Send ECH acceptance signal from backend server.

This CL implements the backend server behavior described in Section 7.2
of draft-ietf-tls-esni-09.

Bug: 275
Change-Id: I2e162673ce564db0cb75fc9b71ef11ed15037f4b
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/43924
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

ssl/handshake.cc

@@ -126,6 +126,8 @@ BSSL_NAMESPACE_BEGIN
 
 SSL_HANDSHAKE::SSL_HANDSHAKE(SSL *ssl_arg)
     : ssl(ssl_arg),
+      ech_present(false),
+      ech_is_inner_present(false),
       scts_requested(false),
       needs_psk_binder(false),
       handshake_finalized(false),

ssl/handshake_server.cc

@@ -644,6 +644,12 @@ static enum ssl_hs_wait_t do_read_client_hello(SSL_HANDSHAKE *hs) {
     return ssl_hs_error;
   }
 
+  if (hs->ech_present && hs->ech_is_inner_present) {
+    OPENSSL_PUT_ERROR(SSL, SSL_R_UNEXPECTED_EXTENSION);
+    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_ILLEGAL_PARAMETER);
+    return ssl_hs_error;
+  }
+
   hs->state = state12_select_certificate;
   return ssl_hs_ok;
 }

ssl/internal.h

@@ -1419,6 +1419,15 @@ bool tls13_verify_psk_binder(SSL_HANDSHAKE *hs, SSL_SESSION *session,
                              const SSLMessage &msg, CBS *binders);
 
 
+// Encrypted Client Hello.
+
+// tls13_ech_accept_confirmation computes the server's ECH acceptance signal,
+// writing it to |out|. It returns true on success, and false on failure.
+bool tls13_ech_accept_confirmation(
+    SSL_HANDSHAKE *hs, bssl::Span<uint8_t> out,
+    bssl::Span<const uint8_t> server_hello_ech_conf);
+
+
 // Handshake functions.
 
 enum ssl_hs_wait_t {
@@ -1720,6 +1729,14 @@ struct SSL_HANDSHAKE {
   // key_block is the record-layer key block for TLS 1.2 and earlier.
   Array<uint8_t> key_block;
 
+  // ech_present, on the server, indicates whether the ClientHello contained an
+  // encrypted_client_hello extension.
+  bool ech_present : 1;
+
+  // ech_is_inner_present, on the server, indicates whether the ClientHello
+  // contained an ech_is_inner extension.
+  bool ech_is_inner_present : 1;
+
   // scts_requested is true if the SCT extension is in the ClientHello.
   bool scts_requested : 1;
 
@@ -1886,7 +1903,8 @@ bool ssl_ext_key_share_parse_serverhello(SSL_HANDSHAKE *hs,
 bool ssl_ext_key_share_parse_clienthello(SSL_HANDSHAKE *hs, bool *out_found,
                                          Array<uint8_t> *out_secret,
                                          uint8_t *out_alert, CBS *contents);
-bool ssl_ext_key_share_add_serverhello(SSL_HANDSHAKE *hs, CBB *out);
+bool ssl_ext_key_share_add_serverhello(SSL_HANDSHAKE *hs, CBB *out,
+                                       bool dry_run);
 
 bool ssl_ext_pre_shared_key_parse_serverhello(SSL_HANDSHAKE *hs,
                                               uint8_t *out_alert,

ssl/t1_lib.cc

@@ -739,6 +739,33 @@ static bool ext_ech_parse_serverhello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
   return true;
 }
 
+static bool ext_ech_parse_clienthello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
+                                      CBS *contents) {
+  if (contents != nullptr) {
+    hs->ech_present = true;
+    return true;
+  }
+  return true;
+}
+
+static bool ext_ech_is_inner_add_clienthello(SSL_HANDSHAKE *hs, CBB *out) {
+  return true;
+}
+
+static bool ext_ech_is_inner_parse_clienthello(SSL_HANDSHAKE *hs,
+                                               uint8_t *out_alert,
+                                               CBS *contents) {
+  if (contents == nullptr) {
+    return true;
+  }
+  if (CBS_len(contents) > 0) {
+    *out_alert = SSL_AD_ILLEGAL_PARAMETER;
+    return false;
+  }
+  hs->ech_is_inner_present = true;
+  return true;
+}
+
 
