Merge pull request #95 from cjpatton/subcerts

Delegated credentials for TLS

13.go

@@ -403,6 +403,16 @@ func (hs *serverHandshakeState) sendCertificate13() error {
 	if len(certEntries) > 0 && hs.clientHello.scts {
 		certEntries[0].sctList = hs.cert.SignedCertificateTimestamps
 	}
+
+	// If hs.delegatedCredential is set (see hs.readClientHello()) then the
+	// server is using the delegated credential extension. In TLS 1.3, the DC is
+	// added as an extension to the end-entity certificate, i.e., the last
+	// CertificateEntry of Certificate.certficate_list (see
+	// https://tools.ietf.org/html/draft-ietf-tls-subcerts).
+	if len(certEntries) > 0 && hs.clientHello.delegatedCredential && hs.delegatedCredential != nil {
+		certEntries[0].delegatedCredential = hs.delegatedCredential
+	}
+
 	certMsg := &certificateMsg13{certificates: certEntries}
 
 	hs.keySchedule.write(certMsg.marshal())
@@ -423,7 +433,7 @@ func (hs *serverHandshakeState) sendCertificate13() error {
 	}
 
 	toSign := prepareDigitallySigned(sigHash, "TLS 1.3, server CertificateVerify", hs.keySchedule.transcriptHash.Sum(nil))
-	signature, err := hs.cert.PrivateKey.(crypto.Signer).Sign(c.config.rand(), toSign[:], opts)
+	signature, err := hs.privateKey.(crypto.Signer).Sign(c.config.rand(), toSign[:], opts)
 	if err != nil {
 		c.sendAlert(alertInternalError)
 		return err
@@ -468,9 +478,9 @@ func (c *Conn) handleEndOfEarlyData() error {
 // See https://tools.ietf.org/html/draft-ietf-tls-tls13-18#section-4.4.1.2
 func (hs *serverHandshakeState) selectTLS13SignatureScheme() (sigScheme SignatureScheme, err error) {
 	var supportedSchemes []SignatureScheme
-	signer, ok := hs.cert.PrivateKey.(crypto.Signer)
+	signer, ok := hs.privateKey.(crypto.Signer)
 	if !ok {
-		return 0, errors.New("tls: certificate private key does not implement crypto.Signer")
+		return 0, errors.New("tls: private key does not implement crypto.Signer")
 	}
 	pk := signer.Public()
 	if _, ok := pk.(*rsa.PublicKey); ok {
@@ -1034,12 +1044,34 @@ func (hs *clientHandshakeState) doTLS13Handshake() error {
 		return unexpectedMessageError(certMsg, msg)
 	}
 	hs.keySchedule.write(certMsg.marshal())
+
 	// Validate certificates.
 	certs := getCertsFromEntries(certMsg.certificates)
 	if err := hs.processCertsFromServer(certs); err != nil {
 		return err
 	}
 
+	// Validate the DC if present. The DC is only processed if the extension was
+	// indicated by the ClientHello; otherwise this call will result in an
+	// "illegal_parameter" alert. The call also asserts that the DC extension
+	// did not appear in the ServerHello.
+	if len(certMsg.certificates) > 0 {
+		if err := hs.processDelegatedCredentialFromServer(
+			certMsg.certificates[0].delegatedCredential); err != nil {
+			return err
+		}
+	}
+
+	// Set the public key used to verify the handshake.
+	pk := hs.c.peerCertificates[0].PublicKey
+
+	// If the delegated credential extension has successfully been negotiated,
+	// then the  CertificateVerify signature will have been produced with the
+	// DelegatedCredential's private key.
+	if hs.c.verifiedDc != nil {
+		pk = hs.c.verifiedDc.PublicKey
+	}
+
 	// Receive CertificateVerify message.
 	msg, err = c.readHandshake()
 	if err != nil {
@@ -1052,7 +1084,7 @@ func (hs *clientHandshakeState) doTLS13Handshake() error {
 	}
 	err, alertCode := verifyPeerHandshakeSignature(
 		certVerifyMsg,
-		hs.c.peerCertificates[0].PublicKey,
+		pk,
 		hs.hello.supportedSignatureAlgorithms,
 		hs.keySchedule.transcriptHash.Sum(nil),
 		"TLS 1.3, server CertificateVerify")

cipher_suites.go

@@ -5,6 +5,7 @@
 package tls
 
 import (
+	"crypto"
 	"crypto/aes"
 	"crypto/cipher"
 	"crypto/des"
@@ -12,7 +13,6 @@ import (
 	"crypto/rc4"
 	"crypto/sha1"
 	"crypto/sha256"
-	"crypto/x509"
 	"hash"
 
 	"golang_org/x/crypto/chacha20poly1305"
@@ -26,15 +26,15 @@ type keyAgreement interface {
 	// In the case that the key agreement protocol doesn't use a
 	// ServerKeyExchange message, generateServerKeyExchange can return nil,
 	// nil.
-	generateServerKeyExchange(*Config, *Certificate, *clientHelloMsg, *serverHelloMsg) (*serverKeyExchangeMsg, error)
+	generateServerKeyExchange(*Config, crypto.PrivateKey, *clientHelloMsg, *serverHelloMsg) (*serverKeyExchangeMsg, error)
-	processClientKeyExchange(*Config, *Certificate, *clientKeyExchangeMsg, uint16) ([]byte, error)
+	processClientKeyExchange(*Config, crypto.PrivateKey, *clientKeyExchangeMsg, uint16) ([]byte, error)
 
 	// On the client side, the next two methods are called in order.
 
