crypto/tls: replace all usages of BytesOrPanic

Message marshalling makes use of BytesOrPanic a lot, under the
assumption that it will never panic. This assumption was incorrect, and
specifically crafted handshakes could trigger panics. Rather than just
surgically replacing the usages of BytesOrPanic in paths that could
panic, replace all usages of it with proper error returns in case there
are other ways of triggering panics which we didn't find.

In one specific case, the tree routed by expandLabel, we replace the
usage of BytesOrPanic, but retain a panic. This function already
explicitly panicked elsewhere, and returning an error from it becomes
rather painful because it requires changing a large number of APIs.
The marshalling is unlikely to ever panic, as the inputs are all either
fixed length, or already limited to the sizes required. If it were to
panic, it'd likely only be during development. A close inspection shows
no paths for a user to cause a panic currently.

This patches ends up being rather large, since it requires routing
errors back through functions which previously had no error returns.
Where possible I've tried to use helpers that reduce the verbosity
of frequently repeated stanzas, and to make the diffs as minimal as
possible.

Thanks to Marten Seemann for reporting this issue.

Fixes #58001
Fixes CVE-2022-41724

Change-Id: Ieb55867ef0a3e1e867b33f09421932510cb58851
Reviewed-on: 1679436
Reviewed-by: Julie Qiu <julieqiu@google.com>
TryBot-Result: Security TryBots <security-trybots@go-security-trybots.iam.gserviceaccount.com>
Run-TryBot: Roland Shoemaker <bracewell@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/468125
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Michael Pratt <mpratt@google.com>

boring_test.go

@@ -269,7 +269,7 @@ func TestBoringClientHello(t *testing.T) {
 
 	go Client(c, clientConfig).Handshake()
 	srv := Server(s, testConfig)
-	msg, err := srv.readHandshake()
+	msg, err := srv.readHandshake(nil)
 	if err != nil {
 		t.Fatal(err)
 	}

common.go

@@ -1394,7 +1394,7 @@ func (c *Certificate) leaf() (*x509.Certificate, error) {
 }
 
 type handshakeMessage interface {
-	marshal() []byte
+	marshal() ([]byte, error)
 	unmarshal([]byte) bool
 }
 

conn.go

@@ -1004,18 +1004,37 @@ func (c *Conn) writeRecordLocked(typ recordType, data []byte) (int, error) {
 	return n, nil
 }
 
-// writeRecord writes a TLS record with the given type and payload to the
-// connection and updates the record layer state.
-func (c *Conn) writeRecord(typ recordType, data []byte) (int, error) {
+// writeHandshakeRecord writes a handshake message to the connection and updates
+// the record layer state. If transcript is non-nil the marshalled message is
+// written to it.
+func (c *Conn) writeHandshakeRecord(msg handshakeMessage, transcript transcriptHash) (int, error) {
 	c.out.Lock()
 	defer c.out.Unlock()
 
-	return c.writeRecordLocked(typ, data)
+	data, err := msg.marshal()
+	if err != nil {
+		return 0, err
+	}
+	if transcript != nil {
+		transcript.Write(data)
+	}
+
+	return c.writeRecordLocked(recordTypeHandshake, data)
+}
+
+// writeChangeCipherRecord writes a ChangeCipherSpec message to the connection and
+// updates the record layer state.
+func (c *Conn) writeChangeCipherRecord() error {
+	c.out.Lock()
+	defer c.out.Unlock()
+	_, err := c.writeRecordLocked(recordTypeChangeCipherSpec, []byte{1})
+	return err
 }
 
 // readHandshake reads the next handshake message from
-// the record layer.
-func (c *Conn) readHandshake() (any, error) {
+// the record layer. If transcript is non-nil, the message
+// is written to the passed transcriptHash.
+func (c *Conn) readHandshake(transcript transcriptHash) (any, error) {
 	for c.hand.Len() < 4 {
 		if err := c.readRecord(); err != nil {
 			return nil, err
@@ -1094,6 +1113,11 @@ func (c *Conn) readHandshake() (any, error) {
 	if !m.unmarshal(data) {
 		return nil, c.in.setErrorLocked(c.sendAlert(alertUnexpectedMessage))
 	}
+
+	if transcript != nil {
+		transcript.Write(data)
+	}
+
 	return m, nil
 }
 
@@ -1169,7 +1193,7 @@ func (c *Conn) handleRenegotiation() error {
 		return errors.New("tls: internal error: unexpected renegotiation")
 	}
 
-	msg, err := c.readHandshake()
+	msg, err := c.readHandshake(nil)
 	if err != nil {
 		return err
 	}
@@ -1215,7 +1239,7 @@ func (c *Conn) handlePostHandshakeMessage() error {
 		return c.handleRenegotiation()
 	}
 
-	msg, err := c.readHandshake()
+	msg, err := c.readHandshake(nil)
 	if err != nil {
 		return err
 	}
@@ -1251,7 +1275,11 @@ func (c *Conn) handleKeyUpdate(keyUpdate *keyUpdateMsg) error {
 		defer c.out.Unlock()
 
 		msg := &keyUpdateMsg{}
-		_, err := c.writeRecordLocked(recordTypeHandshake, msg.marshal())
+		msgBytes, err := msg.marshal()
+		if err != nil {
+			return err
+		}
+		_, err = c.writeRecordLocked(recordTypeHandshake, msgBytes)
 		if err != nil {
 			// Surface the error at the next write.
 			c.out.setErrorLocked(err)

handshake_client.go

@@ -162,7 +162,10 @@ func (c *Conn) clientHandshake(ctx context.Context) (err error) {
 	}
 	c.serverName = hello.serverName
 
-	cacheKey, session, earlySecret, binderKey := c.loadSession(hello)
+	cacheKey, session, earlySecret, binderKey, err := c.loadSession(hello)
+	if err != nil {
+		return err
+	}
 	if cacheKey != "" && session != nil {
 		defer func() {
 			// If we got a handshake failure when resuming a session, throw away
@@ -177,11 +180,12 @@ func (c *Conn) clientHandshake(ctx context.Context) (err error) {
 		}()
 	}
 
-	if _, err := c.writeRecord(recordTypeHandshake, hello.marshal()); err != nil {
+	if _, err := c.writeHandshakeRecord(hello, nil); err != nil {
 		return err
 	}
 
-	msg, err := c.readHandshake()
+	// serverHelloMsg is not included in the transcript
+	msg, err := c.readHandshake(nil)
 	if err != nil {
 		return err
 	}
@@ -246,9 +250,9 @@ func (c *Conn) clientHandshake(ctx context.Context) (err error) {
 }
 
 func (c *Conn) loadSession(hello *clientHelloMsg) (cacheKey string,
-	session *ClientSessionState, earlySecret, binderKey []byte) {
+	session *ClientSessionState, earlySecret, binderKey []byte, err error) {
 	if c.config.SessionTicketsDisabled || c.config.ClientSessionCache == nil {
-		return "", nil, nil, nil
+		return "", nil, nil, nil, nil
 	}
 
 	hello.ticketSupported = true
@@ -263,14 +267,14 @@ func (c *Conn) loadSession(hello *clientHelloMsg) (cacheKey string,
 	// renegotiation is primarily used to allow a client to send a client
 	// certificate, which would be skipped if session resumption occurred.
 	if c.handshakes != 0 {
-		return "", nil, nil, nil
+		return "", nil, nil, nil, nil
 	}
 
 	// Try to resume a previously negotiated TLS session, if available.
 	cacheKey = clientSessionCacheKey(c.conn.RemoteAddr(), c.config)
 	session, ok := c.config.ClientSessionCache.Get(cacheKey)
 	if !ok || session == nil {
-		return cacheKey, nil, nil, nil
+		return cacheKey, nil, nil, nil, nil
 	}
 
 	// Check that version used for the previous session is still valid.
@@ -282,7 +286,7 @@ func (c *Conn) loadSession(hello *clientHelloMsg) (cacheKey string,
 		}
 	}
 	if !versOk {
-		return cacheKey, nil, nil, nil
+		return cacheKey, nil, nil, nil, nil
 	}
 
 	// Check that the cached server certificate is not expired, and that it's
@@ -291,16 +295,16 @@ func (c *Conn) loadSession(hello *clientHelloMsg) (cacheKey string,
 	if !c.config.InsecureSkipVerify {
 		if len(session.verifiedChains) == 0 {
 			// The original connection had InsecureSkipVerify, while this doesn't.
-			return cacheKey, nil, nil, nil
+			return cacheKey, nil, nil, nil, nil
 		}
 		serverCert := session.serverCertificates[0]
 		if c.config.time().After(serverCert.NotAfter) {
 			// Expired certificate, delete the entry.
 			c.config.ClientSessionCache.Put(cacheKey, nil)
-			return cacheKey, nil, nil, nil
+			return cacheKey, nil, nil, nil, nil
 		}
 		if err := serverCert.VerifyHostname(c.config.ServerName); err != nil {
-			return cacheKey, nil, nil, nil
+			return cacheKey, nil, nil, nil, nil
 		}
 	}
 
@@ -308,7 +312,7 @@ func (c *Conn) loadSession(hello *clientHelloMsg) (cacheKey string,
 		// In TLS 1.2 the cipher suite must match the resumed session. Ensure we
 		// are still offering it.
 		if mutualCipherSuite(hello.cipherSuites, session.cipherSuite) == nil {
-			return cacheKey, nil, nil, nil
+			return cacheKey, nil, nil, nil, nil
 		}
 
 		hello.sessionTicket = session.sessionTicket
@@ -318,14 +322,14 @@ func (c *Conn) loadSession(hello *clientHelloMsg) (cacheKey string,
 	// Check that the session ticket is not expired.
 	if c.config.time().After(session.useBy) {
 		c.config.ClientSessionCache.Put(cacheKey, nil)
-		return cacheKey, nil, nil, nil
+		return cacheKey, nil, nil, nil, nil
 	}
 
 	// In TLS 1.3 the KDF hash must match the resumed session. Ensure we
 	// offer at least one cipher suite with that hash.
 	cipherSuite := cipherSuiteTLS13ByID(session.cipherSuite)
 	if cipherSuite == nil {
-		return cacheKey, nil, nil, nil
+		return cacheKey, nil, nil, nil, nil
 	}
 	cipherSuiteOk := false
 	for _, offeredID := range hello.cipherSuites {
@@ -336,7 +340,7 @@ func (c *Conn) loadSession(hello *clientHelloMsg) (cacheKey string,
 		}
 	}
 	if !cipherSuiteOk {
-		return cacheKey, nil, nil, nil
+		return cacheKey, nil, nil, nil, nil
 	}
 
