feat: add GREASEEncryptedClientHelloExtension (#266)

* dicttls: update ECH-related entries * wip: GREASE ECH extension * new: GREASE ECH extension * fix: GREASE ECH Read must succeed with io.EOF * new: GREASE ECH multiple payload len * new: parse ECH in EncryptedExtensions * fix: ECHConfig Length always 0 * new: GREASE ECH parrots * new: (*Config).ECHConfigs Add (*Config).ECHConfigs for future full ECH extension. * new: add GREASE ECH example Add an incomplete example of using GREASE ECH extension (Chrome 120 parrot). * fix: invalid httpGetOverConn call fix a problem in old example where httpGetOverConn was called with uTlsConn.HandshakeState.ServerHello.AlpnProtocol, which will not be populated in case TLS 1.3 is used. * new: possible InnerClientHello length
2025-04-01 02:57:36 +03:00 · 2023-12-13 19:50:50 -07:00 · 2023-12-13 19:50:50 -07:00 · b4de442d02
commit b4de442d02
parent 9521fba944
19 changed files with 925 additions and 51 deletions
--- a/alert.go
+++ b/alert.go
@ -58,6 +58,7 @@ const (
 	alertUnknownPSKIdentity           alert = 115
 	alertCertificateRequired          alert = 116
 	alertNoApplicationProtocol        alert = 120
+	alertECHRequired                  alert = 121
 )

 var alertText = map[alert]string{
@ -94,6 +95,7 @@ var alertText = map[alert]string{
 	alertUnknownPSKIdentity:           "unknown PSK identity",
 	alertCertificateRequired:          "certificate required",
 	alertNoApplicationProtocol:        "no application protocol",
+	alertECHRequired:                  "ECH required",
 }

 func (e alert) String() string {
--- a/common.go
+++ b/common.go
@ -306,6 +306,11 @@ type ConnectionState struct {

 	// ekm is a closure exposed via ExportKeyingMaterial.
 	ekm func(label string, context []byte, length int) ([]byte, error)
+
+	// ECHRetryConfigs contains the ECH retry configurations sent by the server in
+	// EncryptedExtensions message. It is only populated if the server sent the
+	// ech extension in EncryptedExtensions message.
+	ECHRetryConfigs []ECHConfig // [uTLS]
 }

 // ExportKeyingMaterial returns length bytes of exported key material in a new
@ -836,6 +841,17 @@ type Config struct {
 	// autoSessionTicketKeys is like sessionTicketKeys but is owned by the
 	// auto-rotation logic. See Config.ticketKeys.
 	autoSessionTicketKeys []ticketKey
+
+	// ECHConfigs contains the ECH configurations to be used by the ECH
+	// extension if any.
+	// It could either be distributed by the server in EncryptedExtensions
+	// message or out-of-band.
+	//
+	// If ECHConfigs is nil and an ECH extension is present, GREASEd ECH
+	// extension will be sent.
+	//
+	// If GREASE ECH extension is present, this field will be ignored.
+	ECHConfigs []ECHConfig // [uTLS]
 }

 const (
@ -921,6 +937,7 @@ func (c *Config) Clone() *Config {
 		autoSessionTicketKeys:       c.autoSessionTicketKeys,

 		PreferSkipResumptionOnNilExtension: c.PreferSkipResumptionOnNilExtension, // [UTLS]
+		ECHConfigs:                         c.ECHConfigs,                         // [uTLS]
 	}
 }

--- a/dicttls/hpke_aead_identifiers.go
+++ b/dicttls/hpke_aead_identifiers.go
@ -0,0 +1,19 @@
+package dicttls
+
+// source: https://www.iana.org/assignments/hpke/hpke.xhtml
+// last updated: December 2023
+
+const (
+	AEAD_AES_128_GCM       uint16 = 0x0001 // NIST Special Publication 800-38D
+	AEAD_AES_256_GCM       uint16 = 0x0002 // NIST Special Publication 800-38D
+	AEAD_CHACHA20_POLY1305 uint16 = 0x0003 // RFC 8439
+	AEAD_EXPORT_ONLY       uint16 = 0xFFFF // RFC 9180
+)
+
+var DictAEADIdentifierValueIndexed = map[uint16]string{
+	0x0000: "Reserved", // RFC 9180
+	0x0001: "AES-128-GCM",
+	0x0002: "AES-256-GCM",
+	0x0003: "ChaCha20Poly1305",
+	0xFFFF: "Export-only", // RFC 9180
+}
--- a/dicttls/hpke_kdf_identifiers.go
+++ b/dicttls/hpke_kdf_identifiers.go
@ -1,19 +1,24 @@
 package dicttls

-// source: https://www.iana.org/assignments/tls-parameters/tls-kdf-ids.csv
-// last updated: March 2023
+// source: https://www.iana.org/assignments/hpke/hpke.xhtml
+// last updated: December 2023

 const (
 	HKDF_SHA256 uint16 = 0x0001
 	HKDF_SHA384 uint16 = 0x0002
+	HKDF_SHA512 uint16 = 0x0003
 )

 var DictKDFIdentifierValueIndexed = map[uint16]string{
+	0x0000: "Reserved", // RFC 9180
 	0x0001: "HKDF_SHA256",
 	0x0002: "HKDF_SHA384",
+	0x0003: "HKDF_SHA512",
 }