 // Renegotiation indication.
 //
@@ -2435,7 +2462,8 @@ bool ssl_ext_key_share_parse_clienthello(SSL_HANDSHAKE *hs, bool *out_found,
   return true;
 }
 
-bool ssl_ext_key_share_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
+bool ssl_ext_key_share_add_serverhello(SSL_HANDSHAKE *hs, CBB *out,
+                                       bool dry_run) {
   uint16_t group_id;
   CBB kse_bytes, public_key;
   if (!tls1_get_shared_group(hs, &group_id) ||
@@ -2448,10 +2476,10 @@ bool ssl_ext_key_share_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
       !CBB_flush(out)) {
     return false;
   }
-
-  hs->ecdh_public_key.Reset();
-
-  hs->new_session->group_id = group_id;
+  if (!dry_run) {
+    hs->ecdh_public_key.Reset();
+    hs->new_session->group_id = group_id;
+  }
   return true;
 }
 
@@ -3128,7 +3156,15 @@ static const struct tls_extension kExtensions[] = {
     NULL,
     ext_ech_add_clienthello,
     ext_ech_parse_serverhello,
-    ignore_parse_clienthello,
+    ext_ech_parse_clienthello,
+    dont_add_serverhello,
+  },
+  {
+    TLSEXT_TYPE_ech_is_inner,
+    NULL,
+    ext_ech_is_inner_add_clienthello,
+    forbid_parse_serverhello,
+    ext_ech_is_inner_parse_clienthello,
     dont_add_serverhello,
   },
   {

ssl/test/runner/common.go

@@ -128,6 +128,7 @@ const (
 	extensionDelegatedCredentials       uint16 = 0x22   // draft-ietf-tls-subcerts-06
 	extensionDuplicate                  uint16 = 0xffff // not IANA assigned
 	extensionEncryptedClientHello       uint16 = 0xfe09 // not IANA assigned
+	extensionECHIsInner                 uint16 = 0xda09 // not IANA assigned
 )
 
 // TLS signaling cipher suite values
@@ -799,10 +800,24 @@ type ProtocolBugs struct {
 	// encrypted_client_hello extension containing a ClientECH structure.
 	ExpectClientECH bool
 
+	// ExpectServerAcceptECH causes the client to expect that the server will
+	// indicate ECH acceptance in the ServerHello.
+	ExpectServerAcceptECH bool
+
 	// SendECHRetryConfigs, if not empty, contains the ECH server's serialized
 	// retry configs.
 	SendECHRetryConfigs []byte
 
+	// SendEncryptedClientHello, when true, causes the client to send a
+	// placeholder encrypted_client_hello extension on the ClientHelloOuter
+	// message.
+	SendPlaceholderEncryptedClientHello bool
+
+	// SendECHIsInner, when non-nil, causes the client to send an ech_is_inner
+	// extension on the ClientHelloOuter message. When nil, the extension will
+	// be omitted.
+	SendECHIsInner []byte
+
 	// SwapNPNAndALPN switches the relative order between NPN and ALPN in
 	// both ClientHello and ServerHello.
 	SwapNPNAndALPN bool

ssl/test/runner/handshake_client.go

@@ -19,6 +19,8 @@ import (
 	"math/big"
 	"net"
 	"time"
+
+	"boringssl.googlesource.com/boringssl/ssl/test/runner/hpke"
 )
 
 type clientHandshakeState struct {
@@ -122,6 +124,7 @@ func (c *Conn) clientHandshake() error {
 		ocspStapling:            !c.config.Bugs.NoOCSPStapling,
 		sctListSupported:        !c.config.Bugs.NoSignedCertificateTimestamps,
 		serverName:              c.config.ServerName,
+		echIsInner:              c.config.Bugs.SendECHIsInner,
 		supportedCurves:         c.config.curvePreferences(),
 		supportedPoints:         []uint8{pointFormatUncompressed},
 		nextProtoNeg:            len(c.config.NextProtos) > 0,
@@ -211,6 +214,16 @@ func (c *Conn) clientHandshake() error {
 		hello.alpsProtocols = append(hello.alpsProtocols, protocol)
 	}
 