 	// This method may not be called if the server doesn't send a
 	// ServerKeyExchange message.
-	processServerKeyExchange(*Config, *clientHelloMsg, *serverHelloMsg, *x509.Certificate, *serverKeyExchangeMsg) error
+	processServerKeyExchange(*Config, *clientHelloMsg, *serverHelloMsg, crypto.PublicKey, *serverKeyExchangeMsg) error
-	generateClientKeyExchange(*Config, *clientHelloMsg, *x509.Certificate) ([]byte, *clientKeyExchangeMsg, error)
+	generateClientKeyExchange(*Config, *clientHelloMsg, crypto.PublicKey) ([]byte, *clientKeyExchangeMsg, error)
 }
 
 const (

common.go

@@ -97,6 +97,7 @@ const (
 	extensionKeyShare                uint16 = 51
 	extensionNextProtoNeg            uint16 = 13172 // not IANA assigned
 	extensionRenegotiationInfo       uint16 = 0xff01
+	extensionDelegatedCredential     uint16 = 0xff02 // TODO(any) Get IANA assignment
 )
 
 // TLS signaling cipher suite values
@@ -356,6 +357,10 @@ type ClientHelloInfo struct {
 	// immediately available for Read.
 	Offered0RTTData bool
 
+	// AcceptsDelegatedCredential is true if the client indicated willingness
+	// to negotiate the delegated credential extension.
+	AcceptsDelegatedCredential bool
+
 	// The Fingerprint is an sequence of bytes unique to this Client Hello.
 	// It can be used to prevent or mitigate 0-RTT data replays as it's
 	// guaranteed that a replayed connection will have the same Fingerprint.
@@ -609,6 +614,22 @@ type Config struct {
 	// session tickets, instead of SessionTicketKey.
 	SessionTicketSealer SessionTicketSealer
 
+	// AcceptDelegatedCredential is true if the client is willing to negotiate
+	// the delegated credential extension.
+	//
+	// This value has no meaning for the server.
+	//
+	// See https://tools.ietf.org/html/draft-ietf-tls-subcerts.
+	AcceptDelegatedCredential bool
+
+	// GetDelegatedCredential returns a DelegatedCredential for use with the
+	// delegated credential extension based on the ClientHello and TLS version
+	// selected for the session. If this is nil, then the server will not offer
+	// a DelegatedCredential.
+	//
+	// This value has no meaning for the client.
+	GetDelegatedCredential func(*ClientHelloInfo, uint16) (*DelegatedCredential, crypto.PrivateKey, error)
+
 	serverInitOnce sync.Once // guards calling (*Config).serverInit
 
 	// mutex protects sessionTicketKeys.
@@ -685,6 +706,8 @@ func (c *Config) Clone() *Config {
 		Accept0RTTData:              c.Accept0RTTData,
 		Max0RTTDataSize:             c.Max0RTTDataSize,
 		SessionTicketSealer:         c.SessionTicketSealer,
+		AcceptDelegatedCredential:   c.AcceptDelegatedCredential,
+		GetDelegatedCredential:      c.GetDelegatedCredential,
 		sessionTicketKeys:           sessionTicketKeys,
 	}
 }

conn.go

@@ -51,11 +51,14 @@ type Conn struct {
 	didResume        bool // whether this connection was a session resumption
 	cipherSuite      uint16
 	ocspResponse     []byte   // stapled OCSP response
-	scts             [][]byte // signed certificate timestamps from server
+	scts             [][]byte // Signed certificate timestamps from server
 	peerCertificates []*x509.Certificate
 	// verifiedChains contains the certificate chains that we built, as
 	// opposed to the ones presented by the server.
 	verifiedChains [][]*x509.Certificate
+	// verifiedDc is set by a client who negotiates the use of a valid delegated
+	// credential.
+	verifiedDc *DelegatedCredential
 	// serverName contains the server name indicated by the client, if any.
 	serverName string
 	// secureRenegotiation is true if the server echoed the secure

generate_cert.go

@@ -14,6 +14,7 @@ import (
 	"crypto/elliptic"
 	"crypto/rand"
 	"crypto/rsa"
+	"crypto/tls"
 	"crypto/x509"
 	"crypto/x509/pkix"
 	"encoding/pem"
@@ -34,6 +35,7 @@ var (
 	isCA       = flag.Bool("ca", false, "whether this cert should be its own Certificate Authority")
 	rsaBits    = flag.Int("rsa-bits", 2048, "Size of RSA key to generate. Ignored if --ecdsa-curve is set")
 	ecdsaCurve = flag.String("ecdsa-curve", "", "ECDSA curve to use to generate a key. Valid values are P224, P256 (recommended), P384, P521")
+	isDC       = flag.Bool("dc", false, "whether this cert can be used with delegated credentials")
 )
 
 func publicKey(priv interface{}) interface{} {
@@ -137,6 +139,11 @@ func main() {
 		template.KeyUsage |= x509.KeyUsageCertSign
 	}
 
+	if *isDC {
+		template.ExtraExtensions = append(
+			template.ExtraExtensions, *tls.CreateDelegationUsagePKIXExtension())
+	}
+
 	derBytes, err := x509.CreateCertificate(rand.Reader, &template, &template, publicKey(priv), priv)
 	if err != nil {
 		log.Fatalf("Failed to create certificate: %s", err)

handshake_client.go

@@ -65,6 +65,7 @@ func makeClientHello(config *Config) (*clientHelloMsg, error) {
 		supportedPoints:              []uint8{pointFormatUncompressed},
 		nextProtoNeg:                 len(config.NextProtos) > 0,
 		secureRenegotiationSupported: true,
+		delegatedCredential:          config.AcceptDelegatedCredential,
 		alpnProtocols:                config.NextProtos,
 	}
 	possibleCipherSuites := config.cipherSuites()
@@ -412,6 +413,50 @@ func (hs *clientHandshakeState) processCertsFromServer(certificates [][]byte) er
 	return nil
 }
 
