to self-host.
JSON only, for now. The API is largely stable - fields may be added, but existing fields will not be modified or removed. IANA-assigned codes for TLS parameters and extensions are available at:
=> https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml TLS parameters
=> https://www.iana.org/assignments/tls-extensiontype-values/tls-extensiontype-values.xhtml TLS extensions
Note that these lists do not include draft extensions and GREASE values. Missing values will be documented here as the project evolves.
_____________________
=> https://tildegit.org/nervuri/client-hello-mirror Source (contributions welcome)
=> https://www.gnu.org/licenses/agpl-3.0.en.html License: AGPL-3.0-or-later`
func peek(conn net.Conn, tlsConfig *tls.Config) {
// Copy the Client Hello before starting the TLS handshake.
defer conn.Close()
var buf bytes.Buffer
_, err := io.CopyN(&buf, conn, 5) // TLS record header
if err != nil {
log.Println(err)
return
}
length := binary.BigEndian.Uint16(buf.Bytes()[3:5])
_, err = io.CopyN(&buf, conn, int64(length))
if err != nil {
log.Println(err)
return
}
rawClientHello := buf.Bytes()
pConn := prefixConn{
Conn: conn,
Reader: io.MultiReader(&buf, conn),
}
server := tls.Server(pConn, tlsConfig)
err = server.Handshake()
if err != nil {
log.Println(err)
return
}
tlsHandler(server, rawClientHello)
}
func tlsHandler(conn *tls.Conn, rawClientHello []byte) {
defer conn.Close()
scanner := bufio.NewScanner(conn)
scanner.Scan()
line := scanner.Text()
err := scanner.Err()
if err != nil {
log.Println(err)
return
}
//log.Println(line)
var protocol string
var path string // requested page
// Detect protocol by checking the first few characters.
if strings.HasPrefix(line, "gemini://") {
protocol = "Gemini"
u, err := url.Parse(line)
if err != nil {
log.Println(err)
return
}
path = u.Path
if path == "" {
path = "/"
}
if path == "/" {
_, err = conn.Write([]byte("20 text/gemini\r\n"))
} else if path == "/json/v1" {
_, err = conn.Write([]byte("20 application/json\r\n"))
}
} else if strings.HasPrefix(line, "GET ") ||
strings.HasPrefix(line, "POST ") ||
strings.HasPrefix(line, "HEAD ") {
protocol = "HTTP"
path = strings.Split(line, " ")[1]
if path == "/" {
_, err = conn.Write([]byte("HTTP/1.1 200 OK\r\n"))
_, err = conn.Write([]byte("Content-Type: text/html; charset=utf-8\r\n"))
_, err = conn.Write([]byte("Cache-Control: no-store\r\n"))
_, err = conn.Write([]byte("Vary: *\r\n"))
} else if path == "/json/v1" {
_, err = conn.Write([]byte("HTTP/1.1 200 OK\r\n"))
_, err = conn.Write([]byte("Content-Type: application/json\r\n"))
_, err = conn.Write([]byte("Cache-Control: no-store\r\n"))
_, err = conn.Write([]byte("Vary: *\r\n"))
} else {
_, err = conn.Write([]byte("HTTP/1.1 404 Not Found\r\n"))
}
_, err = conn.Write([]byte("\r\n")) // end headers
if strings.HasPrefix(line, "HEAD ") {
return // headers only
}
} else {
log.Println("Unknown protocol")
return
}
var clientHello clientHelloMsg
if !clientHello.unmarshal(rawClientHello) {
log.Println("Failed to parse Client Hello")
return
}
connectionState := conn.ConnectionState() // TLS
tlsConnInfo := tlsConnectionInfo{
connectionState.Version,
connectionState.CipherSuite,
connectionState.DidResume,
connectionState.NegotiatedProtocol,
}
if path == "/" {
if protocol == "HTTP" {
_, err = conn.Write([]byte(html))
} else if protocol == "Gemini" {
_, err = conn.Write([]byte(gemtext))
} else {
log.Println("Unknown protocol")
return
}
} else if path == "/json/v1" {
output := struct {
ClientHello clientHelloMsg `json:"client_hello"`
TlsConnectionInfo tlsConnectionInfo `json:"connection_info"`
}{
clientHello,
tlsConnInfo,
}
outputJSON, err := json.MarshalIndent(output, "", " ")
if err != nil {
log.Println(err)
return
}
_, err = conn.Write(outputJSON)
}
if err != nil {
log.Println(err)
return
}
}
func main() {
var certFile, keyFile string
var hostAndPort string
// Parse arguments
if len(os.Args) != 6 {
fmt.Println("usage: client-hello-mirror -c cert.pem -k key.pem host:port")
return
}
flag.StringVar(&certFile, "c", "", "path to certificate file")
flag.StringVar(&keyFile, "k", "", "path to private key file")
flag.Parse()
hostAndPort = flag.Arg(0)
// Listen for connections
ln, err := net.Listen("tcp", hostAndPort)
if err != nil {
log.Println(err)
return
}
defer ln.Close()
// Load cert
cert, err := tls.LoadX509KeyPair(certFile, keyFile)
if err != nil {
log.Fatal(err)
return
}
// TLS config
tlsConfig := tls.Config{
Certificates: []tls.Certificate{cert},
//MaxVersion: tls.VersionTLS12,
}
log.Println("Server started")
for {
conn, err := ln.Accept()
if err != nil {
log.Println("Error accepting: ", err.Error())
continue
}
conn.SetReadDeadline(time.Now().Add(5 * time.Second)) // timeout
go peek(conn, &tlsConfig)
}
}