 	// Set the pre_shared_key extension. See RFC 8446, Section 4.2.11.1.
@@ -354,9 +358,15 @@ func (c *Conn) loadSession(hello *clientHelloMsg) (cacheKey string,
 	earlySecret = cipherSuite.extract(psk, nil)
 	binderKey = cipherSuite.deriveSecret(earlySecret, resumptionBinderLabel, nil)
 	transcript := cipherSuite.hash.New()
-	transcript.Write(hello.marshalWithoutBinders())
+	helloBytes, err := hello.marshalWithoutBinders()
+	if err != nil {
+		return "", nil, nil, nil, err
+	}
+	transcript.Write(helloBytes)
 	pskBinders := [][]byte{cipherSuite.finishedHash(binderKey, transcript)}
-	hello.updateBinders(pskBinders)
+	if err := hello.updateBinders(pskBinders); err != nil {
+		return "", nil, nil, nil, err
+	}
 
 	return
 }
@@ -401,8 +411,12 @@ func (hs *clientHandshakeState) handshake() error {
 		hs.finishedHash.discardHandshakeBuffer()
 	}
 
-	hs.finishedHash.Write(hs.hello.marshal())
-	hs.finishedHash.Write(hs.serverHello.marshal())
+	if err := transcriptMsg(hs.hello, &hs.finishedHash); err != nil {
+		return err
+	}
+	if err := transcriptMsg(hs.serverHello, &hs.finishedHash); err != nil {
+		return err
+	}
 
 	c.buffering = true
 	c.didResume = isResume
@@ -473,7 +487,7 @@ func (hs *clientHandshakeState) pickCipherSuite() error {
 func (hs *clientHandshakeState) doFullHandshake() error {
 	c := hs.c
 
-	msg, err := c.readHandshake()
+	msg, err := c.readHandshake(&hs.finishedHash)
 	if err != nil {
 		return err
 	}
@@ -482,9 +496,8 @@ func (hs *clientHandshakeState) doFullHandshake() error {
 		c.sendAlert(alertUnexpectedMessage)
 		return unexpectedMessageError(certMsg, msg)
 	}
-	hs.finishedHash.Write(certMsg.marshal())
 
-	msg, err = c.readHandshake()
+	msg, err = c.readHandshake(&hs.finishedHash)
 	if err != nil {
 		return err
 	}
@@ -502,11 +515,10 @@ func (hs *clientHandshakeState) doFullHandshake() error {
 			c.sendAlert(alertUnexpectedMessage)
 			return errors.New("tls: received unexpected CertificateStatus message")
 		}
-		hs.finishedHash.Write(cs.marshal())
 
 		c.ocspResponse = cs.response
 
-		msg, err = c.readHandshake()
+		msg, err = c.readHandshake(&hs.finishedHash)
 		if err != nil {
 			return err
 		}
@@ -535,14 +547,13 @@ func (hs *clientHandshakeState) doFullHandshake() error {
 
 	skx, ok := msg.(*serverKeyExchangeMsg)
 	if ok {
-		hs.finishedHash.Write(skx.marshal())
 		err = keyAgreement.processServerKeyExchange(c.config, hs.hello, hs.serverHello, c.peerCertificates[0], skx)
 		if err != nil {
 			c.sendAlert(alertUnexpectedMessage)
 			return err
 		}
 
-		msg, err = c.readHandshake()
+		msg, err = c.readHandshake(&hs.finishedHash)
 		if err != nil {
 			return err
 		}
@@ -553,7 +564,6 @@ func (hs *clientHandshakeState) doFullHandshake() error {
 	certReq, ok := msg.(*certificateRequestMsg)
 	if ok {
 		certRequested = true
-		hs.finishedHash.Write(certReq.marshal())
 
 		cri := certificateRequestInfoFromMsg(hs.ctx, c.vers, certReq)
 		if chainToSend, err = c.getClientCertificate(cri); err != nil {
@@ -561,7 +571,7 @@ func (hs *clientHandshakeState) doFullHandshake() error {
 			return err
 		}
 
-		msg, err = c.readHandshake()
+		msg, err = c.readHandshake(&hs.finishedHash)
 		if err != nil {
 			return err
 		}
@@ -572,7 +582,6 @@ func (hs *clientHandshakeState) doFullHandshake() error {
 		c.sendAlert(alertUnexpectedMessage)
 		return unexpectedMessageError(shd, msg)
 	}
-	hs.finishedHash.Write(shd.marshal())
 
 	// If the server requested a certificate then we have to send a
 	// Certificate message, even if it's empty because we don't have a
@@ -580,8 +589,7 @@ func (hs *clientHandshakeState) doFullHandshake() error {
 	if certRequested {
 		certMsg = new(certificateMsg)
 		certMsg.certificates = chainToSend.Certificate
-		hs.finishedHash.Write(certMsg.marshal())
-		if _, err := c.writeRecord(recordTypeHandshake, certMsg.marshal()); err != nil {
+		if _, err := hs.c.writeHandshakeRecord(certMsg, &hs.finishedHash); err != nil {
 			return err
 		}
 	}
@@ -592,8 +600,7 @@ func (hs *clientHandshakeState) doFullHandshake() error {
 		return err
 	}
 	if ckx != nil {
-		hs.finishedHash.Write(ckx.marshal())
-		if _, err := c.writeRecord(recordTypeHandshake, ckx.marshal()); err != nil {
+		if _, err := hs.c.writeHandshakeRecord(ckx, &hs.finishedHash); err != nil {
 			return err
 		}
 	}
@@ -640,8 +647,7 @@ func (hs *clientHandshakeState) doFullHandshake() error {
 			return err
 		}
 
-		hs.finishedHash.Write(certVerify.marshal())
-		if _, err := c.writeRecord(recordTypeHandshake, certVerify.marshal()); err != nil {
+		if _, err := hs.c.writeHandshakeRecord(certVerify, &hs.finishedHash); err != nil {
 			return err
 		}
 	}
@@ -776,7 +782,10 @@ func (hs *clientHandshakeState) readFinished(out []byte) error {
 		return err
 	}
 
-	msg, err := c.readHandshake()
+	// finishedMsg is included in the transcript, but not until after we
+	// check the client version, since the state before this message was
+	// sent is used during verification.
+	msg, err := c.readHandshake(nil)
 	if err != nil {
 		return err
 	}
@@ -792,7 +801,11 @@ func (hs *clientHandshakeState) readFinished(out []byte) error {
 		c.sendAlert(alertHandshakeFailure)
 		return errors.New("tls: server's Finished message was incorrect")
 	}
-	hs.finishedHash.Write(serverFinished.marshal())
+
+	if err := transcriptMsg(serverFinished, &hs.finishedHash); err != nil {
+		return err
+	}
+
 	copy(out, verify)
 	return nil
 }
@@ -803,7 +816,7 @@ func (hs *clientHandshakeState) readSessionTicket() error {
 	}
 
 	c := hs.c
-	msg, err := c.readHandshake()
+	msg, err := c.readHandshake(&hs.finishedHash)
 	if err != nil {
 		return err
 	}
@@ -812,7 +825,6 @@ func (hs *clientHandshakeState) readSessionTicket() error {
 		c.sendAlert(alertUnexpectedMessage)
 		return unexpectedMessageError(sessionTicketMsg, msg)
 	}
-	hs.finishedHash.Write(sessionTicketMsg.marshal())
 
 	hs.session = &ClientSessionState{
 		sessionTicket:      sessionTicketMsg.ticket,
@@ -832,14 +844,13 @@ func (hs *clientHandshakeState) readSessionTicket() error {
 func (hs *clientHandshakeState) sendFinished(out []byte) error {
 	c := hs.c
 
-	if _, err := c.writeRecord(recordTypeChangeCipherSpec, []byte{1}); err != nil {
+	if err := c.writeChangeCipherRecord(); err != nil {
 		return err
 	}
 
 	finished := new(finishedMsg)
 	finished.verifyData = hs.finishedHash.clientSum(hs.masterSecret)
-	hs.finishedHash.Write(finished.marshal())
-	if _, err := c.writeRecord(recordTypeHandshake, finished.marshal()); err != nil {
+	if _, err := hs.c.writeHandshakeRecord(finished, &hs.finishedHash); err != nil {
 		return err
 	}
 	copy(out, finished.verifyData)

handshake_client_test.go

@@ -1257,7 +1257,7 @@ func TestServerSelectingUnconfiguredApplicationProtocol(t *testing.T) {
 		cipherSuite:  TLS_RSA_WITH_AES_128_GCM_SHA256,
 		alpnProtocol: "how-about-this",
 	}
-	serverHelloBytes := serverHello.marshal()
+	serverHelloBytes := mustMarshal(t, serverHello)
 
 	s.Write([]byte{
 		byte(recordTypeHandshake),
@@ -1500,7 +1500,7 @@ func TestServerSelectingUnconfiguredCipherSuite(t *testing.T) {
 		random:      make([]byte, 32),
 		cipherSuite: TLS_RSA_WITH_AES_256_GCM_SHA384,
 	}
-	serverHelloBytes := serverHello.marshal()
+	serverHelloBytes := mustMarshal(t, serverHello)
 
 	s.Write([]byte{
 		byte(recordTypeHandshake),

handshake_client_tls13.go

@@ -62,7 +62,10 @@ func (hs *clientHandshakeStateTLS13) handshake() error {
 	}
 
 	hs.transcript = hs.suite.hash.New()
-	hs.transcript.Write(hs.hello.marshal())
+
+	if err := transcriptMsg(hs.hello, hs.transcript); err != nil {
+		return err
+	}
 
 	if bytes.Equal(hs.serverHello.random, helloRetryRequestRandom) {
 		if err := hs.sendDummyChangeCipherSpec(); err != nil {
@@ -73,7 +76,9 @@ func (hs *clientHandshakeStateTLS13) handshake() error {
 		}
 	}
 
-	hs.transcript.Write(hs.serverHello.marshal())
+	if err := transcriptMsg(hs.serverHello, hs.transcript); err != nil {
+		return err
+	}
 
 	c.buffering = true
 	if err := hs.processServerHello(); err != nil {
@@ -172,8 +177,7 @@ func (hs *clientHandshakeStateTLS13) sendDummyChangeCipherSpec() error {
 	}
 	hs.sentDummyCCS = true
 
-	_, err := hs.c.writeRecord(recordTypeChangeCipherSpec, []byte{1})
-	return err
+	return hs.c.writeChangeCipherRecord()
 }
 
 // processHelloRetryRequest handles the HRR in hs.serverHello, modifies and
@@ -188,7 +192,9 @@ func (hs *clientHandshakeStateTLS13) processHelloRetryRequest() error {
 	hs.transcript.Reset()
 	hs.transcript.Write([]byte{typeMessageHash, 0, 0, uint8(len(chHash))})
 	hs.transcript.Write(chHash)
-	hs.transcript.Write(hs.serverHello.marshal())
+	if err := transcriptMsg(hs.serverHello, hs.transcript); err != nil {
+		return err
+	}
 