+// processDelegatedCredentialFromServer unmarshals the DelegatedCredential
+// offered by the server (if present) and validates it using the peer
+// certificate.
+func (hs *clientHandshakeState) processDelegatedCredentialFromServer(dc []byte) error {
+	c := hs.c
+
+	var cred *DelegatedCredential
+	var err error
+	if dc != nil {
+
+		// Assert that the DC extension was indicated by the client.
+		if !hs.hello.delegatedCredential {
+			c.sendAlert(alertUnexpectedMessage)
+			return errors.New("tls: got delegated credential extension without indication")
+		}
+
+		// Assert that the DC was sent in the ServerHello in (and only in)
+		// version 1.2.
+		if hs.serverHello.delegatedCredential != nil && hs.serverHello.vers != VersionTLS12 {
+			c.sendAlert(alertIllegalParameter)
+			return errors.New("tls: ServerHello with delegated credential extension in TLS != 1.2")
+		}
+
+		cred, err = UnmarshalDelegatedCredential(dc)
+		if err != nil {
+			c.sendAlert(alertDecodeError)
+			return fmt.Errorf("tls: delegated credential: %s", err)
+		}
+	}
+
+	if cred != nil && !c.config.InsecureSkipVerify {
+		if v, err := cred.Validate(c.peerCertificates[0], hs.c.vers, c.config.time()); err != nil {
+			c.sendAlert(alertIllegalParameter)
+			return fmt.Errorf("delegated credential: %s", err)
+		} else if !v {
+			c.sendAlert(alertIllegalParameter)
+			return errors.New("delegated credential: signature invalid")
+		}
+	}
+
+	c.verifiedDc = cred
+	return nil
+}
+
 func (hs *clientHandshakeState) doFullHandshake() error {
 	c := hs.c
 
@@ -432,6 +477,13 @@ func (hs *clientHandshakeState) doFullHandshake() error {
 		if err := hs.processCertsFromServer(certMsg.certificates); err != nil {
 			return err
 		}
+		// Validate the DC if present. The DC is only processed if the
+		// extension was indicated by the ClientHello; otherwise this call will
+		// result in an "illegal_parameter" alert. It also asserts that the DC
+		// was sent in the ServerHello if and only if TLS 1.2 is in use.
+		if err := hs.processDelegatedCredentialFromServer(hs.serverHello.delegatedCredential); err != nil {
+			return err
+		}
 	} else {
 		// This is a renegotiation handshake. We require that the
 		// server's identity (i.e. leaf certificate) is unchanged and
@@ -477,10 +529,20 @@ func (hs *clientHandshakeState) doFullHandshake() error {
 
 	keyAgreement := hs.suite.ka(c.vers)
 
+	// Set the public key used to verify the handshake.
+	pk := c.peerCertificates[0].PublicKey
+
+	// If the delegated credential extension has successfully been negotiated,
+	// then the ServerKeyExchange DelegatedCredential's private key.
+	if c.verifiedDc != nil {
+		pk = c.verifiedDc.PublicKey
+	}
+
 	skx, ok := msg.(*serverKeyExchangeMsg)
 	if ok {
 		hs.finishedHash.Write(skx.marshal())
-		err = keyAgreement.processServerKeyExchange(c.config, hs.hello, hs.serverHello, c.peerCertificates[0], skx)
+
+		err = keyAgreement.processServerKeyExchange(c.config, hs.hello, hs.serverHello, pk, skx)
 		if err != nil {
 			c.sendAlert(alertUnexpectedMessage)
 			return err
@@ -529,7 +591,7 @@ func (hs *clientHandshakeState) doFullHandshake() error {
 		}
 	}
 
-	preMasterSecret, ckx, err := keyAgreement.generateClientKeyExchange(c.config, hs.hello, c.peerCertificates[0])
+	preMasterSecret, ckx, err := keyAgreement.generateClientKeyExchange(c.config, hs.hello, pk)
 	if err != nil {
 		c.sendAlert(alertInternalError)
 		return err

handshake_messages.go

@@ -49,12 +49,14 @@ type clientHelloMsg struct {
 	psks                             []psk
 	pskKeyExchangeModes              []uint8
 	earlyData                        bool
+	delegatedCredential              bool
 }
 
-// Helpers
+// Function used for signature_algorithms and signature_algorithrms_cert
-// Function used for signature_algorithms and signature_algorithrms_cert extensions only
+// extensions only (for more details, see TLS 1.3 draft 28, 4.2.3).
-// (for more details, see TLS 1.3 draft 28, 4.2.3)
+//
-// Function advances data slice and returns it, so that it can be used for further processing
+// It advances data slice and returns it, so that it can be used for further
+// processing
 func marshalExtensionSignatureAlgorithms(extension uint16, data []byte, schemes []SignatureScheme) []byte {
 	algNum := uint16(len(schemes))
 	if algNum == 0 {
@@ -126,7 +128,8 @@ func (m *clientHelloMsg) equal(i interface{}) bool {
 		eqStrings(m.alpnProtocols, m1.alpnProtocols) &&
 		eqKeyShares(m.keyShares, m1.keyShares) &&
 		eqUint16s(m.supportedVersions, m1.supportedVersions) &&
-		m.earlyData == m1.earlyData
+		m.earlyData == m1.earlyData &&
+		m.delegatedCredential == m1.delegatedCredential
 }
 
 func (m *clientHelloMsg) marshal() []byte {
@@ -202,6 +205,9 @@ func (m *clientHelloMsg) marshal() []byte {
 	if m.earlyData {
 		numExtensions++
 	}
+	if m.delegatedCredential {
+		numExtensions++
+	}
 	if numExtensions > 0 {
 		extensionsLength += 4 * numExtensions
 		length += 2 + extensionsLength
@@ -419,6 +425,10 @@ func (m *clientHelloMsg) marshal() []byte {
 		z[1] = byte(extensionEarlyData)
 		z = z[4:]
 	}
+	if m.delegatedCredential {
+		binary.BigEndian.PutUint16(z, extensionDelegatedCredential)
+		z = z[4:]
+	}
 
 	m.raw = x
 
@@ -483,6 +493,7 @@ func (m *clientHelloMsg) unmarshal(data []byte) alert {
 	m.psks = nil
 	m.pskKeyExchangeModes = nil
 	m.earlyData = false
+	m.delegatedCredential = false
 
 	if len(data) == 0 {
 		// ClientHello is optionally followed by extension data
@@ -747,6 +758,9 @@ func (m *clientHelloMsg) unmarshal(data []byte) alert {
 		case extensionEarlyData:
 			// https://tools.ietf.org/html/draft-ietf-tls-tls13-18#section-4.2.8
 			m.earlyData = true
+		case extensionDelegatedCredential:
+			// https://tools.ietf.org/html/draft-ietf-tls-subcerts
+			m.delegatedCredential = true
 		}
 		data = data[length:]
 		bindersOffset += length
@@ -775,6 +789,10 @@ type serverHelloMsg struct {
 	secureRenegotiationSupported bool
 	alpnProtocol                 string
 