 var DictKDFIdentifierNameIndexed = map[string]uint16{
+	"Reserved":    0x0000, // RFC 9180
 	"HKDF_SHA256": 0x0001,
 	"HKDF_SHA384": 0x0002,
+	"HKDF_SHA512": 0x0003,
 }
--- a/dicttls/hpke_kem_identifiers.go
+++ b/dicttls/hpke_kem_identifiers.go
@ -0,0 +1,53 @@
+package dicttls
+
+// source: https://www.iana.org/assignments/hpke/hpke.xhtml
+// last updated: December 2023
+
+const (
+	DHKEM_P256_HKDF_SHA256      uint16 = 0x0010 // RFC 5869
+	DHKEM_P384_HKDF_SHA384      uint16 = 0x0011 // RFC 5869
+	DHKEM_P521_HKDF_SHA512      uint16 = 0x0012 // RFC 5869
+	DHKEM_CP256_HKDF_SHA256     uint16 = 0x0013 // RFC 6090
+	DHKEM_CP384_HKDF_SHA384     uint16 = 0x0014 // RFC 6090
+	DHKEM_CP521_HKDF_SHA512     uint16 = 0x0015 // RFC 6090
+	DHKEM_SECP256K1_HKDF_SHA256 uint16 = 0x0016 // draft-wahby-cfrg-hpke-kem-secp256k1-01
+
+	DHKEM_X25519_HKDF_SHA256 uint16 = 0x0020 // RFC 7748
+	DHKEM_X448_HKDF_SHA512   uint16 = 0x0021 // RFC 7748
+
+	X25519_KYBER768_DRAFT00 uint16 = 0x0030 // draft-westerbaan-cfrg-hpke-xyber768d00-02
+)
+
+var DictKEMIdentifierValueIndexed = map[uint16]string{
+	0x0000: "Reserved", // RFC 9180
+
+	0x0010: "DHKEM(P-256, HKDF-SHA256)",
+	0x0011: "DHKEM(P-384, HKDF-SHA384)",
+	0x0012: "DHKEM(P-521, HKDF-SHA512)",
+	0x0013: "DHKEM(CP-256, HKDF-SHA256)",
+	0x0014: "DHKEM(CP-384, HKDF-SHA384)",
+	0x0015: "DHKEM(CP-521, HKDF-SHA512)",
+	0x0016: "DHKEM(secp256k1, HKDF-SHA256)",
+
+	0x0020: "DHKEM(X25519, HKDF-SHA256)",
+	0x0021: "DHKEM(X448, HKDF-SHA512)",
+
+	0x0030: "X25519Kyber768Draft00",
+}
+
+var DictKEMIdentifierNameIndexed = map[string]uint16{
+	"Reserved": 0x0000, // RFC 9180
+
+	"DHKEM(P-256, HKDF-SHA256)":     0x0010,
+	"DHKEM(P-384, HKDF-SHA384)":     0x0011,
+	"DHKEM(P-521, HKDF-SHA512)":     0x0012,
+	"DHKEM(CP-256, HKDF-SHA256)":    0x0013,
+	"DHKEM(CP-384, HKDF-SHA384)":    0x0014,
+	"DHKEM(CP-521, HKDF-SHA512)":    0x0015,
+	"DHKEM(secp256k1, HKDF-SHA256)": 0x0016,
+
+	"DHKEM(X25519, HKDF-SHA256)": 0x0020,
+	"DHKEM(X448, HKDF-SHA512)":   0x0021,
+
+	"X25519Kyber768Draft00": 0x0030,
+}
--- a/dicttls/kem_identifiers.go
+++ b/dicttls/kem_identifiers.go
@ -1,35 +0,0 @@
-package dicttls
-
-// source: https://www.rfc-editor.org/rfc/rfc9180
-// last updated: December 2023
-
-const (
-	DHKEM_P256_HKDF_SHA256 uint16 = 0x0010 // RFC 5869
-	DHKEM_P384_HKDF_SHA384 uint16 = 0x0011 // RFC 5869
-	DHKEM_P521_HKDF_SHA512 uint16 = 0x0012 // RFC 5869
-
-	DHKEM_X25519_HKDF_SHA256 uint16 = 0x0020 // RFC 7748
-	DHKEM_X448_HKDF_SHA512   uint16 = 0x0021 // RFC 7748
-)
-
-var DictKEMIdentifierValueIndexed = map[uint16]string{
-	0x0000: "Reserved", // RFC 9180
-
-	0x0010: "DHKEM_P256_HKDF_SHA256",
-	0x0011: "DHKEM_P384_HKDF_SHA384",
-	0x0012: "DHKEM_P521_HKDF_SHA512",
-
-	0x0020: "DHKEM_X25519_HKDF_SHA256",
-	0x0021: "DHKEM_X448_HKDF_SHA512",
-}
-
-var DictKEMIdentifierNameIndexed = map[string]uint16{
-	"Reserved": 0x0000, // RFC 9180
-
-	"DHKEM_P256_HKDF_SHA256": 0x0010,
-	"DHKEM_P384_HKDF_SHA384": 0x0011,
-	"DHKEM_P521_HKDF_SHA512": 0x0012,
-
-	"DHKEM_X25519_HKDF_SHA256": 0x0020,
-	"DHKEM_X448_HKDF_SHA512":   0x0021,
-}
--- a/examples/ech/main.go
+++ b/examples/ech/main.go
@ -0,0 +1,127 @@
+package main
+
+import (
+	"bufio"
+	"errors"
+	"fmt"
+	"io"
+	"log"
+	"net"
+	"net/http"
+	"net/url"
+	"os"
+	"time"
+
+	tls "github.com/refraction-networking/utls"
+	"golang.org/x/net/http2"
+)
+
+var (
+	dialTimeout = time.Duration(15) * time.Second
+)
+
+// var requestHostname = "crypto.cloudflare.com" // speaks http2 and TLS 1.3 and ECH and PQ
+// var requestAddr = "crypto.cloudflare.com:443"
+// var requestPath = "/cdn-cgi/trace"
+
+// var requestHostname = "tls-ech.dev" // speaks http2 and TLS 1.3 and ECH and PQ
+// var requestAddr = "tls-ech.dev:443"
+// var requestPath = "/"
+
+var requestHostname = "defo.ie" // speaks http2 and TLS 1.3 and ECH and PQ
+var requestAddr = "defo.ie:443"
+var requestPath = "/ech-check.php"
+
+// var requestHostname = "client.tlsfingerprint.io" // speaks http2 and TLS 1.3 and ECH and PQ
+// var requestAddr = "client.tlsfingerprint.io:443"
+// var requestPath = "/"
+
+func HttpGetCustom(hostname string, addr string) (*http.Response, error) {
+	klw, err := os.OpenFile("./sslkeylogging.log", os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0600)
+	if err != nil {
+		return nil, fmt.Errorf("os.OpenFile error: %+v", err)
+	}
+	config := tls.Config{
+		ServerName:   hostname,
+		KeyLogWriter: klw,
+	}
+	dialConn, err := net.DialTimeout("tcp", addr, dialTimeout)
+	if err != nil {
+		return nil, fmt.Errorf("net.DialTimeout error: %+v", err)
+	}
+	uTlsConn := tls.UClient(dialConn, &config, tls.HelloCustom)
+	defer uTlsConn.Close()
+
+	// do not use this particular spec in production
+	// make sure to generate a separate copy of ClientHelloSpec for every connection
+	spec, err := tls.UTLSIdToSpec(tls.HelloChrome_120)
+	// spec, err := tls.UTLSIdToSpec(tls.HelloFirefox_120)
+	if err != nil {
+		return nil, fmt.Errorf("tls.UTLSIdToSpec error: %+v", err)
+	}
+
+	err = uTlsConn.ApplyPreset(&spec)
+	if err != nil {
+		return nil, fmt.Errorf("uTlsConn.Handshake() error: %+v", err)
+	}
+
+	err = uTlsConn.Handshake()
+	if err != nil {
+		return nil, fmt.Errorf("uTlsConn.Handshake() error: %+v", err)
+	}
+
+	return httpGetOverConn(uTlsConn, uTlsConn.ConnectionState().NegotiatedProtocol)
+}
+
+func httpGetOverConn(conn net.Conn, alpn string) (*http.Response, error) {
+	req := &http.Request{
+		Method: "GET",
+		URL:    &url.URL{Scheme: "https", Host: requestHostname, Path: requestPath},
+		Header: make(http.Header),
+		Host:   requestHostname,
+	}
+
+	switch alpn {
+	case "h2":
+		log.Println("HTTP/2 enabled")
+		req.Proto = "HTTP/2.0"
+		req.ProtoMajor = 2
+		req.ProtoMinor = 0
+
+		tr := http2.Transport{}
+		cConn, err := tr.NewClientConn(conn)
+		if err != nil {
+			return nil, err
+		}
+		return cConn.RoundTrip(req)
+	case "http/1.1", "":
+		log.Println("Using HTTP/1.1")
+		req.Proto = "HTTP/1.1"
+		req.ProtoMajor = 1
+		req.ProtoMinor = 1
+
+		err := req.Write(conn)
+		if err != nil {
+			return nil, err
+		}
+		return http.ReadResponse(bufio.NewReader(conn), req)
+	default:
+		return nil, fmt.Errorf("unsupported ALPN: %v", alpn)
+	}
+}
+
+func main() {
+	resp, err := HttpGetCustom(requestHostname, requestAddr)
+	if err != nil {
+		panic(err)
+	}
+	fmt.Printf("Response: %+v\n", resp)
+	// read from resp.Body
+	body := make([]byte, 65535)
+	n, err := resp.Body.Read(body)
+	if err != nil && !errors.Is(err, io.EOF) {
+		panic(err)
+	}
+
+	fmt.Printf("Body: %s\n", body[:n])
+}
--- a/examples/old/examples.go
+++ b/examples/old/examples.go
@ -49,7 +49,7 @@ func HttpGetByHelloID(hostname string, addr string, helloID tls.ClientHelloID) (
 		return nil, fmt.Errorf("uTlsConn.Handshake() error: %+v", err)
 	}