 	// The only HelloRetryRequest extensions we support are key_share and
 	// cookie, and clients must abort the handshake if the HRR would not result
@@ -253,10 +259,18 @@ func (hs *clientHandshakeStateTLS13) processHelloRetryRequest() error {
 			transcript := hs.suite.hash.New()
 			transcript.Write([]byte{typeMessageHash, 0, 0, uint8(len(chHash))})
 			transcript.Write(chHash)
-			transcript.Write(hs.serverHello.marshal())
-			transcript.Write(hs.hello.marshalWithoutBinders())
+			if err := transcriptMsg(hs.serverHello, hs.transcript); err != nil {
+				return err
+			}
+			helloBytes, err := hs.hello.marshalWithoutBinders()
+			if err != nil {
+				return err
+			}
+			transcript.Write(helloBytes)
 			pskBinders := [][]byte{hs.suite.finishedHash(hs.binderKey, transcript)}
-			hs.hello.updateBinders(pskBinders)
+			if err := hs.hello.updateBinders(pskBinders); err != nil {
+				return err
+			}
 		} else {
 			// Server selected a cipher suite incompatible with the PSK.
 			hs.hello.pskIdentities = nil
@@ -264,12 +278,12 @@ func (hs *clientHandshakeStateTLS13) processHelloRetryRequest() error {
 		}
 	}
 
-	hs.transcript.Write(hs.hello.marshal())
-	if _, err := c.writeRecord(recordTypeHandshake, hs.hello.marshal()); err != nil {
+	if _, err := hs.c.writeHandshakeRecord(hs.hello, hs.transcript); err != nil {
 		return err
 	}
 
-	msg, err := c.readHandshake()
+	// serverHelloMsg is not included in the transcript
+	msg, err := c.readHandshake(nil)
 	if err != nil {
 		return err
 	}
@@ -363,6 +377,7 @@ func (hs *clientHandshakeStateTLS13) establishHandshakeKeys() error {
 	if !hs.usingPSK {
 		earlySecret = hs.suite.extract(nil, nil)
 	}
+
 	handshakeSecret := hs.suite.extract(sharedKey,
 		hs.suite.deriveSecret(earlySecret, "derived", nil))
 
@@ -393,7 +408,7 @@ func (hs *clientHandshakeStateTLS13) establishHandshakeKeys() error {
 func (hs *clientHandshakeStateTLS13) readServerParameters() error {
 	c := hs.c
 
-	msg, err := c.readHandshake()
+	msg, err := c.readHandshake(hs.transcript)
 	if err != nil {
 		return err
 	}
@@ -403,7 +418,6 @@ func (hs *clientHandshakeStateTLS13) readServerParameters() error {
 		c.sendAlert(alertUnexpectedMessage)
 		return unexpectedMessageError(encryptedExtensions, msg)
 	}
-	hs.transcript.Write(encryptedExtensions.marshal())
 
 	if err := checkALPN(hs.hello.alpnProtocols, encryptedExtensions.alpnProtocol); err != nil {
 		c.sendAlert(alertUnsupportedExtension)
@@ -432,18 +446,16 @@ func (hs *clientHandshakeStateTLS13) readServerCertificate() error {
 		return nil
 	}
 
-	msg, err := c.readHandshake()
+	msg, err := c.readHandshake(hs.transcript)
 	if err != nil {
 		return err
 	}
 
 	certReq, ok := msg.(*certificateRequestMsgTLS13)
 	if ok {
-		hs.transcript.Write(certReq.marshal())
-
 		hs.certReq = certReq
 
-		msg, err = c.readHandshake()
+		msg, err = c.readHandshake(hs.transcript)
 		if err != nil {
 			return err
 		}
@@ -458,7 +470,6 @@ func (hs *clientHandshakeStateTLS13) readServerCertificate() error {
 		c.sendAlert(alertDecodeError)
 		return errors.New("tls: received empty certificates message")
 	}
-	hs.transcript.Write(certMsg.marshal())
 
 	c.scts = certMsg.certificate.SignedCertificateTimestamps
 	c.ocspResponse = certMsg.certificate.OCSPStaple
@@ -467,7 +478,10 @@ func (hs *clientHandshakeStateTLS13) readServerCertificate() error {
 		return err
 	}
 
-	msg, err = c.readHandshake()
+	// certificateVerifyMsg is included in the transcript, but not until
+	// after we verify the handshake signature, since the state before
+	// this message was sent is used.
+	msg, err = c.readHandshake(nil)
 	if err != nil {
 		return err
 	}
@@ -498,7 +512,9 @@ func (hs *clientHandshakeStateTLS13) readServerCertificate() error {
 		return errors.New("tls: invalid signature by the server certificate: " + err.Error())
 	}
 
-	hs.transcript.Write(certVerify.marshal())
+	if err := transcriptMsg(certVerify, hs.transcript); err != nil {
+		return err
+	}
 
 	return nil
 }
@@ -506,7 +522,10 @@ func (hs *clientHandshakeStateTLS13) readServerCertificate() error {
 func (hs *clientHandshakeStateTLS13) readServerFinished() error {
 	c := hs.c
 
-	msg, err := c.readHandshake()
+	// finishedMsg is included in the transcript, but not until after we
+	// check the client version, since the state before this message was
+	// sent is used during verification.
+	msg, err := c.readHandshake(nil)
 	if err != nil {
 		return err
 	}
@@ -523,7 +542,9 @@ func (hs *clientHandshakeStateTLS13) readServerFinished() error {
 		return errors.New("tls: invalid server finished hash")
 	}
 
-	hs.transcript.Write(finished.marshal())
+	if err := transcriptMsg(finished, hs.transcript); err != nil {
+		return err
+	}
 
 	// Derive secrets that take context through the server Finished.
 
@@ -572,8 +593,7 @@ func (hs *clientHandshakeStateTLS13) sendClientCertificate() error {
 	certMsg.scts = hs.certReq.scts && len(cert.SignedCertificateTimestamps) > 0
 	certMsg.ocspStapling = hs.certReq.ocspStapling && len(cert.OCSPStaple) > 0
 
-	hs.transcript.Write(certMsg.marshal())
-	if _, err := c.writeRecord(recordTypeHandshake, certMsg.marshal()); err != nil {
+	if _, err := hs.c.writeHandshakeRecord(certMsg, hs.transcript); err != nil {
 		return err
 	}
 
@@ -610,8 +630,7 @@ func (hs *clientHandshakeStateTLS13) sendClientCertificate() error {
 	}
 	certVerifyMsg.signature = sig
 
-	hs.transcript.Write(certVerifyMsg.marshal())
-	if _, err := c.writeRecord(recordTypeHandshake, certVerifyMsg.marshal()); err != nil {
+	if _, err := hs.c.writeHandshakeRecord(certVerifyMsg, hs.transcript); err != nil {
 		return err
 	}
 
@@ -625,8 +644,7 @@ func (hs *clientHandshakeStateTLS13) sendClientFinished() error {
 		verifyData: hs.suite.finishedHash(c.out.trafficSecret, hs.transcript),
 	}
 
-	hs.transcript.Write(finished.marshal())
-	if _, err := c.writeRecord(recordTypeHandshake, finished.marshal()); err != nil {
+	if _, err := hs.c.writeHandshakeRecord(finished, hs.transcript); err != nil {
 		return err
 	}
 

handshake_messages.go

@@ -5,6 +5,7 @@
 package tls
 
 import (
+	"errors"
 	"fmt"
 	"strings"
 
@@ -94,9 +95,181 @@ type clientHelloMsg struct {
 	pskBinders                       [][]byte
 }
 
-func (m *clientHelloMsg) marshal() []byte {
+func (m *clientHelloMsg) marshal() ([]byte, error) {
 	if m.raw != nil {
-		return m.raw
+		return m.raw, nil
+	}
+
+	var exts cryptobyte.Builder
+	if len(m.serverName) > 0 {
+		// RFC 6066, Section 3
+		exts.AddUint16(extensionServerName)
+		exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+			exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+				exts.AddUint8(0) // name_type = host_name
+				exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+					exts.AddBytes([]byte(m.serverName))
+				})
+			})
+		})
+	}
+	if m.ocspStapling {
+		// RFC 4366, Section 3.6
+		exts.AddUint16(extensionStatusRequest)
+		exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+			exts.AddUint8(1)  // status_type = ocsp
+			exts.AddUint16(0) // empty responder_id_list
+			exts.AddUint16(0) // empty request_extensions
+		})
+	}
+	if len(m.supportedCurves) > 0 {
+		// RFC 4492, sections 5.1.1 and RFC 8446, Section 4.2.7
+		exts.AddUint16(extensionSupportedCurves)
+		exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+			exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+				for _, curve := range m.supportedCurves {
+					exts.AddUint16(uint16(curve))
+				}
+			})
+		})
+	}
+	if len(m.supportedPoints) > 0 {
+		// RFC 4492, Section 5.1.2
+		exts.AddUint16(extensionSupportedPoints)
+		exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+			exts.AddUint8LengthPrefixed(func(exts *cryptobyte.Builder) {
+				exts.AddBytes(m.supportedPoints)
+			})
+		})
+	}
+	if m.ticketSupported {
+		// RFC 5077, Section 3.2
+		exts.AddUint16(extensionSessionTicket)
+		exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+			exts.AddBytes(m.sessionTicket)
+		})
+	}
+	if len(m.supportedSignatureAlgorithms) > 0 {
+		// RFC 5246, Section 7.4.1.4.1
+		exts.AddUint16(extensionSignatureAlgorithms)
+		exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+			exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+				for _, sigAlgo := range m.supportedSignatureAlgorithms {
+					exts.AddUint16(uint16(sigAlgo))
+				}
+			})
+		})
+	}
+	if len(m.supportedSignatureAlgorithmsCert) > 0 {
+		// RFC 8446, Section 4.2.3
+		exts.AddUint16(extensionSignatureAlgorithmsCert)
+		exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+			exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+				for _, sigAlgo := range m.supportedSignatureAlgorithmsCert {
+					exts.AddUint16(uint16(sigAlgo))
+				}
+			})
+		})
+	}
+	if m.secureRenegotiationSupported {
+		// RFC 5746, Section 3.2
+		exts.AddUint16(extensionRenegotiationInfo)
+		exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+			exts.AddUint8LengthPrefixed(func(exts *cryptobyte.Builder) {
+				exts.AddBytes(m.secureRenegotiation)
+			})
+		})
+	}
+	if len(m.alpnProtocols) > 0 {
+		// RFC 7301, Section 3.1
+		exts.AddUint16(extensionALPN)
+		exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+			exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+				for _, proto := range m.alpnProtocols {
+					exts.AddUint8LengthPrefixed(func(exts *cryptobyte.Builder) {
+						exts.AddBytes([]byte(proto))
+					})
+				}
+			})
+		})
+	}
+	if m.scts {
+		// RFC 6962, Section 3.3.1
+		exts.AddUint16(extensionSCT)
+		exts.AddUint16(0) // empty extension_data
+	}
+	if len(m.supportedVersions) > 0 {
+		// RFC 8446, Section 4.2.1
+		exts.AddUint16(extensionSupportedVersions)
+		exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+			exts.AddUint8LengthPrefixed(func(exts *cryptobyte.Builder) {
+				for _, vers := range m.supportedVersions {
+					exts.AddUint16(vers)
+				}
+			})
+		})
+	}
+	if len(m.cookie) > 0 {
+		// RFC 8446, Section 4.2.2
+		exts.AddUint16(extensionCookie)
+		exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+			exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+				exts.AddBytes(m.cookie)
+			})
+		})
+	}
+	if len(m.keyShares) > 0 {
+		// RFC 8446, Section 4.2.8
+		exts.AddUint16(extensionKeyShare)
+		exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+			exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+				for _, ks := range m.keyShares {
+					exts.AddUint16(uint16(ks.group))
+					exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+						exts.AddBytes(ks.data)
+					})
+				}
+			})
+		})
+	}
+	if m.earlyData {
+		// RFC 8446, Section 4.2.10
+		exts.AddUint16(extensionEarlyData)
+		exts.AddUint16(0) // empty extension_data
+	}
+	if len(m.pskModes) > 0 {
+		// RFC 8446, Section 4.2.9
+		exts.AddUint16(extensionPSKModes)
+		exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+			exts.AddUint8LengthPrefixed(func(exts *cryptobyte.Builder) {
+				exts.AddBytes(m.pskModes)
+			})
+		})
+	}
+	if len(m.pskIdentities) > 0 { // pre_shared_key must be the last extension
+		// RFC 8446, Section 4.2.11
+		exts.AddUint16(extensionPreSharedKey)
+		exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+			exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+				for _, psk := range m.pskIdentities {
+					exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+						exts.AddBytes(psk.label)
+					})
+					exts.AddUint32(psk.obfuscatedTicketAge)
+				}
+			})
+			exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+				for _, binder := range m.pskBinders {
+					exts.AddUint8LengthPrefixed(func(exts *cryptobyte.Builder) {
+						exts.AddBytes(binder)
+					})
+				}
+			})
+		})
+	}
+	extBytes, err := exts.Bytes()
+	if err != nil {
+		return nil, err
 	}
 