+	// TLS 1.2. In TLS 1.3, the DC extension is included in of the end-entity
+	// certificate in the Certificate message.
+	delegatedCredential []byte
+
 	// TLS 1.3
 	keyShare    keyShare
 	psk         bool
@@ -810,6 +828,7 @@ func (m *serverHelloMsg) equal(i interface{}) bool {
 		bytes.Equal(m.secureRenegotiation, m1.secureRenegotiation) &&
 		m.alpnProtocol == m1.alpnProtocol &&
 		m.keyShare.group == m1.keyShare.group &&
+		bytes.Equal(m.delegatedCredential, m1.delegatedCredential) &&
 		bytes.Equal(m.keyShare.data, m1.keyShare.data) &&
 		m.psk == m1.psk &&
 		m.pskIdentity == m1.pskIdentity
@@ -863,6 +882,10 @@ func (m *serverHelloMsg) marshal() []byte {
 		extensionsLength += 2 + sctLen
 		numExtensions++
 	}
+	if dcLen := len(m.delegatedCredential); dcLen > 0 && m.vers == VersionTLS12 {
+		extensionsLength += 4 + dcLen
+		numExtensions++
+	}
 	if m.keyShare.group != 0 {
 		extensionsLength += 4 + len(m.keyShare.data)
 		numExtensions++
@@ -992,7 +1015,13 @@ func (m *serverHelloMsg) marshal() []byte {
 			z = z[len(sct)+2:]
 		}
 	}
-
+	if dcLen := len(m.delegatedCredential); dcLen > 0 && m.vers == VersionTLS12 {
+		binary.BigEndian.PutUint16(z, extensionDelegatedCredential)
+		binary.BigEndian.PutUint16(z[2:], uint16(dcLen))
+		z = z[4:]
+		copy(z, m.delegatedCredential)
+		z = z[dcLen:]
+	}
 	if m.keyShare.group != 0 {
 		z[0] = uint8(extensionKeyShare >> 8)
 		z[1] = uint8(extensionKeyShare)
@@ -1180,6 +1209,11 @@ func (m *serverHelloMsg) unmarshal(data []byte) alert {
 				m.scts = append(m.scts, d[:sctLen])
 				d = d[sctLen:]
 			}
+		case extensionDelegatedCredential:
+			if m.vers != VersionTLS12 {
+				return alertUnexpectedMessage
+			}
+			m.delegatedCredential = data[:length]
 		case extensionKeyShare:
 			d := data[:length]
 
@@ -1425,6 +1459,7 @@ type certificateEntry struct {
 	data                []byte
 	ocspStaple          []byte
 	sctList             [][]byte
+	delegatedCredential []byte
 }
 
 type certificateMsg13 struct {
@@ -1446,6 +1481,7 @@ func (m *certificateMsg13) equal(i interface{}) bool {
 		ok := bytes.Equal(m.certificates[i].data, m1.certificates[i].data)
 		ok = ok && bytes.Equal(m.certificates[i].ocspStaple, m1.certificates[i].ocspStaple)
 		ok = ok && eqByteSlices(m.certificates[i].sctList, m1.certificates[i].sctList)
+		ok = ok && bytes.Equal(m.certificates[i].delegatedCredential, m1.certificates[i].delegatedCredential)
 		if !ok {
 			return false
 		}
@@ -1472,6 +1508,9 @@ func (m *certificateMsg13) marshal() (x []byte) {
 				i += 2 + len(sct)
 			}
 		}
+		if len(cert.delegatedCredential) != 0 {
+			i += 4 + len(cert.delegatedCredential)
+		}
 	}
 
 	length := 3 + 3*len(m.certificates) + i
@@ -1546,6 +1585,15 @@ func (m *certificateMsg13) marshal() (x []byte) {
 			sctLenPos[2] = uint8(sctLen >> 8)
 			sctLenPos[3] = uint8(sctLen)
 		}
+		if len(cert.delegatedCredential) != 0 {
+			binary.BigEndian.PutUint16(z, extensionDelegatedCredential)
+			binary.BigEndian.PutUint16(z[2:], uint16(len(cert.delegatedCredential)))
+			z = z[4:]
+			copy(z, cert.delegatedCredential)
+			z = z[len(cert.delegatedCredential):]
+			extensionLen += 4 + len(cert.delegatedCredential)
+		}
+
 		extLenPos[0] = uint8(extensionLen >> 8)
 		extLenPos[1] = uint8(extensionLen)
 	}
@@ -1651,6 +1699,8 @@ func (m *certificateMsg13) unmarshal(data []byte) alert {
 					m.certificates[i].sctList = append(m.certificates[i].sctList, body[2:2+sctLen])
 					body = body[2+sctLen:]
 				}
+			case extensionDelegatedCredential:
+				m.certificates[i].delegatedCredential = body
 			}
 		}
 	}

handshake_server.go

@@ -30,6 +30,11 @@ type serverHandshakeState struct {
 	clientHello           *clientHelloMsg
 	hello                 *serverHelloMsg
 	cert                  *Certificate
+	privateKey            crypto.PrivateKey
+
+	// A marshalled DelegatedCredential to be sent to the client in the
+	// handshake.
+	delegatedCredential []byte
 