-	return httpGetOverConn(uTlsConn, uTlsConn.HandshakeState.ServerHello.AlpnProtocol)
+	return httpGetOverConn(uTlsConn, uTlsConn.ConnectionState().NegotiatedProtocol)
 }

 // this example generates a randomized fingeprint, then re-uses it in a follow-up connection
@ -80,7 +80,7 @@ func HttpGetConsistentRandomized(hostname string, addr string) (*http.Response,
 		return nil, fmt.Errorf("uTlsConn.Handshake() error: %+v", err)
 	}

-	return httpGetOverConn(uTlsConn2, uTlsConn2.HandshakeState.ServerHello.AlpnProtocol)
+	return httpGetOverConn(uTlsConn2, uTlsConn2.ConnectionState().NegotiatedProtocol)
 }

 func HttpGetExplicitRandom(hostname string, addr string) (*http.Response, error) {
@ -112,7 +112,7 @@ func HttpGetExplicitRandom(hostname string, addr string) (*http.Response, error)
 	fmt.Printf("#> ClientHello Random:\n%s", hex.Dump(uTlsConn.HandshakeState.Hello.Random))
 	fmt.Printf("#> ServerHello Random:\n%s", hex.Dump(uTlsConn.HandshakeState.ServerHello.Random))

-	return httpGetOverConn(uTlsConn, uTlsConn.HandshakeState.ServerHello.AlpnProtocol)
+	return httpGetOverConn(uTlsConn, uTlsConn.ConnectionState().NegotiatedProtocol)
 }

 // Note that the server will reject the fake ticket(unless you set up your server to accept them) and do full handshake
@ -152,7 +152,7 @@ func HttpGetTicket(hostname string, addr string) (*http.Response, error) {
 	fmt.Println("#> This is how client hello with session ticket looked:")
 	fmt.Print(hex.Dump(uTlsConn.HandshakeState.Hello.Raw))

-	return httpGetOverConn(uTlsConn, uTlsConn.HandshakeState.ServerHello.AlpnProtocol)
+	return httpGetOverConn(uTlsConn, uTlsConn.ConnectionState().NegotiatedProtocol)
 }

 // Note that the server will reject the fake ticket(unless you set up your server to accept them) and do full handshake
@ -183,7 +183,7 @@ func HttpGetTicketHelloID(hostname string, addr string, helloID tls.ClientHelloI
 	fmt.Println("#> This is how client hello with session ticket looked:")
 	fmt.Print(hex.Dump(uTlsConn.HandshakeState.Hello.Raw))

-	return httpGetOverConn(uTlsConn, uTlsConn.HandshakeState.ServerHello.AlpnProtocol)
+	return httpGetOverConn(uTlsConn, uTlsConn.ConnectionState().NegotiatedProtocol)
 }

 func HttpGetCustom(hostname string, addr string) (*http.Response, error) {
@ -253,7 +253,7 @@ func HttpGetCustom(hostname string, addr string) (*http.Response, error) {
 		return nil, fmt.Errorf("uTlsConn.Handshake() error: %+v", err)
 	}

-	return httpGetOverConn(uTlsConn, uTlsConn.HandshakeState.ServerHello.AlpnProtocol)
+	return httpGetOverConn(uTlsConn, uTlsConn.ConnectionState().NegotiatedProtocol)
 }

 var roller *tls.Roller
@ -277,7 +277,7 @@ func HttpGetGoogleWithRoller() (*http.Response, error) {
 		return nil, err
 	}

-	return httpGetOverConn(c, c.HandshakeState.ServerHello.AlpnProtocol)
+	return httpGetOverConn(c, c.ConnectionState().NegotiatedProtocol)
 }