 	var b cryptobyte.Builder
@@ -116,219 +289,53 @@ func (m *clientHelloMsg) marshal() []byte {
 			b.AddBytes(m.compressionMethods)
 		})
 
-		// If extensions aren't present, omit them.
-		var extensionsPresent bool
-		bWithoutExtensions := *b
-
-		b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-			if len(m.serverName) > 0 {
-				// RFC 6066, Section 3
-				b.AddUint16(extensionServerName)
-				b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-					b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-						b.AddUint8(0) // name_type = host_name
-						b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-							b.AddBytes([]byte(m.serverName))
-						})
-					})
-				})
-			}
-			if m.ocspStapling {
-				// RFC 4366, Section 3.6
-				b.AddUint16(extensionStatusRequest)
-				b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-					b.AddUint8(1)  // status_type = ocsp
-					b.AddUint16(0) // empty responder_id_list
-					b.AddUint16(0) // empty request_extensions
-				})
-			}
-			if len(m.supportedCurves) > 0 {
-				// RFC 4492, sections 5.1.1 and RFC 8446, Section 4.2.7
-				b.AddUint16(extensionSupportedCurves)
-				b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-					b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-						for _, curve := range m.supportedCurves {
-							b.AddUint16(uint16(curve))
-						}
-					})
-				})
-			}
-			if len(m.supportedPoints) > 0 {
-				// RFC 4492, Section 5.1.2
-				b.AddUint16(extensionSupportedPoints)
-				b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-					b.AddUint8LengthPrefixed(func(b *cryptobyte.Builder) {
-						b.AddBytes(m.supportedPoints)
-					})
-				})
-			}
-			if m.ticketSupported {
-				// RFC 5077, Section 3.2
-				b.AddUint16(extensionSessionTicket)
-				b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-					b.AddBytes(m.sessionTicket)
-				})
-			}
-			if len(m.supportedSignatureAlgorithms) > 0 {
-				// RFC 5246, Section 7.4.1.4.1
-				b.AddUint16(extensionSignatureAlgorithms)
-				b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-					b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-						for _, sigAlgo := range m.supportedSignatureAlgorithms {
-							b.AddUint16(uint16(sigAlgo))
-						}
-					})
-				})
-			}
-			if len(m.supportedSignatureAlgorithmsCert) > 0 {
-				// RFC 8446, Section 4.2.3
-				b.AddUint16(extensionSignatureAlgorithmsCert)
-				b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-					b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-						for _, sigAlgo := range m.supportedSignatureAlgorithmsCert {
-							b.AddUint16(uint16(sigAlgo))
-						}
-					})
-				})
-			}
-			if m.secureRenegotiationSupported {
-				// RFC 5746, Section 3.2
-				b.AddUint16(extensionRenegotiationInfo)
-				b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-					b.AddUint8LengthPrefixed(func(b *cryptobyte.Builder) {
-						b.AddBytes(m.secureRenegotiation)
-					})
-				})
-			}
-			if len(m.alpnProtocols) > 0 {
-				// RFC 7301, Section 3.1
-				b.AddUint16(extensionALPN)
-				b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-					b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-						for _, proto := range m.alpnProtocols {
-							b.AddUint8LengthPrefixed(func(b *cryptobyte.Builder) {
-								b.AddBytes([]byte(proto))
-							})
-						}
-					})
-				})
-			}
-			if m.scts {
-				// RFC 6962, Section 3.3.1
-				b.AddUint16(extensionSCT)
-				b.AddUint16(0) // empty extension_data
-			}
-			if len(m.supportedVersions) > 0 {
-				// RFC 8446, Section 4.2.1
-				b.AddUint16(extensionSupportedVersions)
-				b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-					b.AddUint8LengthPrefixed(func(b *cryptobyte.Builder) {
-						for _, vers := range m.supportedVersions {
-							b.AddUint16(vers)
-						}
-					})
-				})
-			}
-			if len(m.cookie) > 0 {
-				// RFC 8446, Section 4.2.2
-				b.AddUint16(extensionCookie)
-				b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-					b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-						b.AddBytes(m.cookie)
-					})
-				})
-			}
-			if len(m.keyShares) > 0 {
-				// RFC 8446, Section 4.2.8
-				b.AddUint16(extensionKeyShare)
-				b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-					b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-						for _, ks := range m.keyShares {
-							b.AddUint16(uint16(ks.group))
-							b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-								b.AddBytes(ks.data)
-							})
-						}
-					})
-				})
-			}
-			if m.earlyData {
-				// RFC 8446, Section 4.2.10
-				b.AddUint16(extensionEarlyData)
-				b.AddUint16(0) // empty extension_data
-			}
-			if len(m.pskModes) > 0 {
-				// RFC 8446, Section 4.2.9
-				b.AddUint16(extensionPSKModes)
-				b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-					b.AddUint8LengthPrefixed(func(b *cryptobyte.Builder) {
-						b.AddBytes(m.pskModes)
-					})
-				})
-			}
-			if len(m.pskIdentities) > 0 { // pre_shared_key must be the last extension
-				// RFC 8446, Section 4.2.11
-				b.AddUint16(extensionPreSharedKey)
-				b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-					b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-						for _, psk := range m.pskIdentities {
-							b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-								b.AddBytes(psk.label)
-							})
-							b.AddUint32(psk.obfuscatedTicketAge)
-						}
-					})
-					b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-						for _, binder := range m.pskBinders {
-							b.AddUint8LengthPrefixed(func(b *cryptobyte.Builder) {
-								b.AddBytes(binder)
-							})
-						}
-					})
-				})
-			}
-
-			extensionsPresent = len(b.BytesOrPanic()) > 2
-		})
-
-		if !extensionsPresent {
-			*b = bWithoutExtensions
+		if len(extBytes) > 0 {
+			b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
+				b.AddBytes(extBytes)
+			})
 		}
 	})
 
-	m.raw = b.BytesOrPanic()
-	return m.raw
+	m.raw, err = b.Bytes()
+	return m.raw, err
 }
 
 // marshalWithoutBinders returns the ClientHello through the
 // PreSharedKeyExtension.identities field, according to RFC 8446, Section
 // 4.2.11.2. Note that m.pskBinders must be set to slices of the correct length.
-func (m *clientHelloMsg) marshalWithoutBinders() []byte {
+func (m *clientHelloMsg) marshalWithoutBinders() ([]byte, error) {
 	bindersLen := 2 // uint16 length prefix
 	for _, binder := range m.pskBinders {
 		bindersLen += 1 // uint8 length prefix
 		bindersLen += len(binder)
 	}
 
-	fullMessage := m.marshal()
-	return fullMessage[:len(fullMessage)-bindersLen]
+	fullMessage, err := m.marshal()
+	if err != nil {
+		return nil, err
+	}
+	return fullMessage[:len(fullMessage)-bindersLen], nil
 }
 
 // updateBinders updates the m.pskBinders field, if necessary updating the
 // cached marshaled representation. The supplied binders must have the same
 // length as the current m.pskBinders.
-func (m *clientHelloMsg) updateBinders(pskBinders [][]byte) {
+func (m *clientHelloMsg) updateBinders(pskBinders [][]byte) error {
 	if len(pskBinders) != len(m.pskBinders) {
-		panic("tls: internal error: pskBinders length mismatch")
+		return errors.New("tls: internal error: pskBinders length mismatch")
 	}
 	for i := range m.pskBinders {
 		if len(pskBinders[i]) != len(m.pskBinders[i]) {
-			panic("tls: internal error: pskBinders length mismatch")
+			return errors.New("tls: internal error: pskBinders length mismatch")
 		}
 	}
 	m.pskBinders = pskBinders
 	if m.raw != nil {
-		lenWithoutBinders := len(m.marshalWithoutBinders())
+		helloBytes, err := m.marshalWithoutBinders()
+		if err != nil {
+			return err
+		}
+		lenWithoutBinders := len(helloBytes)
 		b := cryptobyte.NewFixedBuilder(m.raw[:lenWithoutBinders])
 		b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
 			for _, binder := range m.pskBinders {
@@ -338,9 +345,11 @@ func (m *clientHelloMsg) updateBinders(pskBinders [][]byte) {
 			}
 		})
 		if out, err := b.Bytes(); err != nil || len(out) != len(m.raw) {
-			panic("tls: internal error: failed to update binders")
+			return errors.New("tls: internal error: failed to update binders")
 		}
 	}
+
+	return nil
 }
 
 func (m *clientHelloMsg) unmarshal(data []byte) bool {
@@ -618,9 +627,98 @@ type serverHelloMsg struct {
 	selectedGroup CurveID
 }
 