 	// TLS 1.0-1.2 fields
 	ellipticOk      bool
@@ -299,11 +304,48 @@ Curves:
 		c.sendAlert(alertInternalError)
 		return false, err
 	}
-	if hs.clientHello.scts && hs.hello != nil {
+
+	// Set the private key for this handshake to the certificate's secret key.
+	hs.privateKey = hs.cert.PrivateKey
+
+	if hs.clientHello.scts {
 		hs.hello.scts = hs.cert.SignedCertificateTimestamps
 	}
 
-	if priv, ok := hs.cert.PrivateKey.(crypto.Signer); ok {
+	// Set the private key to the DC private key if the client and server are
+	// willing to negotiate the delegated credential extension.
+	//
+	// Check to see if a DelegatedCredential is available and should be used.
+	// If one is available, the session is using TLS >= 1.2, and the client
+	// accepts the delegated credential extension, then set the handshake
+	// private key to the DC private key.
+	if c.config.GetDelegatedCredential != nil && hs.clientHello.delegatedCredential && c.vers >= VersionTLS12 {
+		dc, sk, err := c.config.GetDelegatedCredential(hs.clientHelloInfo(), c.vers)
+		if err != nil {
+			c.sendAlert(alertInternalError)
+			return false, err
+		}
+
+		// Set the handshake private key.
+		if dc != nil {
+			hs.privateKey = sk
+			if dc.Raw == nil {
+				dc.Raw, err = dc.Marshal()
+				if err != nil {
+					c.sendAlert(alertInternalError)
+					return false, err
+				}
+			}
+			hs.delegatedCredential = dc.Raw
+
+			// For TLS 1.2, the DC is an extension to the ServerHello.
+			if c.vers == VersionTLS12 {
+				hs.hello.delegatedCredential = hs.delegatedCredential
+			}
+		}
+	}
+
+	if priv, ok := hs.privateKey.(crypto.Signer); ok {
 		switch priv.Public().(type) {
 		case *ecdsa.PublicKey:
 			hs.ecdsaOk = true
@@ -314,7 +356,7 @@ Curves:
 			return false, fmt.Errorf("tls: unsupported signing key type (%T)", priv.Public())
 		}
 	}
-	if priv, ok := hs.cert.PrivateKey.(crypto.Decrypter); ok {
+	if priv, ok := hs.privateKey.(crypto.Decrypter); ok {
 		switch priv.Public().(type) {
 		case *rsa.PublicKey:
 			hs.rsaDecryptOk = true
@@ -479,7 +521,7 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 	}
 
 	keyAgreement := hs.suite.ka(c.vers)
-	skx, err := keyAgreement.generateServerKeyExchange(c.config, hs.cert, hs.clientHello, hs.hello)
+	skx, err := keyAgreement.generateServerKeyExchange(c.config, hs.privateKey, hs.clientHello, hs.hello)
 	if err != nil {
 		c.sendAlert(alertHandshakeFailure)
 		return err
@@ -572,7 +614,7 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 	}
 	hs.finishedHash.Write(ckx.marshal())
 
-	preMasterSecret, err := keyAgreement.processClientKeyExchange(c.config, hs.cert, ckx, c.vers)
+	preMasterSecret, err := keyAgreement.processClientKeyExchange(c.config, hs.privateKey, ckx, c.vers)
 	if err != nil {
 		if err == errClientKeyExchange {
 			c.sendAlert(alertDecodeError)
@@ -889,6 +931,7 @@ func (hs *serverHandshakeState) clientHelloInfo() *ClientHelloInfo {
 		SupportedVersions:          supportedVersions,
 		Conn:                       hs.c.conn,
 		Offered0RTTData:            hs.clientHello.earlyData,
+		AcceptsDelegatedCredential: hs.clientHello.delegatedCredential,
 		Fingerprint:                pskBinder,
 	}
 

key_agreement.go

@@ -10,7 +10,6 @@ import (
 	"crypto/md5"
 	"crypto/rsa"
 	"crypto/sha1"
-	"crypto/x509"
 	"errors"
 	"io"
 	"math/big"
@@ -25,11 +24,11 @@ var errServerKeyExchange = errors.New("tls: invalid ServerKeyExchange message")
 // encrypts the pre-master secret to the server's public key.
 type rsaKeyAgreement struct{}
 
-func (ka rsaKeyAgreement) generateServerKeyExchange(config *Config, cert *Certificate, clientHello *clientHelloMsg, hello *serverHelloMsg) (*serverKeyExchangeMsg, error) {
+func (ka rsaKeyAgreement) generateServerKeyExchange(config *Config, sk crypto.PrivateKey, clientHello *clientHelloMsg, hello *serverHelloMsg) (*serverKeyExchangeMsg, error) {
 	return nil, nil
 }
 
-func (ka rsaKeyAgreement) processClientKeyExchange(config *Config, cert *Certificate, ckx *clientKeyExchangeMsg, version uint16) ([]byte, error) {
+func (ka rsaKeyAgreement) processClientKeyExchange(config *Config, sk crypto.PrivateKey, ckx *clientKeyExchangeMsg, version uint16) ([]byte, error) {
 	if len(ckx.ciphertext) < 2 {
 		return nil, errClientKeyExchange
 	}
@@ -42,7 +41,7 @@ func (ka rsaKeyAgreement) processClientKeyExchange(config *Config, cert *Certifi
 		}
 		ciphertext = ckx.ciphertext[2:]
 	}
-	priv, ok := cert.PrivateKey.(crypto.Decrypter)
+	priv, ok := sk.(crypto.Decrypter)
 	if !ok {
 		return nil, errors.New("tls: certificate private key does not implement crypto.Decrypter")
 	}
@@ -60,11 +59,11 @@ func (ka rsaKeyAgreement) processClientKeyExchange(config *Config, cert *Certifi
 	return preMasterSecret, nil
 }
 