+	if c.config.Bugs.SendPlaceholderEncryptedClientHello {
+		hello.clientECH = &clientECH{
+			hpkeKDF:  hpke.HKDFSHA256,
+			hpkeAEAD: hpke.AES128GCM,
+			configID: []byte{0x02, 0x0d, 0xe8, 0xae, 0xf5, 0x8b, 0x59, 0xb5},
+			enc:      []byte{0xe2, 0xf0, 0x96, 0x64, 0x18, 0x35, 0x10, 0xb3},
+			payload:  []byte{0xa8, 0x53, 0x3a, 0x8d, 0xe8, 0x5f, 0x7c, 0xd7},
+		}
+	}
+
 	var keyShares map[CurveID]ecdhCurve
 	if maxVersion >= VersionTLS13 {
 		keyShares = make(map[CurveID]ecdhCurve)
@@ -740,13 +753,13 @@ NextCipherSuite:
 		hs.writeServerHash(helloRetryRequest.marshal())
 		hs.writeClientHash(secondHelloBytes)
 	}
-	hs.writeServerHash(hs.serverHello.marshal())
 
 	if c.vers >= VersionTLS13 {
 		if err := hs.doTLS13Handshake(); err != nil {
 			return err
 		}
 	} else {
+		hs.writeServerHash(hs.serverHello.marshal())
 		if c.config.Bugs.EarlyChangeCipherSpec > 0 {
 			hs.establishKeys()
 			c.writeRecord(recordTypeChangeCipherSpec, []byte{1})
@@ -839,10 +852,6 @@ func (hs *clientHandshakeState) doTLS13Handshake() error {
 		return errors.New("tls: session IDs did not match.")
 	}
 
-	// Once the PRF hash is known, TLS 1.3 does not require a handshake
-	// buffer.
-	hs.finishedHash.discardHandshakeBuffer()
-
 	zeroSecret := hs.finishedHash.zeroSecret()
 
 	// Resolve PSK and compute the early secret.
@@ -891,6 +900,25 @@ func (hs *clientHandshakeState) doTLS13Handshake() error {
 		hs.finishedHash.addEntropy(zeroSecret)
 	}
 
+	// Determine whether the server indicated ECH acceptance.
+
+	// Generate ServerHelloECHConf, which is identical to the ServerHello except
+	// that the last 8 bytes of the random value are zeroes.
+	echAcceptConfirmation := hs.finishedHash.deriveSecretPeek([]byte("ech accept confirmation"), hs.serverHello.marshalForECHConf())
+	serverAcceptedECH := bytes.Equal(echAcceptConfirmation[:8], hs.serverHello.random[24:])
+	if c.config.Bugs.ExpectServerAcceptECH && !serverAcceptedECH {
+		return errors.New("tls: server did not indicate ECH acceptance")
+	}
+	if !c.config.Bugs.ExpectServerAcceptECH && serverAcceptedECH {
+		return errors.New("tls: server indicated ECH acceptance")
+	}
+
+	// Once the PRF hash is known, TLS 1.3 does not require a handshake
+	// buffer.
+	hs.finishedHash.discardHandshakeBuffer()
+
+	hs.writeServerHash(hs.serverHello.marshal())
+
 	// Derive handshake traffic keys and switch read key to handshake
 	// traffic key.
 	clientHandshakeTrafficSecret := hs.finishedHash.deriveSecret(clientHandshakeTrafficLabel)