-func (m *serverHelloMsg) marshal() []byte {
+func (m *serverHelloMsg) marshal() ([]byte, error) {
 	if m.raw != nil {
-		return m.raw
+		return m.raw, nil
+	}
+
+	var exts cryptobyte.Builder
+	if m.ocspStapling {
+		exts.AddUint16(extensionStatusRequest)
+		exts.AddUint16(0) // empty extension_data
+	}
+	if m.ticketSupported {
+		exts.AddUint16(extensionSessionTicket)
+		exts.AddUint16(0) // empty extension_data
+	}
+	if m.secureRenegotiationSupported {
+		exts.AddUint16(extensionRenegotiationInfo)
+		exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+			exts.AddUint8LengthPrefixed(func(exts *cryptobyte.Builder) {
+				exts.AddBytes(m.secureRenegotiation)
+			})
+		})
+	}
+	if len(m.alpnProtocol) > 0 {
+		exts.AddUint16(extensionALPN)
+		exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+			exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+				exts.AddUint8LengthPrefixed(func(exts *cryptobyte.Builder) {
+					exts.AddBytes([]byte(m.alpnProtocol))
+				})
+			})
+		})
+	}
+	if len(m.scts) > 0 {
+		exts.AddUint16(extensionSCT)
+		exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+			exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+				for _, sct := range m.scts {
+					exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+						exts.AddBytes(sct)
+					})
+				}
+			})
+		})
+	}
+	if m.supportedVersion != 0 {
+		exts.AddUint16(extensionSupportedVersions)
+		exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+			exts.AddUint16(m.supportedVersion)
+		})
+	}
+	if m.serverShare.group != 0 {
+		exts.AddUint16(extensionKeyShare)
+		exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+			exts.AddUint16(uint16(m.serverShare.group))
+			exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+				exts.AddBytes(m.serverShare.data)
+			})
+		})
+	}
+	if m.selectedIdentityPresent {
+		exts.AddUint16(extensionPreSharedKey)
+		exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+			exts.AddUint16(m.selectedIdentity)
+		})
+	}
+
+	if len(m.cookie) > 0 {
+		exts.AddUint16(extensionCookie)
+		exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+			exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+				exts.AddBytes(m.cookie)
+			})
+		})
+	}
+	if m.selectedGroup != 0 {
+		exts.AddUint16(extensionKeyShare)
+		exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+			exts.AddUint16(uint16(m.selectedGroup))
+		})
+	}
+	if len(m.supportedPoints) > 0 {
+		exts.AddUint16(extensionSupportedPoints)
+		exts.AddUint16LengthPrefixed(func(exts *cryptobyte.Builder) {
+			exts.AddUint8LengthPrefixed(func(exts *cryptobyte.Builder) {
+				exts.AddBytes(m.supportedPoints)
+			})
+		})
+	}
+
+	extBytes, err := exts.Bytes()
+	if err != nil {
+		return nil, err
 	}
 
 	var b cryptobyte.Builder
@@ -634,104 +732,15 @@ func (m *serverHelloMsg) marshal() []byte {
 		b.AddUint16(m.cipherSuite)
 		b.AddUint8(m.compressionMethod)
 
-		// If extensions aren't present, omit them.
-		var extensionsPresent bool
-		bWithoutExtensions := *b
-
-		b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-			if m.ocspStapling {
-				b.AddUint16(extensionStatusRequest)
-				b.AddUint16(0) // empty extension_data
-			}
-			if m.ticketSupported {
-				b.AddUint16(extensionSessionTicket)
-				b.AddUint16(0) // empty extension_data
-			}
-			if m.secureRenegotiationSupported {
-				b.AddUint16(extensionRenegotiationInfo)
-				b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-					b.AddUint8LengthPrefixed(func(b *cryptobyte.Builder) {
-						b.AddBytes(m.secureRenegotiation)
-					})
-				})
-			}
-			if len(m.alpnProtocol) > 0 {
-				b.AddUint16(extensionALPN)
-				b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-					b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-						b.AddUint8LengthPrefixed(func(b *cryptobyte.Builder) {
-							b.AddBytes([]byte(m.alpnProtocol))
-						})
-					})
-				})
-			}
-			if len(m.scts) > 0 {
-				b.AddUint16(extensionSCT)
-				b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-					b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-						for _, sct := range m.scts {
-							b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-								b.AddBytes(sct)
-							})
-						}
-					})
-				})
-			}
-			if m.supportedVersion != 0 {
-				b.AddUint16(extensionSupportedVersions)
-				b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-					b.AddUint16(m.supportedVersion)
-				})
-			}
-			if m.serverShare.group != 0 {
-				b.AddUint16(extensionKeyShare)
-				b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-					b.AddUint16(uint16(m.serverShare.group))
-					b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-						b.AddBytes(m.serverShare.data)
-					})
-				})
-			}
-			if m.selectedIdentityPresent {
-				b.AddUint16(extensionPreSharedKey)
-				b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-					b.AddUint16(m.selectedIdentity)
-				})
-			}
-
-			if len(m.cookie) > 0 {
-				b.AddUint16(extensionCookie)
-				b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-					b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-						b.AddBytes(m.cookie)
-					})
-				})
-			}
-			if m.selectedGroup != 0 {
-				b.AddUint16(extensionKeyShare)
-				b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-					b.AddUint16(uint16(m.selectedGroup))
-				})
-			}
-			if len(m.supportedPoints) > 0 {
-				b.AddUint16(extensionSupportedPoints)
-				b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
-					b.AddUint8LengthPrefixed(func(b *cryptobyte.Builder) {
-						b.AddBytes(m.supportedPoints)
-					})
-				})
-			}
-
-			extensionsPresent = len(b.BytesOrPanic()) > 2
-		})
-
-		if !extensionsPresent {
-			*b = bWithoutExtensions
+		if len(extBytes) > 0 {
+			b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) {
+				b.AddBytes(extBytes)
+			})
 		}
 	})
 
-	m.raw = b.BytesOrPanic()
-	return m.raw
+	m.raw, err = b.Bytes()
+	return m.raw, err
 }
 
 func (m *serverHelloMsg) unmarshal(data []byte) bool {
@@ -855,9 +864,9 @@ type encryptedExtensionsMsg struct {
 	alpnProtocol string
 }
 
-func (m *encryptedExtensionsMsg) marshal() []byte {
+func (m *encryptedExtensionsMsg) marshal() ([]byte, error) {
 	if m.raw != nil {
-		return m.raw
+		return m.raw, nil
 	}
 
 	var b cryptobyte.Builder
@@ -877,8 +886,9 @@ func (m *encryptedExtensionsMsg) marshal() []byte {
 		})
 	})
 
-	m.raw = b.BytesOrPanic()
-	return m.raw
+	var err error
+	m.raw, err = b.Bytes()
+	return m.raw, err
 }
 
 func (m *encryptedExtensionsMsg) unmarshal(data []byte) bool {
@@ -926,10 +936,10 @@ func (m *encryptedExtensionsMsg) unmarshal(data []byte) bool {
 
 type endOfEarlyDataMsg struct{}
 
-func (m *endOfEarlyDataMsg) marshal() []byte {
+func (m *endOfEarlyDataMsg) marshal() ([]byte, error) {
 	x := make([]byte, 4)
 	x[0] = typeEndOfEarlyData
-	return x
+	return x, nil
 }
 
 func (m *endOfEarlyDataMsg) unmarshal(data []byte) bool {
@@ -941,9 +951,9 @@ type keyUpdateMsg struct {
 	updateRequested bool
 }
 
-func (m *keyUpdateMsg) marshal() []byte {
+func (m *keyUpdateMsg) marshal() ([]byte, error) {
 	if m.raw != nil {
-		return m.raw
+		return m.raw, nil
 	}
 
 	var b cryptobyte.Builder
@@ -956,8 +966,9 @@ func (m *keyUpdateMsg) marshal() []byte {
 		}
 	})
 
-	m.raw = b.BytesOrPanic()
-	return m.raw
+	var err error
+	m.raw, err = b.Bytes()
+	return m.raw, err
 }
 
 func (m *keyUpdateMsg) unmarshal(data []byte) bool {
@@ -989,9 +1000,9 @@ type newSessionTicketMsgTLS13 struct {
 	maxEarlyData uint32
 }
 
-func (m *newSessionTicketMsgTLS13) marshal() []byte {
+func (m *newSessionTicketMsgTLS13) marshal() ([]byte, error) {
 	if m.raw != nil {
-		return m.raw
+		return m.raw, nil
 	}
 
 	var b cryptobyte.Builder
@@ -1016,8 +1027,9 @@ func (m *newSessionTicketMsgTLS13) marshal() []byte {
 		})
 	})
 
-	m.raw = b.BytesOrPanic()
-	return m.raw
+	var err error
+	m.raw, err = b.Bytes()
+	return m.raw, err
 }
 
 func (m *newSessionTicketMsgTLS13) unmarshal(data []byte) bool {
@@ -1070,9 +1082,9 @@ type certificateRequestMsgTLS13 struct {
 	certificateAuthorities           [][]byte
 }
 
-func (m *certificateRequestMsgTLS13) marshal() []byte {
+func (m *certificateRequestMsgTLS13) marshal() ([]byte, error) {
 	if m.raw != nil {
-		return m.raw
+		return m.raw, nil
 	}
 
 	var b cryptobyte.Builder
@@ -1131,8 +1143,9 @@ func (m *certificateRequestMsgTLS13) marshal() []byte {
 		})
 	})
 
-	m.raw = b.BytesOrPanic()
-	return m.raw
+	var err error
+	m.raw, err = b.Bytes()
+	return m.raw, err
 }
 
 func (m *certificateRequestMsgTLS13) unmarshal(data []byte) bool {
@@ -1216,9 +1229,9 @@ type certificateMsg struct {
 	certificates [][]byte
 }
 
-func (m *certificateMsg) marshal() (x []byte) {
+func (m *certificateMsg) marshal() ([]byte, error) {
 	if m.raw != nil {
-		return m.raw
+		return m.raw, nil
 	}
 
 	var i int
@@ -1227,7 +1240,7 @@ func (m *certificateMsg) marshal() (x []byte) {
 	}
 
 	length := 3 + 3*len(m.certificates) + i
-	x = make([]byte, 4+length)
+	x := make([]byte, 4+length)
 	x[0] = typeCertificate
 	x[1] = uint8(length >> 16)
 	x[2] = uint8(length >> 8)
@@ -1248,7 +1261,7 @@ func (m *certificateMsg) marshal() (x []byte) {
 	}
 
 	m.raw = x
-	return
+	return m.raw, nil
 }
 
 func (m *certificateMsg) unmarshal(data []byte) bool {
@@ -1295,9 +1308,9 @@ type certificateMsgTLS13 struct {
 	scts         bool
 }
 
-func (m *certificateMsgTLS13) marshal() []byte {
+func (m *certificateMsgTLS13) marshal() ([]byte, error) {
 	if m.raw != nil {
-		return m.raw
+		return m.raw, nil
 	}
 
 	var b cryptobyte.Builder
@@ -1315,8 +1328,9 @@ func (m *certificateMsgTLS13) marshal() []byte {
 		marshalCertificate(b, certificate)
 	})
 
-	m.raw = b.BytesOrPanic()
-	return m.raw
+	var err error
+	m.raw, err = b.Bytes()
+	return m.raw, err
 }
 
 func marshalCertificate(b *cryptobyte.Builder, certificate Certificate) {
@@ -1439,9 +1453,9 @@ type serverKeyExchangeMsg struct {
 	key []byte
 }
 