-func (ka rsaKeyAgreement) processServerKeyExchange(config *Config, clientHello *clientHelloMsg, serverHello *serverHelloMsg, cert *x509.Certificate, skx *serverKeyExchangeMsg) error {
+func (ka rsaKeyAgreement) processServerKeyExchange(config *Config, clientHello *clientHelloMsg, serverHello *serverHelloMsg, pk crypto.PublicKey, skx *serverKeyExchangeMsg) error {
 	return errors.New("tls: unexpected ServerKeyExchange")
 }
 
-func (ka rsaKeyAgreement) generateClientKeyExchange(config *Config, clientHello *clientHelloMsg, cert *x509.Certificate) ([]byte, *clientKeyExchangeMsg, error) {
+func (ka rsaKeyAgreement) generateClientKeyExchange(config *Config, clientHello *clientHelloMsg, pk crypto.PublicKey) ([]byte, *clientKeyExchangeMsg, error) {
 	preMasterSecret := make([]byte, 48)
 	preMasterSecret[0] = byte(clientHello.vers >> 8)
 	preMasterSecret[1] = byte(clientHello.vers)
@@ -73,7 +72,7 @@ func (ka rsaKeyAgreement) generateClientKeyExchange(config *Config, clientHello
 		return nil, nil, err
 	}
 
-	encrypted, err := rsa.EncryptPKCS1v15(config.rand(), cert.PublicKey.(*rsa.PublicKey), preMasterSecret)
+	encrypted, err := rsa.EncryptPKCS1v15(config.rand(), pk.(*rsa.PublicKey), preMasterSecret)
 	if err != nil {
 		return nil, nil, err
 	}
@@ -156,7 +155,7 @@ type ecdheKeyAgreement struct {
 	x, y *big.Int
 }
 
-func (ka *ecdheKeyAgreement) generateServerKeyExchange(config *Config, cert *Certificate, clientHello *clientHelloMsg, hello *serverHelloMsg) (*serverKeyExchangeMsg, error) {
+func (ka *ecdheKeyAgreement) generateServerKeyExchange(config *Config, sk crypto.PrivateKey, clientHello *clientHelloMsg, hello *serverHelloMsg) (*serverKeyExchangeMsg, error) {
 	preferredCurves := config.curvePreferences()
 
 NextCandidate:
@@ -207,7 +206,7 @@ NextCandidate:
 	serverECDHParams[3] = byte(len(ecdhePublic))
 	copy(serverECDHParams[4:], ecdhePublic)
 
-	priv, ok := cert.PrivateKey.(crypto.Signer)
+	priv, ok := sk.(crypto.Signer)
 	if !ok {
 		return nil, errors.New("tls: certificate private key does not implement crypto.Signer")
 	}
@@ -255,7 +254,7 @@ NextCandidate:
 	return skx, nil
 }
 
-func (ka *ecdheKeyAgreement) processClientKeyExchange(config *Config, cert *Certificate, ckx *clientKeyExchangeMsg, version uint16) ([]byte, error) {
+func (ka *ecdheKeyAgreement) processClientKeyExchange(config *Config, sk crypto.PrivateKey, ckx *clientKeyExchangeMsg, version uint16) ([]byte, error) {
 	if len(ckx.ciphertext) == 0 || int(ckx.ciphertext[0]) != len(ckx.ciphertext)-1 {
 		return nil, errClientKeyExchange
 	}
@@ -291,7 +290,7 @@ func (ka *ecdheKeyAgreement) processClientKeyExchange(config *Config, cert *Cert
 	return preMasterSecret, nil
 }
 
-func (ka *ecdheKeyAgreement) processServerKeyExchange(config *Config, clientHello *clientHelloMsg, serverHello *serverHelloMsg, cert *x509.Certificate, skx *serverKeyExchangeMsg) error {
+func (ka *ecdheKeyAgreement) processServerKeyExchange(config *Config, clientHello *clientHelloMsg, serverHello *serverHelloMsg, pk crypto.PublicKey, skx *serverKeyExchangeMsg) error {
 	if len(skx.key) < 4 {
 		return errServerKeyExchange
 	}
@@ -337,7 +336,7 @@ func (ka *ecdheKeyAgreement) processServerKeyExchange(config *Config, clientHell
 			return errServerKeyExchange
 		}
 	}
-	_, sigType, hashFunc, err := pickSignatureAlgorithm(cert.PublicKey, []SignatureScheme{signatureAlgorithm}, clientHello.supportedSignatureAlgorithms, ka.version)
+	_, sigType, hashFunc, err := pickSignatureAlgorithm(pk, []SignatureScheme{signatureAlgorithm}, clientHello.supportedSignatureAlgorithms, ka.version)
 	if err != nil {
 		return err
 	}
@@ -355,10 +354,10 @@ func (ka *ecdheKeyAgreement) processServerKeyExchange(config *Config, clientHell
 	if err != nil {
 		return err
 	}
-	return verifyHandshakeSignature(sigType, cert.PublicKey, hashFunc, digest, sig)
+	return verifyHandshakeSignature(sigType, pk, hashFunc, digest, sig)
 }
 
-func (ka *ecdheKeyAgreement) generateClientKeyExchange(config *Config, clientHello *clientHelloMsg, cert *x509.Certificate) ([]byte, *clientKeyExchangeMsg, error) {
+func (ka *ecdheKeyAgreement) generateClientKeyExchange(config *Config, clientHello *clientHelloMsg, pk crypto.PublicKey) ([]byte, *clientKeyExchangeMsg, error) {
 	if ka.curveid == 0 {
 		return nil, nil, errors.New("tls: missing ServerKeyExchange message")
 	}