ssl/test/runner/handshake_messages.go

@@ -303,6 +303,7 @@ type clientHelloMsg struct {
 	nextProtoNeg            bool
 	serverName              string
 	clientECH               *clientECH
+	echIsInner              []byte
 	ocspStapling            bool
 	supportedCurves         []CurveID
 	supportedPoints         []uint8
@@ -422,7 +423,6 @@ func (m *clientHelloMsg) marshal() []byte {
 		})
 	}
 	if m.clientECH != nil {
-		// https://tools.ietf.org/html/draft-ietf-tls-esni-09
 		body := newByteBuilder()
 		body.addU16(m.clientECH.hpkeKDF)
 		body.addU16(m.clientECH.hpkeAEAD)
@@ -435,6 +435,12 @@ func (m *clientHelloMsg) marshal() []byte {
 			body: body.finish(),
 		})
 	}
+	if m.echIsInner != nil {
+		extensions = append(extensions, extension{
+			id:   extensionECHIsInner,
+			body: m.echIsInner,
+		})
+	}
 	if m.ocspStapling {
 		certificateStatusRequest := newByteBuilder()
 		// RFC 4366, section 3.6
@@ -1269,6 +1275,16 @@ func (m *serverHelloMsg) unmarshal(data []byte) bool {
 	return true
 }
 
+// marshalForECHConf marshals |m|, but zeroes out the last 8 bytes of the
+// ServerHello.random.
+func (m *serverHelloMsg) marshalForECHConf() []byte {
+	serverHelloECHConf := *m
+	serverHelloECHConf.raw = nil
+	serverHelloECHConf.random = make([]byte, 32)
+	copy(serverHelloECHConf.random[:24], m.random)
+	return serverHelloECHConf.marshal()
+}
+
 type encryptedExtensionsMsg struct {
 	raw        []byte
 	extensions serverExtensions

ssl/test/runner/prf.go

@@ -457,6 +457,16 @@ func (h *finishedHash) deriveSecret(label []byte) []byte {
 	return hkdfExpandLabel(h.hash, h.secret, label, h.appendContextHashes(nil), h.hash.Size())
 }
 
+// deriveSecretPeek is the same as deriveSecret, but it enables the caller to
+// tentatively append messages to the transcript. The |extraMessages| parameter
+// contains the bytes of these tentative messages.
+func (h *finishedHash) deriveSecretPeek(label []byte, extraMessages []byte) []byte {
+	hashPeek := h.hash.New()
+	hashPeek.Write(h.buffer)
+	hashPeek.Write(extraMessages)
+	return hkdfExpandLabel(h.hash, h.secret, label, hashPeek.Sum(nil), h.hash.Size())
+}
+
 // The following are context strings for CertificateVerify in TLS 1.3.
 var (
 	clientCertificateVerifyContextTLS13 = []byte("TLS 1.3, client CertificateVerify")