-func (m *serverKeyExchangeMsg) marshal() []byte {
+func (m *serverKeyExchangeMsg) marshal() ([]byte, error) {
 	if m.raw != nil {
-		return m.raw
+		return m.raw, nil
 	}
 	length := len(m.key)
 	x := make([]byte, length+4)
@@ -1452,7 +1466,7 @@ func (m *serverKeyExchangeMsg) marshal() []byte {
 	copy(x[4:], m.key)
 
 	m.raw = x
-	return x
+	return x, nil
 }
 
 func (m *serverKeyExchangeMsg) unmarshal(data []byte) bool {
@@ -1469,9 +1483,9 @@ type certificateStatusMsg struct {
 	response []byte
 }
 
-func (m *certificateStatusMsg) marshal() []byte {
+func (m *certificateStatusMsg) marshal() ([]byte, error) {
 	if m.raw != nil {
-		return m.raw
+		return m.raw, nil
 	}
 
 	var b cryptobyte.Builder
@@ -1483,8 +1497,9 @@ func (m *certificateStatusMsg) marshal() []byte {
 		})
 	})
 
-	m.raw = b.BytesOrPanic()
-	return m.raw
+	var err error
+	m.raw, err = b.Bytes()
+	return m.raw, err
 }
 
 func (m *certificateStatusMsg) unmarshal(data []byte) bool {
@@ -1503,10 +1518,10 @@ func (m *certificateStatusMsg) unmarshal(data []byte) bool {
 
 type serverHelloDoneMsg struct{}
 
-func (m *serverHelloDoneMsg) marshal() []byte {
+func (m *serverHelloDoneMsg) marshal() ([]byte, error) {
 	x := make([]byte, 4)
 	x[0] = typeServerHelloDone
-	return x
+	return x, nil
 }
 
 func (m *serverHelloDoneMsg) unmarshal(data []byte) bool {
@@ -1518,9 +1533,9 @@ type clientKeyExchangeMsg struct {
 	ciphertext []byte
 }
 
-func (m *clientKeyExchangeMsg) marshal() []byte {
+func (m *clientKeyExchangeMsg) marshal() ([]byte, error) {
 	if m.raw != nil {
-		return m.raw
+		return m.raw, nil
 	}
 	length := len(m.ciphertext)
 	x := make([]byte, length+4)
@@ -1531,7 +1546,7 @@ func (m *clientKeyExchangeMsg) marshal() []byte {
 	copy(x[4:], m.ciphertext)
 
 	m.raw = x
-	return x
+	return x, nil
 }
 
 func (m *clientKeyExchangeMsg) unmarshal(data []byte) bool {
@@ -1552,9 +1567,9 @@ type finishedMsg struct {
 	verifyData []byte
 }
 
-func (m *finishedMsg) marshal() []byte {
+func (m *finishedMsg) marshal() ([]byte, error) {
 	if m.raw != nil {
-		return m.raw
+		return m.raw, nil
 	}
 
 	var b cryptobyte.Builder
@@ -1563,8 +1578,9 @@ func (m *finishedMsg) marshal() []byte {
 		b.AddBytes(m.verifyData)
 	})
 
-	m.raw = b.BytesOrPanic()
-	return m.raw
+	var err error
+	m.raw, err = b.Bytes()
+	return m.raw, err
 }
 
 func (m *finishedMsg) unmarshal(data []byte) bool {
@@ -1586,9 +1602,9 @@ type certificateRequestMsg struct {
 	certificateAuthorities       [][]byte
 }
 
-func (m *certificateRequestMsg) marshal() (x []byte) {
+func (m *certificateRequestMsg) marshal() ([]byte, error) {
 	if m.raw != nil {
-		return m.raw
+		return m.raw, nil
 	}
 
 	// See RFC 4346, Section 7.4.4.
@@ -1603,7 +1619,7 @@ func (m *certificateRequestMsg) marshal() (x []byte) {
 		length += 2 + 2*len(m.supportedSignatureAlgorithms)
 	}
 
-	x = make([]byte, 4+length)
+	x := make([]byte, 4+length)
 	x[0] = typeCertificateRequest
 	x[1] = uint8(length >> 16)
 	x[2] = uint8(length >> 8)
@@ -1638,7 +1654,7 @@ func (m *certificateRequestMsg) marshal() (x []byte) {
 	}
 
 	m.raw = x
-	return
+	return m.raw, nil
 }
 
 func (m *certificateRequestMsg) unmarshal(data []byte) bool {
@@ -1724,9 +1740,9 @@ type certificateVerifyMsg struct {
 	signature             []byte
 }
 
-func (m *certificateVerifyMsg) marshal() (x []byte) {
+func (m *certificateVerifyMsg) marshal() ([]byte, error) {
 	if m.raw != nil {
-		return m.raw
+		return m.raw, nil
 	}
 
 	var b cryptobyte.Builder
@@ -1740,8 +1756,9 @@ func (m *certificateVerifyMsg) marshal() (x []byte) {
 		})
 	})
 
-	m.raw = b.BytesOrPanic()
-	return m.raw
+	var err error
+	m.raw, err = b.Bytes()
+	return m.raw, err
 }
 
 func (m *certificateVerifyMsg) unmarshal(data []byte) bool {
@@ -1764,15 +1781,15 @@ type newSessionTicketMsg struct {
 	ticket []byte
 }
 
-func (m *newSessionTicketMsg) marshal() (x []byte) {
+func (m *newSessionTicketMsg) marshal() ([]byte, error) {
 	if m.raw != nil {
-		return m.raw
+		return m.raw, nil
 	}
 
 	// See RFC 5077, Section 3.3.
 	ticketLen := len(m.ticket)
 	length := 2 + 4 + ticketLen
-	x = make([]byte, 4+length)
+	x := make([]byte, 4+length)
 	x[0] = typeNewSessionTicket
 	x[1] = uint8(length >> 16)
 	x[2] = uint8(length >> 8)
@@ -1783,7 +1800,7 @@ func (m *newSessionTicketMsg) marshal() (x []byte) {
 
 	m.raw = x
 
-	return
+	return m.raw, nil
 }
 
 func (m *newSessionTicketMsg) unmarshal(data []byte) bool {
@@ -1811,10 +1828,25 @@ func (m *newSessionTicketMsg) unmarshal(data []byte) bool {
 type helloRequestMsg struct {
 }
 
-func (*helloRequestMsg) marshal() []byte {
-	return []byte{typeHelloRequest, 0, 0, 0}
+func (*helloRequestMsg) marshal() ([]byte, error) {
+	return []byte{typeHelloRequest, 0, 0, 0}, nil
 }
 
 func (*helloRequestMsg) unmarshal(data []byte) bool {
 	return len(data) == 4
 }
+
+type transcriptHash interface {
+	Write([]byte) (int, error)
+}
+
+// transcriptMsg is a helper used to marshal and hash messages which typically
+// are not written to the wire, and as such aren't hashed during Conn.writeRecord.
+func transcriptMsg(msg handshakeMessage, h transcriptHash) error {
+	data, err := msg.marshal()
+	if err != nil {
+		return err
+	}
+	h.Write(data)
+	return nil
+}

handshake_messages_test.go

@@ -38,6 +38,15 @@ var tests = []any{
 	&certificateMsgTLS13{},
 }
 
+func mustMarshal(t *testing.T, msg handshakeMessage) []byte {
+	t.Helper()
+	b, err := msg.marshal()
+	if err != nil {
+		t.Fatal(err)
+	}
+	return b
+}
+
 func TestMarshalUnmarshal(t *testing.T) {
 	rand := rand.New(rand.NewSource(time.Now().UnixNano()))
 
@@ -56,7 +65,7 @@ func TestMarshalUnmarshal(t *testing.T) {
 			}
 
 			m1 := v.Interface().(handshakeMessage)
-			marshaled := m1.marshal()
+			marshaled := mustMarshal(t, m1)
 			m2 := iface.(handshakeMessage)
 			if !m2.unmarshal(marshaled) {
 				t.Errorf("#%d failed to unmarshal %#v %x", i, m1, marshaled)
@@ -409,12 +418,12 @@ func TestRejectEmptySCTList(t *testing.T) {
 
 	var random [32]byte
 	sct := []byte{0x42, 0x42, 0x42, 0x42}
-	serverHello := serverHelloMsg{
+	serverHello := &serverHelloMsg{
 		vers:   VersionTLS12,
 		random: random[:],
 		scts:   [][]byte{sct},
 	}
-	serverHelloBytes := serverHello.marshal()
+	serverHelloBytes := mustMarshal(t, serverHello)
 
 	var serverHelloCopy serverHelloMsg
 	if !serverHelloCopy.unmarshal(serverHelloBytes) {
@@ -452,12 +461,12 @@ func TestRejectEmptySCT(t *testing.T) {
 	// not be zero length.
 
 	var random [32]byte
-	serverHello := serverHelloMsg{
+	serverHello := &serverHelloMsg{
 		vers:   VersionTLS12,
 		random: random[:],
 		scts:   [][]byte{nil},
 	}
-	serverHelloBytes := serverHello.marshal()
+	serverHelloBytes := mustMarshal(t, serverHello)
 
 	var serverHelloCopy serverHelloMsg
 	if serverHelloCopy.unmarshal(serverHelloBytes) {

handshake_server.go

@@ -128,7 +128,9 @@ func (hs *serverHandshakeState) handshake() error {
 
 // readClientHello reads a ClientHello message and selects the protocol version.
 func (c *Conn) readClientHello(ctx context.Context) (*clientHelloMsg, error) {
-	msg, err := c.readHandshake()
+	// clientHelloMsg is included in the transcript, but we haven't initialized
+	// it yet. The respective handshake functions will record it themselves.
+	msg, err := c.readHandshake(nil)
 	if err != nil {
 		return nil, err
 	}
@@ -462,9 +464,10 @@ func (hs *serverHandshakeState) doResumeHandshake() error {
 	hs.hello.ticketSupported = hs.sessionState.usedOldKey
 	hs.finishedHash = newFinishedHash(c.vers, hs.suite)
 	hs.finishedHash.discardHandshakeBuffer()
-	hs.finishedHash.Write(hs.clientHello.marshal())
-	hs.finishedHash.Write(hs.hello.marshal())
-	if _, err := c.writeRecord(recordTypeHandshake, hs.hello.marshal()); err != nil {
+	if err := transcriptMsg(hs.clientHello, &hs.finishedHash); err != nil {
+		return err
+	}
+	if _, err := hs.c.writeHandshakeRecord(hs.hello, &hs.finishedHash); err != nil {
 		return err
 	}
 