 func forgeConn() {
--- a/go.mod
+++ b/go.mod
@ -9,7 +9,7 @@ retract (

 require (
 	github.com/andybalholm/brotli v1.0.5
-	github.com/cloudflare/circl v1.3.3
+	github.com/cloudflare/circl v1.3.6
 	github.com/klauspost/compress v1.16.7
 	github.com/quic-go/quic-go v0.37.4
 	golang.org/x/crypto v0.14.0
--- a/go.sum
+++ b/go.sum
@ -1,7 +1,7 @@
 github.com/andybalholm/brotli v1.0.5 h1:8uQZIdzKmjc/iuPu7O2ioW48L81FgatrcpfFmiq/cCs=
 github.com/andybalholm/brotli v1.0.5/go.mod h1:fO7iG3H7G2nSZ7m0zPUDn85XEX2GTukHGRSepvi9Eig=
-github.com/cloudflare/circl v1.3.3 h1:fE/Qz0QdIGqeWfnwq0RE0R7MI51s0M2E4Ga9kq5AEMs=
-github.com/cloudflare/circl v1.3.3/go.mod h1:5XYMA4rFBvNIrhs50XuiBJ15vF2pZn4nnUKZrLbUZFA=
+github.com/cloudflare/circl v1.3.6 h1:/xbKIqSHbZXHwkhbrhrt2YOHIwYJlXH94E3tI/gDlUg=
+github.com/cloudflare/circl v1.3.6/go.mod h1:5XYMA4rFBvNIrhs50XuiBJ15vF2pZn4nnUKZrLbUZFA=
 github.com/go-logr/logr v1.2.4 h1:g01GSCwiDw2xSZfjJ2/T9M+S6pFdcNtFYsp+Y43HYDQ=
 github.com/go-task/slim-sprig v0.0.0-20230315185526-52ccab3ef572 h1:tfuBGBXKqDEevZMzYi5KSi8KkcZtzBcTgAUUtapy0OI=
 github.com/google/go-cmp v0.5.9 h1:O2Tfq5qg4qc4AmwVlvv0oLiVAGB7enBSJ2x2DqQFi38=
--- a/tls_test.go
+++ b/tls_test.go
@ -874,6 +874,8 @@ func TestCloneNonFuncFields(t *testing.T) {
 			continue // these are unexported fields that are handled separately
 		case "ApplicationSettings": // [UTLS] ALPS (Application Settings)
 			f.Set(reflect.ValueOf(map[string][]byte{"a": {1}}))
+		case "ECHConfigs": // [UTLS] ECH (Encrypted Client Hello) Configs
+			f.Set(reflect.ValueOf([]ECHConfig{{Version: 1}}))
 		default:
 			t.Errorf("all fields must be accounted for, but saw unknown field %q", fn)
 		}
--- a/u_common.go
+++ b/u_common.go
@ -35,9 +35,11 @@ const (
 	extensionNextProtoNeg uint16 = 13172 // not IANA assigned. Removed by crypto/tls since Nov 2019

 	utlsExtensionPadding             uint16 = 21
-	utlsExtensionCompressCertificate uint16 = 27    // https://datatracker.ietf.org/doc/html/rfc8879#section-7.1
-	utlsExtensionApplicationSettings uint16 = 17513 // not IANA assigned
-	utlsFakeExtensionCustom          uint16 = 1234  // not IANA assigned, for ALPS
+	utlsExtensionCompressCertificate uint16 = 27     // https://datatracker.ietf.org/doc/html/rfc8879#section-7.1
+	utlsExtensionApplicationSettings uint16 = 17513  // not IANA assigned
+	utlsFakeExtensionCustom          uint16 = 1234   // not IANA assigned, for ALPS
+	utlsExtensionECH                 uint16 = 0xfe0d // draft-ietf-tls-esni-17
+	utlsExtensionECHOuterExtensions  uint16 = 0xfd00 // draft-ietf-tls-esni-17

 	// extensions with 'fake' prefix break connection, if server echoes them back
 	fakeExtensionEncryptThenMAC       uint16 = 22
@ -593,6 +595,7 @@ var (
 	HelloFirefox_99   = ClientHelloID{helloFirefox, "99", nil, nil}
 	HelloFirefox_102  = ClientHelloID{helloFirefox, "102", nil, nil}
 	HelloFirefox_105  = ClientHelloID{helloFirefox, "105", nil, nil}
+	HelloFirefox_120  = ClientHelloID{helloFirefox, "120", nil, nil}

 	HelloChrome_Auto        = HelloChrome_106_Shuffle
 	HelloChrome_58          = ClientHelloID{helloChrome, "58", nil, nil}
@ -618,6 +621,9 @@ var (
 	HelloChrome_115_PQ     = ClientHelloID{helloChrome, "115_PQ", nil, nil}
 	HelloChrome_115_PQ_PSK = ClientHelloID{helloChrome, "115_PQ_PSK", nil, nil}

+	// Chrome w/ Post-Quantum Key Agreement and Encrypted ClientHello
+	HelloChrome_120 = ClientHelloID{helloChrome, "120", nil, nil}
+
 	HelloIOS_Auto = HelloIOS_14
 	HelloIOS_11_1 = ClientHelloID{helloIOS, "111", nil, nil} // legacy "111" means 11.1
 	HelloIOS_12_1 = ClientHelloID{helloIOS, "12.1", nil, nil}
--- a/u_conn.go
+++ b/u_conn.go
@ -48,6 +48,9 @@ type UConn struct {
 	// algorithms, as specified in the ClientHello. This is only relevant client-side, for the
 	// server certificate. All other forms of certificate compression are unsupported.
 	certCompressionAlgs []CertCompressionAlgo
+
+	// ech extension is a shortcut to the ECH extension in the Extensions slice if there is one.
+	ech ECHExtension
 }

 // UClient returns a new uTLS client, with behavior depending on clientHelloID.
@ -616,13 +619,19 @@ func (uconn *UConn) ApplyConfig() error {
 }

 func (uconn *UConn) MarshalClientHello() error {
+	if uconn.ech != nil {
+		if err := uconn.ech.Configure(uconn.config.ECHConfigs); err != nil {
+			return err
+		}
+		return uconn.ech.MarshalClientHello(uconn)
+	}
 	hello := uconn.HandshakeState.Hello
 	headerLength := 2 + 32 + 1 + len(hello.SessionId) +
 		2 + len(hello.CipherSuites)*2 +
 		1 + len(hello.CompressionMethods)

 	extensionsLen := 0
-	var paddingExt *UtlsPaddingExtension
+	var paddingExt *UtlsPaddingExtension // reference to padding extension, if present
 	for _, ext := range uconn.Extensions {
 		if pe, ok := ext.(*UtlsPaddingExtension); !ok {
 			// If not padding - just add length of extension to total length
@ -859,6 +868,7 @@ func (c *Conn) utlsHandshakeMessageType(msgType byte) (handshakeMessage, error)
 // Extending (*Conn).connectionStateLocked()
 func (c *Conn) utlsConnectionStateLocked(state *ConnectionState) {
 	state.PeerApplicationSettings = c.utls.peerApplicationSettings
+	state.ECHRetryConfigs = c.utls.echRetryConfigs
 }

 type utlsConnExtraFields struct {
@ -867,5 +877,8 @@ type utlsConnExtraFields struct {
 	peerApplicationSettings  []byte
 	localApplicationSettings []byte