subcerts.go

@@ -0,0 +1,521 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package tls
+
+// Delegated credentials for TLS
+// (https://tools.ietf.org/html/draft-ietf-tls-subcerts) is an IETF Internet
+// draft and proposed TLS extension. If the client supports this extension, then
+// the server may use a "delegated credential" as the signing key in the
+// handshake. A delegated credential is a short lived public/secret key pair
+// delegated to the server by an entity trusted by the client. This allows a
+// middlebox to terminate a TLS connection on behalf of the entity; for example,
+// this can be used to delegate TLS termination to a reverse proxy. Credentials
+// can't be revoked; in order to mitigate risk in case the middlebox is
+// compromised, the credential is only valid for a short time (days, hours, or
+// even minutes).
+//
+// BUG(cjpatton) Subcerts: Need to add support for PKCS1, PSS, and EdDSA.
+// Currently delegated credentials only support ECDSA. The delegator must also
+// use an ECDSA key.
+
+import (
+	"bytes"
+	"crypto"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/x509"
+	"crypto/x509/pkix"
+	"encoding/asn1"
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"time"
+)
+
+const (
+	dcMaxTTLSeconds   = 60 * 60 * 24 * 7 // 7 days
+	dcMaxTTL          = time.Duration(dcMaxTTLSeconds * time.Second)
+	dcMaxPublicKeyLen = 1 << 16 // Bytes
+	dcMaxSignatureLen = 1 << 16 // Bytes
+)
+
+var errNoDelegationUsage = errors.New("certificate not authorized for delegation")
+
+// delegationUsageId is the DelegationUsage X.509 extension OID
+//
+// NOTE(cjpatton) This OID is a child of Cloudflare's IANA-assigned OID.
+var delegationUsageId = asn1.ObjectIdentifier{1, 3, 6, 1, 4, 1, 44363, 44}
+
+// CreateDelegationUsagePKIXExtension returns a pkix.Extension that every delegation
+// certificate must have.
+//
+// NOTE(cjpatton) Brendan McMillion suggests adding the delegationUsage
+// extension as a flag `PermitsDelegationUsage` for the `x509.Certificate`
+// structure. But we can't make this change unless tris includes crypto/x509,
+// too. Once we upstream this code, we'll want to do modify x509.Certficate and
+// do away with this function.
+func CreateDelegationUsagePKIXExtension() *pkix.Extension {
+	return &pkix.Extension{
+		Id:       delegationUsageId,
+		Critical: false,
+		Value:    nil,
+	}
+}
+
+// canDelegate returns true if a certificate can be used for delegated
+// credentials.
+func canDelegate(cert *x509.Certificate) bool {
+	// Check that the digitalSignature key usage is set.
+	if (cert.KeyUsage & x509.KeyUsageDigitalSignature) == 0 {
+		return false
+	}
+
+	// Check that the certificate has the DelegationUsage extension and that
+	// it's non-critical (per the spec).
+	for _, extension := range cert.Extensions {
+		if extension.Id.Equal(delegationUsageId) {
+			return true
+		}
+	}
+	return false
+}
+
+// This structure stores the public components of a credential.
+type credential struct {
+	validTime time.Duration
+	publicKey crypto.PublicKey
+	scheme    SignatureScheme
+}
+
+// marshalSubjectPublicKeyInfo returns a DER encoded SubjectPublicKeyInfo structure
+// (as defined in the X.509 standard) for the credential.
+func (cred *credential) marshalSubjectPublicKeyInfo() ([]byte, error) {
+	switch cred.scheme {
+	case ECDSAWithP256AndSHA256,
+		ECDSAWithP384AndSHA384,
+		ECDSAWithP521AndSHA512:
+		serializedPublicKey, err := x509.MarshalPKIXPublicKey(cred.publicKey)
+		if err != nil {
+			return nil, err
+		}
+		return serializedPublicKey, nil
+
+	default:
+		return nil, fmt.Errorf("unsupported signature scheme: 0x%04x", cred.scheme)
+	}
+}
+
+// marshal encodes a credential as per the spec.
+func (cred *credential) marshal() ([]byte, error) {
+	// Write the valid_time field.
+	serialized := make([]byte, 6)
+	binary.BigEndian.PutUint32(serialized, uint32(cred.validTime/time.Second))
+
+	// Encode the public key and assert that the encoding is no longer than 2^16
+	// bytes (per the spect).
+	serializedPublicKey, err := cred.marshalSubjectPublicKeyInfo()
+	if err != nil {
+		return nil, err
+	}
+	if len(serializedPublicKey) > dcMaxPublicKeyLen {
+		return nil, errors.New("public key is too long")
+	}
+
+	// Write the length of the public_key field.
+	binary.BigEndian.PutUint16(serialized[4:], uint16(len(serializedPublicKey)))
+
+	// Write the public key.
+	return append(serialized, serializedPublicKey...), nil
+}
+
+// unmarshalCredential decodes a credential and returns it.
+func unmarshalCredential(serialized []byte) (*credential, error) {
+	// Bytes 0-3 are the validity time field; bytes 4-6 are the length of the
+	// serialized SubjectPublicKeyInfo.
+	if len(serialized) < 6 {
+		return nil, errors.New("credential is too short")
+	}
+
+	// Parse the validity time.
+	validTime := time.Duration(binary.BigEndian.Uint32(serialized)) * time.Second
+
+	// Parse the SubjectPublicKeyInfo.
+	pk, scheme, err := unmarshalSubjectPublicKeyInfo(serialized[6:])
+	if err != nil {
+		return nil, err
+	}
+
+	return &credential{validTime, pk, scheme}, nil
+}
+
+// unmarshalSubjectPublicKeyInfo parses a DER encoded SubjectPublicKeyInfo
+// structure into a public key and its corresponding algorithm.
+func unmarshalSubjectPublicKeyInfo(serialized []byte) (crypto.PublicKey, SignatureScheme, error) {
+	publicKey, err := x509.ParsePKIXPublicKey(serialized)
+	if err != nil {
+		return nil, 0, err
+	}
+
+	switch pk := publicKey.(type) {
+	case *ecdsa.PublicKey:
+		curveName := pk.Curve.Params().Name
+		if curveName == "P-256" {