@@ -502,24 +505,23 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 		// certificates won't be used.
 		hs.finishedHash.discardHandshakeBuffer()
 	}
-	hs.finishedHash.Write(hs.clientHello.marshal())
-	hs.finishedHash.Write(hs.hello.marshal())
-	if _, err := c.writeRecord(recordTypeHandshake, hs.hello.marshal()); err != nil {
+	if err := transcriptMsg(hs.clientHello, &hs.finishedHash); err != nil {
+		return err
+	}
+	if _, err := hs.c.writeHandshakeRecord(hs.hello, &hs.finishedHash); err != nil {
 		return err
 	}
 
 	certMsg := new(certificateMsg)
 	certMsg.certificates = hs.cert.Certificate
-	hs.finishedHash.Write(certMsg.marshal())
-	if _, err := c.writeRecord(recordTypeHandshake, certMsg.marshal()); err != nil {
+	if _, err := hs.c.writeHandshakeRecord(certMsg, &hs.finishedHash); err != nil {
 		return err
 	}
 
 	if hs.hello.ocspStapling {
 		certStatus := new(certificateStatusMsg)
 		certStatus.response = hs.cert.OCSPStaple
-		hs.finishedHash.Write(certStatus.marshal())
-		if _, err := c.writeRecord(recordTypeHandshake, certStatus.marshal()); err != nil {
+		if _, err := hs.c.writeHandshakeRecord(certStatus, &hs.finishedHash); err != nil {
 			return err
 		}
 	}
@@ -531,8 +533,7 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 		return err
 	}
 	if skx != nil {
-		hs.finishedHash.Write(skx.marshal())
-		if _, err := c.writeRecord(recordTypeHandshake, skx.marshal()); err != nil {
+		if _, err := hs.c.writeHandshakeRecord(skx, &hs.finishedHash); err != nil {
 			return err
 		}
 	}
@@ -558,15 +559,13 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 		if c.config.ClientCAs != nil {
 			certReq.certificateAuthorities = c.config.ClientCAs.Subjects()
 		}
-		hs.finishedHash.Write(certReq.marshal())
-		if _, err := c.writeRecord(recordTypeHandshake, certReq.marshal()); err != nil {
+		if _, err := hs.c.writeHandshakeRecord(certReq, &hs.finishedHash); err != nil {
 			return err
 		}
 	}
 
 	helloDone := new(serverHelloDoneMsg)
-	hs.finishedHash.Write(helloDone.marshal())
-	if _, err := c.writeRecord(recordTypeHandshake, helloDone.marshal()); err != nil {
+	if _, err := hs.c.writeHandshakeRecord(helloDone, &hs.finishedHash); err != nil {
 		return err
 	}
 
@@ -576,7 +575,7 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 
 	var pub crypto.PublicKey // public key for client auth, if any
 
-	msg, err := c.readHandshake()
+	msg, err := c.readHandshake(&hs.finishedHash)
 	if err != nil {
 		return err
 	}
@@ -589,7 +588,6 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 			c.sendAlert(alertUnexpectedMessage)
 			return unexpectedMessageError(certMsg, msg)
 		}
-		hs.finishedHash.Write(certMsg.marshal())
 
 		if err := c.processCertsFromClient(Certificate{
 			Certificate: certMsg.certificates,
@@ -600,7 +598,7 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 			pub = c.peerCertificates[0].PublicKey
 		}
 
-		msg, err = c.readHandshake()
+		msg, err = c.readHandshake(&hs.finishedHash)
 		if err != nil {
 			return err
 		}
@@ -618,7 +616,6 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 		c.sendAlert(alertUnexpectedMessage)
 		return unexpectedMessageError(ckx, msg)
 	}
-	hs.finishedHash.Write(ckx.marshal())
 
 	preMasterSecret, err := keyAgreement.processClientKeyExchange(c.config, hs.cert, ckx, c.vers)
 	if err != nil {
@@ -638,7 +635,10 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 	// to the client's certificate. This allows us to verify that the client is in
 	// possession of the private key of the certificate.
 	if len(c.peerCertificates) > 0 {
-		msg, err = c.readHandshake()
+		// certificateVerifyMsg is included in the transcript, but not until
+		// after we verify the handshake signature, since the state before
+		// this message was sent is used.
+		msg, err = c.readHandshake(nil)
 		if err != nil {
 			return err
 		}
@@ -673,7 +673,9 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 			return errors.New("tls: invalid signature by the client certificate: " + err.Error())
 		}
 
-		hs.finishedHash.Write(certVerify.marshal())
+		if err := transcriptMsg(certVerify, &hs.finishedHash); err != nil {
+			return err
+		}
 	}
 
 	hs.finishedHash.discardHandshakeBuffer()
@@ -713,7 +715,10 @@ func (hs *serverHandshakeState) readFinished(out []byte) error {
 		return err
 	}
 
-	msg, err := c.readHandshake()
+	// finishedMsg is included in the transcript, but not until after we
+	// check the client version, since the state before this message was
+	// sent is used during verification.
+	msg, err := c.readHandshake(nil)
 	if err != nil {
 		return err
 	}
@@ -730,7 +735,10 @@ func (hs *serverHandshakeState) readFinished(out []byte) error {
 		return errors.New("tls: client's Finished message is incorrect")
 	}
 
-	hs.finishedHash.Write(clientFinished.marshal())
+	if err := transcriptMsg(clientFinished, &hs.finishedHash); err != nil {
+		return err
+	}
+
 	copy(out, verify)
 	return nil
 }
@@ -764,14 +772,16 @@ func (hs *serverHandshakeState) sendSessionTicket() error {
 		masterSecret: hs.masterSecret,
 		certificates: certsFromClient,
 	}
-	var err error
-	m.ticket, err = c.encryptTicket(state.marshal())
+	stateBytes, err := state.marshal()
+	if err != nil {
+		return err
+	}
+	m.ticket, err = c.encryptTicket(stateBytes)
 	if err != nil {
 		return err
 	}
 
-	hs.finishedHash.Write(m.marshal())
-	if _, err := c.writeRecord(recordTypeHandshake, m.marshal()); err != nil {
+	if _, err := hs.c.writeHandshakeRecord(m, &hs.finishedHash); err != nil {
 		return err
 	}
 
@@ -781,14 +791,13 @@ func (hs *serverHandshakeState) sendSessionTicket() error {
 func (hs *serverHandshakeState) sendFinished(out []byte) error {
 	c := hs.c
 
-	if _, err := c.writeRecord(recordTypeChangeCipherSpec, []byte{1}); err != nil {
+	if err := c.writeChangeCipherRecord(); err != nil {
 		return err
 	}
 
 	finished := new(finishedMsg)
 	finished.verifyData = hs.finishedHash.serverSum(hs.masterSecret)
-	hs.finishedHash.Write(finished.marshal())
-	if _, err := c.writeRecord(recordTypeHandshake, finished.marshal()); err != nil {
+	if _, err := hs.c.writeHandshakeRecord(finished, &hs.finishedHash); err != nil {
 		return err
 	}
 

handshake_server_test.go

@@ -30,6 +30,13 @@ func testClientHello(t *testing.T, serverConfig *Config, m handshakeMessage) {
 	testClientHelloFailure(t, serverConfig, m, "")
 }
 
+// testFatal is a hack to prevent the compiler from complaining that there is a
+// call to t.Fatal from a non-test goroutine
+func testFatal(t *testing.T, err error) {
+	t.Helper()
+	t.Fatal(err)
+}
+
 func testClientHelloFailure(t *testing.T, serverConfig *Config, m handshakeMessage, expectedSubStr string) {
 	c, s := localPipe(t)
 	go func() {
@@ -37,7 +44,9 @@ func testClientHelloFailure(t *testing.T, serverConfig *Config, m handshakeMessa
 		if ch, ok := m.(*clientHelloMsg); ok {
 			cli.vers = ch.vers
 		}
-		cli.writeRecord(recordTypeHandshake, m.marshal())
+		if _, err := cli.writeHandshakeRecord(m, nil); err != nil {
+			testFatal(t, err)
+		}
 		c.Close()
 	}()
 	ctx := context.Background()
@@ -194,7 +203,9 @@ func TestRenegotiationExtension(t *testing.T) {
 	go func() {
 		cli := Client(c, testConfig)
 		cli.vers = clientHello.vers
-		cli.writeRecord(recordTypeHandshake, clientHello.marshal())
+		if _, err := cli.writeHandshakeRecord(clientHello, nil); err != nil {
+			testFatal(t, err)
+		}
 
 		buf := make([]byte, 1024)
 		n, err := c.Read(buf)
@@ -253,8 +264,10 @@ func TestTLS12OnlyCipherSuites(t *testing.T) {
 	go func() {
 		cli := Client(c, testConfig)
 		cli.vers = clientHello.vers
-		cli.writeRecord(recordTypeHandshake, clientHello.marshal())
-		reply, err := cli.readHandshake()
+		if _, err := cli.writeHandshakeRecord(clientHello, nil); err != nil {
+			testFatal(t, err)
+		}
+		reply, err := cli.readHandshake(nil)
 		c.Close()
 		if err != nil {
 			replyChan <- err
@@ -311,8 +324,10 @@ func TestTLSPointFormats(t *testing.T) {
 			go func() {
 				cli := Client(c, testConfig)
 				cli.vers = clientHello.vers
-				cli.writeRecord(recordTypeHandshake, clientHello.marshal())
-				reply, err := cli.readHandshake()
+				if _, err := cli.writeHandshakeRecord(clientHello, nil); err != nil {
+					testFatal(t, err)
+				}
+				reply, err := cli.readHandshake(nil)
 				c.Close()
 				if err != nil {
 					replyChan <- err
@@ -1426,7 +1441,9 @@ func TestSNIGivenOnFailure(t *testing.T) {
 	go func() {
 		cli := Client(c, testConfig)
 		cli.vers = clientHello.vers
-		cli.writeRecord(recordTypeHandshake, clientHello.marshal())
+		if _, err := cli.writeHandshakeRecord(clientHello, nil); err != nil {
+			testFatal(t, err)
+		}
 		c.Close()
 	}()
 	conn := Server(s, serverConfig)

handshake_server_tls13.go

@@ -306,7 +306,12 @@ func (hs *serverHandshakeStateTLS13) checkForResumption() error {
 			c.sendAlert(alertInternalError)
 			return errors.New("tls: internal error: failed to clone hash")
 		}
-		transcript.Write(hs.clientHello.marshalWithoutBinders())
+		clientHelloBytes, err := hs.clientHello.marshalWithoutBinders()
+		if err != nil {
+			c.sendAlert(alertInternalError)
+			return err
+		}
+		transcript.Write(clientHelloBytes)
 		pskBinder := hs.suite.finishedHash(binderKey, transcript)
 		if !hmac.Equal(hs.clientHello.pskBinders[i], pskBinder) {
 			c.sendAlert(alertDecryptError)
@@ -397,8 +402,7 @@ func (hs *serverHandshakeStateTLS13) sendDummyChangeCipherSpec() error {
 	}
 	hs.sentDummyCCS = true
 
-	_, err := hs.c.writeRecord(recordTypeChangeCipherSpec, []byte{1})
-	return err
+	return hs.c.writeChangeCipherRecord()
 }
 
 func (hs *serverHandshakeStateTLS13) doHelloRetryRequest(selectedGroup CurveID) error {
@@ -406,7 +410,9 @@ func (hs *serverHandshakeStateTLS13) doHelloRetryRequest(selectedGroup CurveID)
 