+	// Encrypted Client Hello (ECH)
+	echRetryConfigs []ECHConfig
+
 	sessionController *sessionController
 }
--- a/u_ech.go
+++ b/u_ech.go
@ -0,0 +1,236 @@
+package tls
+
+import (
+	"crypto/rand"
+	"errors"
+	"fmt"
+	"io"
+	"math/big"
+	"sync"
+
+	"github.com/cloudflare/circl/hpke"
+)
+
+// Unstable API: This is a work in progress and may change in the future. Using
+// it in your application may cause your application to break when updating to
+// a new version of uTLS.
+
+const (
+	OuterClientHello byte = 0x00
+	InnerClientHello byte = 0x01
+)
+
+type EncryptedClientHelloExtension interface {
+	// TLSExtension must be implemented by all EncryptedClientHelloExtension implementations.
+	TLSExtension
+
+	// Configure configures the EncryptedClientHelloExtension with the given slice of ECHConfig.
+	Configure([]ECHConfig) error
+
+	// MarshalClientHello is called by (*UConn).MarshalClientHello() when an ECH extension
+	// is present to allow the ECH extension to take control of the generation of the
+	// entire ClientHello message.
+	MarshalClientHello(*UConn) error
+
+	mustEmbedUnimplementedECHExtension()
+}
+
+type ECHExtension = EncryptedClientHelloExtension // alias
+
+// type guard: GREASEEncryptedClientHelloExtension must implement EncryptedClientHelloExtension
+var (
+	_ EncryptedClientHelloExtension = (*GREASEEncryptedClientHelloExtension)(nil)
+
+	_ EncryptedClientHelloExtension = (*UnimplementedECHExtension)(nil)
+)
+
+type GREASEEncryptedClientHelloExtension struct {
+	CandidateCipherSuites []HPKESymmetricCipherSuite
+	cipherSuite           HPKESymmetricCipherSuite // randomly picked from CandidateCipherSuites or generated if empty
+	CandidateConfigIds    []uint8
+	configId              uint8    // randomly picked from CandidateConfigIds or generated if empty
+	EncapsulatedKey       []byte   // if empty, will generate random bytes
+	CandidatePayloadLens  []uint16 // Pre-encryption. If 0, will pick 128(+16=144)
+	payload               []byte   // payload should be calculated ONCE and stored here, HRR will reuse this
+
+	initOnce sync.Once
+
+	UnimplementedECHExtension
+}
+
+type GREASEECHExtension = GREASEEncryptedClientHelloExtension // alias
+
+// init initializes the GREASEEncryptedClientHelloExtension with random values if they are not set.
+//
+// Based on cloudflare/go's echGenerateGreaseExt()
+func (g *GREASEEncryptedClientHelloExtension) init() error {
+	var initErr error
+	g.initOnce.Do(func() {
+		// Set the config_id field to a random byte.
+		//
+		// Note: must not reuse this extension unless for HRR. It is required
+		// to generate new random bytes for config_id for each new ClientHello,
+		// but reuse the same config_id for HRR.
+		if len(g.CandidateConfigIds) == 0 {
+			var b []byte = make([]byte, 1)
+			_, err := rand.Read(b[:])
+			if err != nil {
+				initErr = fmt.Errorf("error generating random byte for config_id: %w", err)
+				return
+			}
+			g.configId = b[0]
+		} else {
+			// randomly pick one from the list
+			rndIndex, err := rand.Int(rand.Reader, big.NewInt(int64(len(g.CandidateConfigIds))))
+			if err != nil {
+				initErr = fmt.Errorf("error generating random index for config_id: %w", err)
+				return
+			}
+			g.configId = g.CandidateConfigIds[rndIndex.Int64()]
+		}
+
+		// Set the cipher_suite field to a supported HpkeSymmetricCipherSuite.
+		// The selection SHOULD vary to exercise all supported configurations,
+		// but MAY be held constant for successive connections to the same server
+		// in the same session.
+		if len(g.CandidateCipherSuites) == 0 {
+			_, kdf, aead := defaultHPKESuite.Params()
+			g.cipherSuite = HPKESymmetricCipherSuite{uint16(kdf), uint16(aead)}
+		} else {
+			// randomly pick one from the list
+			rndIndex, err := rand.Int(rand.Reader, big.NewInt(int64(len(g.CandidateCipherSuites))))
+			if err != nil {
+				initErr = fmt.Errorf("error generating random index for cipher_suite: %w", err)
+				return
+			}
+			g.cipherSuite = HPKESymmetricCipherSuite{
+				g.CandidateCipherSuites[rndIndex.Int64()].KdfId,
+				g.CandidateCipherSuites[rndIndex.Int64()].AeadId,
+			}
+			// aead = hpke.AEAD(g.cipherSuite.AeadId)
+		}
+
+		if len(g.EncapsulatedKey) == 0 {
+			// use default random key from cloudflare/go
+			kem := hpke.KEM_X25519_HKDF_SHA256
+
+			pk, err := kem.Scheme().UnmarshalBinaryPublicKey(dummyX25519PublicKey)
+			if err != nil {
+				initErr = fmt.Errorf("tls: grease ech: failed to parse dummy public key: %w", err)
+				return
+			}
+			sender, err := defaultHPKESuite.NewSender(pk, nil)
+			if err != nil {
+				initErr = fmt.Errorf("tls: grease ech: failed to create sender: %w", err)
+				return
+			}
+
+			g.EncapsulatedKey, _, err = sender.Setup(rand.Reader)
+			if err != nil {
+				initErr = fmt.Errorf("tls: grease ech: failed to setup encapsulated key: %w", err)
+				return
+			}
+		}
+
+		if len(g.payload) == 0 {
+			if len(g.CandidatePayloadLens) == 0 {
+				g.CandidatePayloadLens = []uint16{128}
+			}
+
+			// randomly pick one from the list
+			rndIndex, err := rand.Int(rand.Reader, big.NewInt(int64(len(g.CandidatePayloadLens))))
+			if err != nil {
+				initErr = fmt.Errorf("error generating random index for payload length: %w", err)
+				return
+			}
+
+			initErr = g.randomizePayload(g.CandidatePayloadLens[rndIndex.Int64()])
+		}
+	})
+
+	return initErr
+}
+
+func (g *GREASEEncryptedClientHelloExtension) randomizePayload(encodedHelloInnerLen uint16) error {
+	if len(g.payload) != 0 {
+		return errors.New("tls: grease ech: regenerating payload is forbidden")
+	}
+
+	aead := hpke.AEAD(g.cipherSuite.AeadId)
+	g.payload = make([]byte, int(aead.CipherLen(uint(encodedHelloInnerLen))))
+	_, err := rand.Read(g.payload)
+	if err != nil {
+		return fmt.Errorf("tls: generating grease ech payload: %w", err)
+	}
+	return nil
+}
+
+// For ECH extensions, writeToUConn simply points the ech field in UConn to the extension.
+func (g *GREASEEncryptedClientHelloExtension) writeToUConn(uconn *UConn) error {
+	// uconn.ech = g // don't do this, so we don't intercept the MarshalClientHello() call
+	return nil
+}
+
+func (g *GREASEEncryptedClientHelloExtension) Len() int {
+	g.init()
+	return 2 + 2 + 1 /* ClientHello Type */ + 4 /* CipherSuite */ + 1 /* Config ID */ + 2 + len(g.EncapsulatedKey) + 2 + len(g.payload)
+}
+