ssl/test/runner/runner.go

@@ -16246,6 +16246,95 @@ func addEncryptedClientHelloTests() {
 		expectedLocalError: "remote error: error decoding message",
 		expectedError:      ":ERROR_PARSING_EXTENSION:",
 	})
+
+	// Test that the server responds to an empty ECH extension with the acceptance
+	// confirmation.
+	testCases = append(testCases, testCase{
+		testType: serverTest,
+		name:     "ECH-Server-ECHIsInner",
+		config: Config{
+			MinVersion: VersionTLS13,
+			MaxVersion: VersionTLS13,
+			Bugs: ProtocolBugs{
+				SendECHIsInner:        []byte{},
+				ExpectServerAcceptECH: true,
+			},
+		},
+	})
+
+	// Test that server fails the handshake when it sees a nonempty ech_is_inner
+	// extension.
+	testCases = append(testCases, testCase{
+		testType: serverTest,
+		name:     "ECH-Server-ECHIsInner-NotEmpty",
+		config: Config{
+			MinVersion: VersionTLS13,
+			MaxVersion: VersionTLS13,
+			Bugs: ProtocolBugs{
+				SendECHIsInner: []byte{42, 42, 42},
+			},
+		},
+		shouldFail:         true,
+		expectedLocalError: "remote error: illegal parameter",
+		expectedError:      ":ERROR_PARSING_EXTENSION:",
+	})
+
+	// When ech_is_inner extension is absent, the server should not accept ECH.
+	testCases = append(testCases, testCase{
+		testType: serverTest,
+		name:     "ECH-Server-ECHIsInner-Absent",
+		config: Config{
+			MinVersion: VersionTLS13,
+			MaxVersion: VersionTLS13,
+			Bugs: ProtocolBugs{
+				ExpectServerAcceptECH: false,
+			},
+		},
+	})
+
+	// Test that a TLS 1.3 server that receives an ech_is_inner extension can
+	// negotiate TLS 1.2 without clobbering the downgrade signal.
+	testCases = append(testCases, testCase{
+		testType: serverTest,
+		name:     "ECH-Server-ECHIsInner-Absent-TLS12",
+		config: Config{
+			MinVersion: VersionTLS12,
+			MaxVersion: VersionTLS13,
+			Bugs: ProtocolBugs{
+				// Omit supported_versions extension so the server negotiates
+				// TLS 1.2.
+				OmitSupportedVersions: true,
+				SendECHIsInner:        []byte{},
+			},
+		},
+		// Check that the client sees the TLS 1.3 downgrade signal in
+		// ServerHello.random.
+		shouldFail:         true,
+		expectedLocalError: "tls: downgrade from TLS 1.3 detected",
+	})
+
+	// Test that the handshake fails when the client sends both
+	// encrypted_client_hello and ech_is_inner extensions.
+	//
+	// TODO(dmcardle) Replace this test once runner is capable of sending real
+	// ECH extensions. The equivalent test will send ech_is_inner and a real
+	// encrypted_client_hello extension derived from a key unknown to the
+	// server.
+	testCases = append(testCases, testCase{
+		testType: serverTest,
+		name:     "ECH-Server-EncryptedClientHello-ECHIsInner",
+		config: Config{
+			MinVersion: VersionTLS13,
+			MaxVersion: VersionTLS13,
+			Bugs: ProtocolBugs{
+				SendECHIsInner:                      []byte{},
+				SendPlaceholderEncryptedClientHello: true,
+			},
+		},
+		shouldFail:         true,
+		expectedLocalError: "remote error: illegal parameter",
+		expectedError:      ":UNEXPECTED_EXTENSION:",
+	})
 }
 
 func worker(statusChan chan statusMsg, c chan *testCase, shimPath string, wg *sync.WaitGroup) {

ssl/tls13_enc.cc

@@ -507,4 +507,40 @@ bool tls13_verify_psk_binder(SSL_HANDSHAKE *hs, SSL_SESSION *session,
   return true;
 }
 
+bool tls13_ech_accept_confirmation(
+    SSL_HANDSHAKE *hs, bssl::Span<uint8_t> out,
+    bssl::Span<const uint8_t> server_hello_ech_conf) {
+  // Compute the hash of the transcript concatenated with
+  // |server_hello_ech_conf| without modifying |hs->transcript|.
+  uint8_t context_hash[EVP_MAX_MD_SIZE];
+  unsigned context_hash_len;
+  ScopedEVP_MD_CTX ctx;
+  if (!hs->transcript.CopyToHashContext(ctx.get(), hs->transcript.Digest()) ||
+      !EVP_DigestUpdate(ctx.get(), server_hello_ech_conf.data(),
+                        server_hello_ech_conf.size()) ||
+      !EVP_DigestFinal_ex(ctx.get(), context_hash, &context_hash_len)) {
+    return false;
+  }
+
+  // Per draft-ietf-tls-esni-09, accept_confirmation is computed with
+  // Derive-Secret, which derives a secret of size Hash.length. That value is
+  // then truncated to the first 8 bytes. Note this differs from deriving an
+  // 8-byte secret because the target length is included in the derivation.
+  uint8_t accept_confirmation_buf[EVP_MAX_MD_SIZE];
+  bssl::Span<uint8_t> accept_confirmation =
+      MakeSpan(accept_confirmation_buf, hs->transcript.DigestLen());
+  if (!hkdf_expand_label(accept_confirmation, hs->transcript.Digest(),
+                         hs->secret(), label_to_span("ech accept confirmation"),
+                         MakeConstSpan(context_hash, context_hash_len))) {
+    return false;
+  }
+
+  if (out.size() > accept_confirmation.size()) {
+    OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
+    return false;
+  }
+  OPENSSL_memcpy(out.data(), accept_confirmation.data(), out.size());
+  return true;
+}
+
 BSSL_NAMESPACE_END

ssl/tls13_server.cc

@@ -636,20 +636,58 @@ static enum ssl_hs_wait_t do_read_second_client_hello(SSL_HANDSHAKE *hs) {
 static enum ssl_hs_wait_t do_send_server_hello(SSL_HANDSHAKE *hs) {
   SSL *const ssl = hs->ssl;
 