 	// The first ClientHello gets double-hashed into the transcript upon a
 	// HelloRetryRequest. See RFC 8446, Section 4.4.1.
-	hs.transcript.Write(hs.clientHello.marshal())
+	if err := transcriptMsg(hs.clientHello, hs.transcript); err != nil {
+		return err
+	}
 	chHash := hs.transcript.Sum(nil)
 	hs.transcript.Reset()
 	hs.transcript.Write([]byte{typeMessageHash, 0, 0, uint8(len(chHash))})
@@ -422,8 +428,7 @@ func (hs *serverHandshakeStateTLS13) doHelloRetryRequest(selectedGroup CurveID)
 		selectedGroup:     selectedGroup,
 	}
 
-	hs.transcript.Write(helloRetryRequest.marshal())
-	if _, err := c.writeRecord(recordTypeHandshake, helloRetryRequest.marshal()); err != nil {
+	if _, err := hs.c.writeHandshakeRecord(helloRetryRequest, hs.transcript); err != nil {
 		return err
 	}
 
@@ -431,7 +436,8 @@ func (hs *serverHandshakeStateTLS13) doHelloRetryRequest(selectedGroup CurveID)
 		return err
 	}
 
-	msg, err := c.readHandshake()
+	// clientHelloMsg is not included in the transcript.
+	msg, err := c.readHandshake(nil)
 	if err != nil {
 		return err
 	}
@@ -522,9 +528,10 @@ func illegalClientHelloChange(ch, ch1 *clientHelloMsg) bool {
 func (hs *serverHandshakeStateTLS13) sendServerParameters() error {
 	c := hs.c
 
-	hs.transcript.Write(hs.clientHello.marshal())
-	hs.transcript.Write(hs.hello.marshal())
-	if _, err := c.writeRecord(recordTypeHandshake, hs.hello.marshal()); err != nil {
+	if err := transcriptMsg(hs.clientHello, hs.transcript); err != nil {
+		return err
+	}
+	if _, err := hs.c.writeHandshakeRecord(hs.hello, hs.transcript); err != nil {
 		return err
 	}
 
@@ -567,8 +574,7 @@ func (hs *serverHandshakeStateTLS13) sendServerParameters() error {
 	encryptedExtensions.alpnProtocol = selectedProto
 	c.clientProtocol = selectedProto
 
-	hs.transcript.Write(encryptedExtensions.marshal())
-	if _, err := c.writeRecord(recordTypeHandshake, encryptedExtensions.marshal()); err != nil {
+	if _, err := hs.c.writeHandshakeRecord(encryptedExtensions, hs.transcript); err != nil {
 		return err
 	}
 
@@ -597,8 +603,7 @@ func (hs *serverHandshakeStateTLS13) sendServerCertificate() error {
 			certReq.certificateAuthorities = c.config.ClientCAs.Subjects()
 		}
 
-		hs.transcript.Write(certReq.marshal())
-		if _, err := c.writeRecord(recordTypeHandshake, certReq.marshal()); err != nil {
+		if _, err := hs.c.writeHandshakeRecord(certReq, hs.transcript); err != nil {
 			return err
 		}
 	}
@@ -609,8 +614,7 @@ func (hs *serverHandshakeStateTLS13) sendServerCertificate() error {
 	certMsg.scts = hs.clientHello.scts && len(hs.cert.SignedCertificateTimestamps) > 0
 	certMsg.ocspStapling = hs.clientHello.ocspStapling && len(hs.cert.OCSPStaple) > 0
 
-	hs.transcript.Write(certMsg.marshal())
-	if _, err := c.writeRecord(recordTypeHandshake, certMsg.marshal()); err != nil {
+	if _, err := hs.c.writeHandshakeRecord(certMsg, hs.transcript); err != nil {
 		return err
 	}
 
@@ -641,8 +645,7 @@ func (hs *serverHandshakeStateTLS13) sendServerCertificate() error {
 	}
 	certVerifyMsg.signature = sig
 
-	hs.transcript.Write(certVerifyMsg.marshal())
-	if _, err := c.writeRecord(recordTypeHandshake, certVerifyMsg.marshal()); err != nil {
+	if _, err := hs.c.writeHandshakeRecord(certVerifyMsg, hs.transcript); err != nil {
 		return err
 	}
 
@@ -656,8 +659,7 @@ func (hs *serverHandshakeStateTLS13) sendServerFinished() error {
 		verifyData: hs.suite.finishedHash(c.out.trafficSecret, hs.transcript),
 	}
 
-	hs.transcript.Write(finished.marshal())
-	if _, err := c.writeRecord(recordTypeHandshake, finished.marshal()); err != nil {
+	if _, err := hs.c.writeHandshakeRecord(finished, hs.transcript); err != nil {
 		return err
 	}
 
@@ -718,7 +720,9 @@ func (hs *serverHandshakeStateTLS13) sendSessionTickets() error {
 	finishedMsg := &finishedMsg{
 		verifyData: hs.clientFinished,
 	}
-	hs.transcript.Write(finishedMsg.marshal())
+	if err := transcriptMsg(finishedMsg, hs.transcript); err != nil {
+		return err
+	}
 
 	if !hs.shouldSendSessionTickets() {
 		return nil
@@ -743,8 +747,12 @@ func (hs *serverHandshakeStateTLS13) sendSessionTickets() error {
 			SignedCertificateTimestamps: c.scts,
 		},
 	}
-	var err error
-	m.label, err = c.encryptTicket(state.marshal())
+	stateBytes, err := state.marshal()
+	if err != nil {
+		c.sendAlert(alertInternalError)
+		return err
+	}
+	m.label, err = c.encryptTicket(stateBytes)
 	if err != nil {
 		return err
 	}
@@ -763,7 +771,7 @@ func (hs *serverHandshakeStateTLS13) sendSessionTickets() error {
 	// ticket_nonce, which must be unique per connection, is always left at
 	// zero because we only ever send one ticket per connection.
 
-	if _, err := c.writeRecord(recordTypeHandshake, m.marshal()); err != nil {
+	if _, err := c.writeHandshakeRecord(m, nil); err != nil {
 		return err
 	}
 
@@ -788,7 +796,7 @@ func (hs *serverHandshakeStateTLS13) readClientCertificate() error {
 	// If we requested a client certificate, then the client must send a
 	// certificate message. If it's empty, no CertificateVerify is sent.
 
-	msg, err := c.readHandshake()
+	msg, err := c.readHandshake(hs.transcript)
 	if err != nil {
 		return err
 	}
@@ -798,7 +806,6 @@ func (hs *serverHandshakeStateTLS13) readClientCertificate() error {
 		c.sendAlert(alertUnexpectedMessage)
 		return unexpectedMessageError(certMsg, msg)
 	}
-	hs.transcript.Write(certMsg.marshal())
 
 	if err := c.processCertsFromClient(certMsg.certificate); err != nil {
 		return err
@@ -812,7 +819,10 @@ func (hs *serverHandshakeStateTLS13) readClientCertificate() error {
 	}
 
 	if len(certMsg.certificate.Certificate) != 0 {
-		msg, err = c.readHandshake()
+		// certificateVerifyMsg is included in the transcript, but not until
+		// after we verify the handshake signature, since the state before
+		// this message was sent is used.
+		msg, err = c.readHandshake(nil)
 		if err != nil {
 			return err
 		}
@@ -843,7 +853,9 @@ func (hs *serverHandshakeStateTLS13) readClientCertificate() error {
 			return errors.New("tls: invalid signature by the client certificate: " + err.Error())
 		}
 
-		hs.transcript.Write(certVerify.marshal())
+		if err := transcriptMsg(certVerify, hs.transcript); err != nil {
+			return err
+		}
 	}
 
 	// If we waited until the client certificates to send session tickets, we
@@ -858,7 +870,8 @@ func (hs *serverHandshakeStateTLS13) readClientCertificate() error {
 func (hs *serverHandshakeStateTLS13) readClientFinished() error {
 	c := hs.c
 
-	msg, err := c.readHandshake()
+	// finishedMsg is not included in the transcript.
+	msg, err := c.readHandshake(nil)
 	if err != nil {
 		return err
 	}

key_schedule.go

@@ -8,6 +8,7 @@ import (
 	"crypto/ecdh"
 	"crypto/hmac"
 	"errors"
+	"fmt"
 	"hash"
 	"io"
 
@@ -40,8 +41,24 @@ func (c *cipherSuiteTLS13) expandLabel(secret []byte, label string, context []by
 	hkdfLabel.AddUint8LengthPrefixed(func(b *cryptobyte.Builder) {
 		b.AddBytes(context)
 	})
+	hkdfLabelBytes, err := hkdfLabel.Bytes()
+	if err != nil {
+		// Rather than calling BytesOrPanic, we explicitly handle this error, in
+		// order to provide a reasonable error message. It should be basically
+		// impossible for this to panic, and routing errors back through the
+		// tree rooted in this function is quite painful. The labels are fixed
+		// size, and the context is either a fixed-length computed hash, or
+		// parsed from a field which has the same length limitation. As such, an
+		// error here is likely to only be caused during development.
+		//
+		// NOTE: another reasonable approach here might be to return a
+		// randomized slice if we encounter an error, which would break the
+		// connection, but avoid panicking. This would perhaps be safer but
+		// significantly more confusing to users.
+		panic(fmt.Errorf("failed to construct HKDF label: %s", err))
+	}
 	out := make([]byte, length)
-	n, err := hkdf.Expand(c.hash.New, secret, hkdfLabel.BytesOrPanic()).Read(out)
+	n, err := hkdf.Expand(c.hash.New, secret, hkdfLabelBytes).Read(out)
 	if err != nil || n != length {
 		panic("tls: HKDF-Expand-Label invocation failed unexpectedly")
 	}

ticket.go

@@ -32,7 +32,7 @@ type sessionState struct {
 	usedOldKey bool
 }
 
-func (m *sessionState) marshal() []byte {
+func (m *sessionState) marshal() ([]byte, error) {
 	var b cryptobyte.Builder
 	b.AddUint16(m.vers)
 	b.AddUint16(m.cipherSuite)
@@ -47,7 +47,7 @@ func (m *sessionState) marshal() []byte {
 			})
 		}
 	})
-	return b.BytesOrPanic()
+	return b.Bytes()
 }
 
 func (m *sessionState) unmarshal(data []byte) bool {
@@ -86,7 +86,7 @@ type sessionStateTLS13 struct {
 	certificate      Certificate // CertificateEntry certificate_list<0..2^24-1>;
 }
 
-func (m *sessionStateTLS13) marshal() []byte {
+func (m *sessionStateTLS13) marshal() ([]byte, error) {
 	var b cryptobyte.Builder
 	b.AddUint16(VersionTLS13)
 	b.AddUint8(0) // revision
@@ -96,7 +96,7 @@ func (m *sessionStateTLS13) marshal() []byte {
 		b.AddBytes(m.resumptionSecret)
 	})
 	marshalCertificate(&b, m.certificate)
-	return b.BytesOrPanic()
+	return b.Bytes()
 }
 
 func (m *sessionStateTLS13) unmarshal(data []byte) bool {