+func (g *GREASEEncryptedClientHelloExtension) Read(b []byte) (int, error) {
+	if len(b) < g.Len() {
+		return 0, io.ErrShortBuffer
+	}
+
+	b[0] = byte(utlsExtensionECH >> 8)
+	b[1] = byte(utlsExtensionECH & 0xFF)
+	b[2] = byte((g.Len() - 4) >> 8)
+	b[3] = byte((g.Len() - 4) & 0xFF)
+	b[4] = OuterClientHello
+	b[5] = byte(g.cipherSuite.KdfId >> 8)
+	b[6] = byte(g.cipherSuite.KdfId & 0xFF)
+	b[7] = byte(g.cipherSuite.AeadId >> 8)
+	b[8] = byte(g.cipherSuite.AeadId & 0xFF)
+	b[9] = g.configId
+	b[10] = byte(len(g.EncapsulatedKey) >> 8)
+	b[11] = byte(len(g.EncapsulatedKey) & 0xFF)
+	copy(b[12:], g.EncapsulatedKey)
+	b[12+len(g.EncapsulatedKey)] = byte(len(g.payload) >> 8)
+	b[12+len(g.EncapsulatedKey)+1] = byte(len(g.payload) & 0xFF)
+	copy(b[12+len(g.EncapsulatedKey)+2:], g.payload)
+
+	return g.Len(), io.EOF
+}
+
+func (*GREASEEncryptedClientHelloExtension) Configure([]ECHConfig) error {
+	return errors.New("tls: grease ech: Configure() is not implemented")
+}
+
+func (*GREASEEncryptedClientHelloExtension) MarshalClientHello(*UConn) error {
+	return errors.New("tls: grease ech: MarshalClientHello() is not implemented, use (*UConn).MarshalClientHello() instead")
+}
+
+type UnimplementedECHExtension struct{}
+
+func (*UnimplementedECHExtension) writeToUConn(_ *UConn) error {
+	return errors.New("tls: unimplemented ECHExtension")
+}
+
+func (*UnimplementedECHExtension) Len() int {
+	return 0
+}
+
+func (*UnimplementedECHExtension) Read(_ []byte) (int, error) {
+	return 0, errors.New("tls: unimplemented ECHExtension")
+}
+
+func (*UnimplementedECHExtension) Configure([]ECHConfig) error {
+	return errors.New("tls: unimplemented ECHExtension")
+}
+
+func (*UnimplementedECHExtension) MarshalClientHello(*UConn) error {
+	return errors.New("tls: unimplemented ECHExtension")
+}
+
+func (*UnimplementedECHExtension) mustEmbedUnimplementedECHExtension() {
+	panic("mustEmbedUnimplementedECHExtension() is not implemented")
+}
--- a/u_ech_config.go
+++ b/u_ech_config.go
@ -0,0 +1,135 @@
+package tls
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/cloudflare/circl/hpke"
+	"golang.org/x/crypto/cryptobyte"
+)
+
+type ECHConfigContents struct {
+	KeyConfig         HPKEKeyConfig
+	MaximumNameLength uint8
+	PublicName        []byte
+	// Extensions        []TLSExtension // ignored for now
+	rawExtensions []byte
+}
+
+func UnmarshalECHConfigContents(contents []byte) (ECHConfigContents, error) {
+	var (
+		contentCryptobyte = cryptobyte.String(contents)
+		config            ECHConfigContents
+	)
+
+	// Parse KeyConfig
+	var t cryptobyte.String
+	if !contentCryptobyte.ReadUint8(&config.KeyConfig.ConfigId) ||
+		!contentCryptobyte.ReadUint16(&config.KeyConfig.KemId) ||
+		!contentCryptobyte.ReadUint16LengthPrefixed(&t) ||
+		!t.ReadBytes(&config.KeyConfig.rawPublicKey, len(t)) ||
+		!contentCryptobyte.ReadUint16LengthPrefixed(&t) ||
+		len(t)%4 != 0 {
+		return config, errors.New("error parsing KeyConfig")
+	}
+
+	// Parse all CipherSuites in KeyConfig
+	config.KeyConfig.CipherSuites = nil
+	for !t.Empty() {
+		var kdfId, aeadId uint16
+		if !t.ReadUint16(&kdfId) || !t.ReadUint16(&aeadId) {
+			// This indicates an internal bug.
+			panic("internal error while parsing contents.cipher_suites")
+		}
+		config.KeyConfig.CipherSuites = append(config.KeyConfig.CipherSuites, HPKESymmetricCipherSuite{kdfId, aeadId})
+	}
+
+	if !contentCryptobyte.ReadUint8(&config.MaximumNameLength) ||
+		!contentCryptobyte.ReadUint8LengthPrefixed(&t) ||
+		!t.ReadBytes(&config.PublicName, len(t)) ||
+		!contentCryptobyte.ReadUint16LengthPrefixed(&t) ||
+		!t.ReadBytes(&config.rawExtensions, len(t)) ||
+		!contentCryptobyte.Empty() {
+		return config, errors.New("error parsing ECHConfigContents")
+	}
+	return config, nil
+}
+
+func (echcc *ECHConfigContents) ParsePublicKey() error {
+	var err error
+	kem := hpke.KEM(echcc.KeyConfig.KemId)
+	if !kem.IsValid() {
+		return errors.New("invalid KEM")
+	}
+	echcc.KeyConfig.PublicKey, err = kem.Scheme().UnmarshalBinaryPublicKey(echcc.KeyConfig.rawPublicKey)
+	if err != nil {
+		return fmt.Errorf("error parsing public key: %s", err)
+	}
+	return nil
+}
+
+type ECHConfig struct {
+	Version  uint16
+	Length   uint16
+	Contents ECHConfigContents
+
+	raw []byte
+}
+
+// UnmarshalECHConfigs parses a sequence of ECH configurations.
+//
+// Ported from cloudflare/go
+func UnmarshalECHConfigs(raw []byte) ([]ECHConfig, error) {
+	var (
+		err         error
+		config      ECHConfig
+		t, contents cryptobyte.String
+	)
+	configs := make([]ECHConfig, 0)
+	s := cryptobyte.String(raw)
+	if !s.ReadUint16LengthPrefixed(&t) || !s.Empty() {
+		return configs, errors.New("error parsing configs")
+	}
+	raw = raw[2:]
+ConfigsLoop:
+	for !t.Empty() {
+		l := len(t)
+		if !t.ReadUint16(&config.Version) ||
+			!t.ReadUint16LengthPrefixed(&contents) {
+			return nil, errors.New("error parsing config")
+		}
+		config.Length = uint16(len(contents))
+		n := l - len(t)
+		config.raw = raw[:n]
+		raw = raw[n:]
+
+		if config.Version != utlsExtensionECH {
+			continue ConfigsLoop
+		}
+
+		/**** cloudflare/go original ****/
+		// if !readConfigContents(&contents, &config) {
+		// 	return nil, errors.New("error parsing config contents")
+		// }
+
+		config.Contents, err = UnmarshalECHConfigContents(contents)
+		if err != nil {
+			return nil, fmt.Errorf("error parsing config contents: %s", err)
+		}
+
+		/**** cloudflare/go original ****/
+		// kem := hpke.KEM(config.kemId)
+		// if !kem.IsValid() {
+		// 	continue ConfigsLoop
+		// }
+		// config.pk, err = kem.Scheme().UnmarshalBinaryPublicKey(config.rawPublicKey)
+		// if err != nil {
+		// 	return nil, fmt.Errorf("error parsing public key: %s", err)
+		// }
+
+		config.Contents.ParsePublicKey() // parse the bytes into a public key
+
+		configs = append(configs, config)
+	}
+	return configs, nil
+}
--- a/u_handshake_client.go
+++ b/u_handshake_client.go
@ -141,6 +141,7 @@ func (hs *clientHandshakeStateTLS13) sendClientEncryptedExtensions() error {
 func (hs *clientHandshakeStateTLS13) utlsReadServerParameters(encryptedExtensions *encryptedExtensionsMsg) error {
 	hs.c.utls.hasApplicationSettings = encryptedExtensions.utls.hasApplicationSettings
 	hs.c.utls.peerApplicationSettings = encryptedExtensions.utls.applicationSettings
+	hs.c.utls.echRetryConfigs = encryptedExtensions.utls.echRetryConfigs