+  Span<uint8_t> random(ssl->s3->server_random);
+  RAND_bytes(random.data(), random.size());
+
+  // If the ClientHello has an ech_is_inner extension, we must be the ECH
+  // backend server. In response to ech_is_inner, we will overwrite part of the
+  // ServerHello.random with the ECH acceptance confirmation.
+  if (hs->ech_is_inner_present) {
+    // Construct the ServerHelloECHConf message, which is the same as
+    // ServerHello, except the last 8 bytes of its random field are zeroed out.
+    Span<uint8_t> random_suffix = random.subspan(24);
+    OPENSSL_memset(random_suffix.data(), 0, random_suffix.size());
+
+    ScopedCBB cbb;
+    CBB body, extensions, session_id;
+    if (!ssl->method->init_message(ssl, cbb.get(), &body,
+                                   SSL3_MT_SERVER_HELLO) ||
+        !CBB_add_u16(&body, TLS1_2_VERSION) ||
+        !CBB_add_bytes(&body, random.data(), random.size()) ||
+        !CBB_add_u8_length_prefixed(&body, &session_id) ||
+        !CBB_add_bytes(&session_id, hs->session_id, hs->session_id_len) ||
+        !CBB_add_u16(&body, SSL_CIPHER_get_protocol_id(hs->new_cipher)) ||
+        !CBB_add_u8(&body, 0) ||
+        !CBB_add_u16_length_prefixed(&body, &extensions) ||
+        !ssl_ext_pre_shared_key_add_serverhello(hs, &extensions) ||
+        !ssl_ext_key_share_add_serverhello(hs, &extensions, /*dry_run=*/true) ||
+        !ssl_ext_supported_versions_add_serverhello(hs, &extensions) ||
+        !CBB_flush(cbb.get())) {
+      return ssl_hs_error;
+    }
+
+    // Note that |cbb| includes the message type and length fields, but not the
+    // record layer header.
+    if (!tls13_ech_accept_confirmation(
+            hs, random_suffix,
+            bssl::MakeConstSpan(CBB_data(cbb.get()), CBB_len(cbb.get())))) {
+      return ssl_hs_error;
+    }
+  }
+
   // Send a ServerHello.
   ScopedCBB cbb;
   CBB body, extensions, session_id;
   if (!ssl->method->init_message(ssl, cbb.get(), &body, SSL3_MT_SERVER_HELLO) ||
       !CBB_add_u16(&body, TLS1_2_VERSION) ||
-      !RAND_bytes(ssl->s3->server_random, sizeof(ssl->s3->server_random)) ||
-      !CBB_add_bytes(&body, ssl->s3->server_random, SSL3_RANDOM_SIZE) ||
+      !CBB_add_bytes(&body, random.data(), random.size()) ||
       !CBB_add_u8_length_prefixed(&body, &session_id) ||
       !CBB_add_bytes(&session_id, hs->session_id, hs->session_id_len) ||
       !CBB_add_u16(&body, SSL_CIPHER_get_protocol_id(hs->new_cipher)) ||
       !CBB_add_u8(&body, 0) ||
       !CBB_add_u16_length_prefixed(&body, &extensions) ||
       !ssl_ext_pre_shared_key_add_serverhello(hs, &extensions) ||
-      !ssl_ext_key_share_add_serverhello(hs, &extensions) ||
+      !ssl_ext_key_share_add_serverhello(hs, &extensions, /*dry_run=*/false) ||
       !ssl_ext_supported_versions_add_serverhello(hs, &extensions) ||
       !ssl_add_message_cbb(ssl, cbb.get())) {
     return ssl_hs_error;