 	if hs.c.utls.hasApplicationSettings {
 		if hs.uconn.vers < VersionTLS13 {
@ -160,6 +161,23 @@ func (hs *clientHandshakeStateTLS13) utlsReadServerParameters(encryptedExtension
 		}
 	}

+	if len(hs.c.utls.echRetryConfigs) > 0 {
+		if hs.uconn.vers < VersionTLS13 {
+			return errors.New("tls: server sent ECH retry configs at invalid version")
+		}
+
+		// find ECH extension in ClientHello
+		var echIncluded bool
+		for _, ext := range hs.uconn.Extensions {
+			if _, ok := ext.(ECHExtension); ok {
+				echIncluded = true
+			}
+		}
+		if !echIncluded {
+			return errors.New("tls: server sent ECH retry configs without client sending ECH extension")
+		}
+	}
+
 	return nil
 }

--- a/u_handshake_messages.go
+++ b/u_handshake_messages.go
@ -56,6 +56,7 @@ func (m *utlsCompressedCertificateMsg) unmarshal(data []byte) bool {
 type utlsEncryptedExtensionsMsgExtraFields struct {
 	hasApplicationSettings bool
 	applicationSettings    []byte
+	echRetryConfigs        []ECHConfig
 	customExtension        []byte
 }

@ -64,6 +65,12 @@ func (m *encryptedExtensionsMsg) utlsUnmarshal(extension uint16, extData cryptob
 	case utlsExtensionApplicationSettings:
 		m.utls.hasApplicationSettings = true
 		m.utls.applicationSettings = []byte(extData)
+	case utlsExtensionECH:
+		var err error
+		m.utls.echRetryConfigs, err = UnmarshalECHConfigs([]byte(extData))
+		if err != nil {
+			return false
+		}
 	}
 	return true // success/unknown extension
 }
--- a/u_hpke.go
+++ b/u_hpke.go
@ -0,0 +1,62 @@
+package tls
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/cloudflare/circl/hpke"
+	"github.com/cloudflare/circl/kem"
+)
+
+type HPKERawPublicKey = []byte
+type HPKE_KEM_ID = uint16  // RFC 9180
+type HPKE_KDF_ID = uint16  // RFC 9180
+type HPKE_AEAD_ID = uint16 // RFC 9180
+
+type HPKESymmetricCipherSuite struct {
+	KdfId  HPKE_KDF_ID
+	AeadId HPKE_AEAD_ID
+}
+
+type HPKEKeyConfig struct {
+	ConfigId     uint8
+	KemId        HPKE_KEM_ID
+	PublicKey    kem.PublicKey
+	rawPublicKey HPKERawPublicKey
+	CipherSuites []HPKESymmetricCipherSuite
+}
+
+var defaultHPKESuite hpke.Suite
+
+func init() {
+	var err error
+	defaultHPKESuite, err = hpkeAssembleSuite(
+		uint16(hpke.KEM_X25519_HKDF_SHA256),
+		uint16(hpke.KDF_HKDF_SHA256),
+		uint16(hpke.AEAD_AES128GCM),
+	)
+	if err != nil {
+		panic(fmt.Sprintf("hpke: mandatory-to-implement cipher suite not supported: %s", err))
+	}
+}
+
+func hpkeAssembleSuite(kemId, kdfId, aeadId uint16) (hpke.Suite, error) {
+	kem := hpke.KEM(kemId)
+	if !kem.IsValid() {
+		return hpke.Suite{}, errors.New("KEM is not supported")
+	}
+	kdf := hpke.KDF(kdfId)
+	if !kdf.IsValid() {
+		return hpke.Suite{}, errors.New("KDF is not supported")
+	}
+	aead := hpke.AEAD(aeadId)
+	if !aead.IsValid() {
+		return hpke.Suite{}, errors.New("AEAD is not supported")
+	}
+	return hpke.NewSuite(kem, kdf, aead), nil
+}
+
+var dummyX25519PublicKey = []byte{
+	143, 38, 37, 36, 12, 6, 229, 30, 140, 27, 167, 73, 26, 100, 203, 107, 216,
+	81, 163, 222, 52, 211, 54, 210, 46, 37, 78, 216, 157, 97, 241, 244,
+}
--- a/u_parrots.go
+++ b/u_parrots.go
@ -14,6 +14,8 @@ import (
 	"math/rand"
 	"sort"
 	"strconv"
+
+	"github.com/refraction-networking/utls/dicttls"
 )

 var ErrUnknownClientHelloID = errors.New("tls: unknown ClientHelloID")
@ -656,6 +658,96 @@ func utlsIdToSpec(id ClientHelloID) (ClientHelloSpec, error) {
 				&UtlsPaddingExtension{GetPaddingLen: BoringPaddingStyle},
 			}),
 		}, nil
+	// Chrome w/ Post-Quantum Key Agreement and ECH
+	case HelloChrome_120:
+		return ClientHelloSpec{
+			CipherSuites: []uint16{
+				GREASE_PLACEHOLDER,
+				TLS_AES_128_GCM_SHA256,
+				TLS_AES_256_GCM_SHA384,
+				TLS_CHACHA20_POLY1305_SHA256,
+				TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
+				TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
+				TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,
+				TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
+				TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305,
+				TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305,
+				TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA,
+				TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA,
+				TLS_RSA_WITH_AES_128_GCM_SHA256,
+				TLS_RSA_WITH_AES_256_GCM_SHA384,
+				TLS_RSA_WITH_AES_128_CBC_SHA,
+				TLS_RSA_WITH_AES_256_CBC_SHA,
+			},
+			CompressionMethods: []byte{
+				0x00, // compressionNone
+			},
+			Extensions: ShuffleChromeTLSExtensions([]TLSExtension{
+				&UtlsGREASEExtension{},
+				&SNIExtension{},
+				&ExtendedMasterSecretExtension{},
+				&RenegotiationInfoExtension{Renegotiation: RenegotiateOnceAsClient},
+				&SupportedCurvesExtension{[]CurveID{
+					GREASE_PLACEHOLDER,
+					X25519Kyber768Draft00,
+					X25519,
+					CurveP256,
+					CurveP384,
+				}},
+				&SupportedPointsExtension{SupportedPoints: []byte{
+					0x00, // pointFormatUncompressed
+				}},
+				&SessionTicketExtension{},
+				&ALPNExtension{AlpnProtocols: []string{"h2", "http/1.1"}},
+				&StatusRequestExtension{},
+				&SignatureAlgorithmsExtension{SupportedSignatureAlgorithms: []SignatureScheme{
+					ECDSAWithP256AndSHA256,
+					PSSWithSHA256,
+					PKCS1WithSHA256,
+					ECDSAWithP384AndSHA384,
+					PSSWithSHA384,
+					PKCS1WithSHA384,
+					PSSWithSHA512,
+					PKCS1WithSHA512,
+				}},
+				&SCTExtension{},
+				&KeyShareExtension{[]KeyShare{
+					{Group: CurveID(GREASE_PLACEHOLDER), Data: []byte{0}},
+					{Group: X25519Kyber768Draft00},
+					{Group: X25519},
+				}},
+				&PSKKeyExchangeModesExtension{[]uint8{
+					PskModeDHE,
+				}},
+				&SupportedVersionsExtension{[]uint16{
+					GREASE_PLACEHOLDER,
+					VersionTLS13,
+					VersionTLS12,
+				}},
+				&UtlsCompressCertExtension{[]CertCompressionAlgo{
+					CertCompressionBrotli,
+				}},
+				&ApplicationSettingsExtension{SupportedProtocols: []string{"h2"}},
+				&GREASEEncryptedClientHelloExtension{
+					CandidateCipherSuites: []HPKESymmetricCipherSuite{
+						{
+							KdfId:  dicttls.HKDF_SHA256,
+							AeadId: dicttls.AEAD_AES_128_GCM,
+						},
+						{
+							KdfId:  dicttls.HKDF_SHA256,
+							AeadId: dicttls.AEAD_AES_256_GCM,
+						},
+						{
+							KdfId:  dicttls.HKDF_SHA256,
+							AeadId: dicttls.AEAD_CHACHA20_POLY1305,
+						},
+					},
+					CandidatePayloadLens: []uint16{128, 160}, // +16: 144, 176
+				},
+				&UtlsGREASEExtension{},
+			}),
+		}, nil
 	case HelloFirefox_55, HelloFirefox_56:
 		return ClientHelloSpec{
 			TLSVersMax: VersionTLS12,
@ -1043,6 +1135,121 @@ func utlsIdToSpec(id ClientHelloID) (ClientHelloSpec, error) {
 				},
 			},
 		}, nil
+	case HelloFirefox_120:
+		return ClientHelloSpec{
+			TLSVersMin: VersionTLS12,
+			TLSVersMax: VersionTLS13,
+			CipherSuites: []uint16{
+				TLS_AES_128_GCM_SHA256,
+				TLS_CHACHA20_POLY1305_SHA256,
+				TLS_AES_256_GCM_SHA384,
+				TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
+				TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
+				TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305,
+				TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305,
+				TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,
+				TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
+				TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA,
+				TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA,
+				TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA,
+				TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA,
+				TLS_RSA_WITH_AES_128_GCM_SHA256,
+				TLS_RSA_WITH_AES_256_GCM_SHA384,
+				TLS_RSA_WITH_AES_128_CBC_SHA,
+				TLS_RSA_WITH_AES_256_CBC_SHA,
+			},
+			CompressionMethods: []uint8{
+				0x0, // no compression
+			},
+			Extensions: []TLSExtension{
+				&SNIExtension{},
+				&ExtendedMasterSecretExtension{},
+				&RenegotiationInfoExtension{
+					Renegotiation: RenegotiateOnceAsClient,
+				},
+				&SupportedCurvesExtension{
+					Curves: []CurveID{
+						X25519,
+						CurveP256,
+						CurveP384,
+						CurveP521,
+						256,
+						257,
+					},
+				},
+				&SupportedPointsExtension{
+					SupportedPoints: []uint8{
+						0x0, // uncompressed
+					},
+				},
+				&ALPNExtension{
+					AlpnProtocols: []string{
+						"h2",
+						"http/1.1",
+					},
+				},
+				&StatusRequestExtension{},
+				&FakeDelegatedCredentialsExtension{
+					SupportedSignatureAlgorithms: []SignatureScheme{
+						ECDSAWithP256AndSHA256,
+						ECDSAWithP384AndSHA384,
+						ECDSAWithP521AndSHA512,
+						ECDSAWithSHA1,
+					},
+				},
+				&KeyShareExtension{
+					KeyShares: []KeyShare{
+						{
+							Group: X25519,
+						},
+						{
+							Group: CurveP256,
+						},
+					},
+				},
+				&SupportedVersionsExtension{
+					Versions: []uint16{
+						VersionTLS13,
+						VersionTLS12,
+					},
+				},
+				&SignatureAlgorithmsExtension{
+					SupportedSignatureAlgorithms: []SignatureScheme{
+						ECDSAWithP256AndSHA256,
+						ECDSAWithP384AndSHA384,
+						ECDSAWithP521AndSHA512,
+						PSSWithSHA256,
+						PSSWithSHA384,
+						PSSWithSHA512,
+						PKCS1WithSHA256,
+						PKCS1WithSHA384,
+						PKCS1WithSHA512,
+						ECDSAWithSHA1,
+						PKCS1WithSHA1,
+					},
+				},
+				&FakeRecordSizeLimitExtension{
+					Limit: 0x4001,
+				},
+				&GREASEEncryptedClientHelloExtension{
+					CandidateCipherSuites: []HPKESymmetricCipherSuite{
+						{
+							KdfId:  dicttls.HKDF_SHA256,
+							AeadId: dicttls.AEAD_AES_128_GCM,
+						},
+						{
+							KdfId:  dicttls.HKDF_SHA256,
+							AeadId: dicttls.AEAD_AES_256_GCM,
+						},
+						{
+							KdfId:  dicttls.HKDF_SHA256,
+							AeadId: dicttls.AEAD_CHACHA20_POLY1305,
+						},
+					},
+					CandidatePayloadLens: []uint16{128, 223}, // +16: 144, 239
+				},
+			},
+		}, nil
 	case HelloIOS_11_1:
 		return ClientHelloSpec{
 			TLSVersMax: VersionTLS12,