+			return pk, ECDSAWithP256AndSHA256, nil
+		} else if curveName == "P-384" {
+			return pk, ECDSAWithP384AndSHA384, nil
+		} else if curveName == "P-521" {
+			return pk, ECDSAWithP521AndSHA512, nil
+		} else {
+			return nil, 0, fmt.Errorf("curve %s s not supported", curveName)
+		}
+
+	default:
+		return nil, 0, fmt.Errorf("unsupported delgation key type: %T", pk)
+	}
+}
+
+// getCredentialLen returns the number of bytes comprising the serialized
+// credential that starts at the beginning of the input slice. It returns an
+// error if the input is too short to contain a credential.
+func getCredentialLen(serialized []byte) (int, error) {
+	if len(serialized) < 6 {
+		return 0, errors.New("credential is too short")
+	}
+	// First 4 bytes is the validity time.
+	serialized = serialized[4:]
+
+	// The next 2 bytes are the length of the serialized public key.
+	serializedPublicKeyLen := int(binary.BigEndian.Uint16(serialized))
+	serialized = serialized[2:]
+
+	if len(serialized) < serializedPublicKeyLen {
+		return 0, errors.New("public key of credential is too short")
+	}
+
+	return 6 + serializedPublicKeyLen, nil
+}
+
+// DelegatedCredential stores a credential and its delegation.
+type DelegatedCredential struct {
+	// The serialized form of the credential.
+	Raw []byte
+
+	// The amount of time for which the credential is valid. Specifically, the
+	// the credential expires `ValidTime` seconds after the `notBefore` of the
+	// delegation certificate. The delegator shall not issue delegated
+	// credentials that are valid for more than 7 days from the current time.
+	//
+	// When this data structure is serialized, this value is converted to a
+	// uint32 representing the duration in seconds.
+	ValidTime time.Duration
+
+	// The credential public key.
+	PublicKey crypto.PublicKey
+
+	// The signature scheme associated with the credential public key.
+	publicKeyScheme SignatureScheme
+
+	// The signature scheme used to sign the credential.
+	Scheme SignatureScheme
+
+	// The credential's delegation.
+	Signature []byte
+}
+
+// NewDelegatedCredential creates a new delegated credential using `cert` for
+// delegation. It generates a public/private key pair for the provided signature
+// algorithm (`scheme`), validity interval (defined by `cert.Leaf.notBefore` and
+// `validTime`), and TLS version (`vers`), and signs it using `cert.PrivateKey`.
+func NewDelegatedCredential(cert *Certificate, scheme SignatureScheme, validTime time.Duration, vers uint16) (*DelegatedCredential, crypto.PrivateKey, error) {
+	// The granularity of DC validity is seconds.
+	validTime = validTime.Round(time.Second)
+
+	// Parse the leaf certificate if needed.
+	var err error
+	if cert.Leaf == nil {
+		if len(cert.Certificate[0]) == 0 {
+			return nil, nil, errors.New("missing leaf certificate")
+		}
+		cert.Leaf, err = x509.ParseCertificate(cert.Certificate[0])
+		if err != nil {
+			return nil, nil, err
+		}
+	}
+
+	// Check that the leaf certificate can be used for delegation.
+	if !canDelegate(cert.Leaf) {
+		return nil, nil, errNoDelegationUsage
+	}
+
+	// Extract the delegator signature scheme from the certificate.
+	var delegatorScheme SignatureScheme
+	switch sk := cert.PrivateKey.(type) {
+	case *ecdsa.PrivateKey:
+		// Set scheme.
+		pk := sk.Public().(*ecdsa.PublicKey)
+		curveName := pk.Curve.Params().Name
+		certAlg := cert.Leaf.SignatureAlgorithm
+		if certAlg == x509.ECDSAWithSHA256 && curveName == "P-256" {
+			delegatorScheme = ECDSAWithP256AndSHA256
+		} else if certAlg == x509.ECDSAWithSHA384 && curveName == "P-384" {
+			delegatorScheme = ECDSAWithP384AndSHA384
+		} else if certAlg == x509.ECDSAWithSHA512 && curveName == "P-521" {
+			delegatorScheme = ECDSAWithP521AndSHA512
+		} else {
+			return nil, nil, fmt.Errorf(
+				"using curve %s for %s is not supported",
+				curveName, cert.Leaf.SignatureAlgorithm)
+		}
+
+	default:
+		return nil, nil, fmt.Errorf("unsupported delgation key type: %T", sk)
+	}
+
+	// Generate a new key pair.
+	var sk crypto.PrivateKey
+	var pk crypto.PublicKey
+	switch scheme {
+	case ECDSAWithP256AndSHA256,
+		ECDSAWithP384AndSHA384,
+		ECDSAWithP521AndSHA512:
+		sk, err = ecdsa.GenerateKey(getCurve(scheme), rand.Reader)
+		if err != nil {
+			return nil, nil, err
+		}
+		pk = sk.(*ecdsa.PrivateKey).Public()
+
+	default:
+		return nil, nil, fmt.Errorf("unsupported signature scheme: 0x%04x", scheme)
+	}
+
+	// Prepare the credential for digital signing.
+	hash := getHash(delegatorScheme)
+	cred := &credential{validTime, pk, scheme}
+	in, err := prepareDelegation(hash, cred, cert.Leaf.Raw, delegatorScheme, vers)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	// Sign the credential.
+	var sig []byte
+	switch sk := cert.PrivateKey.(type) {
+	case *ecdsa.PrivateKey:
+		opts := crypto.SignerOpts(hash)
+		sig, err = sk.Sign(rand.Reader, in, opts)
+		if err != nil {
+			return nil, nil, err
+		}
+	default:
+		return nil, nil, fmt.Errorf("unsupported delgation key type: %T", sk)
+	}
+
+	return &DelegatedCredential{
+		ValidTime:       validTime,
+		PublicKey:       pk,
+		publicKeyScheme: scheme,
+		Scheme:          delegatorScheme,
+		Signature:       sig,
+	}, sk, nil
+}
+
+// IsExpired returns true if the credential has expired. The end of the validity
+// interval is defined as the delegator certificate's notBefore field (`start`)
+// plus ValidTime seconds. This function simply checks that the current time
+// (`now`) is before the end of the valdity interval.
+func (dc *DelegatedCredential) IsExpired(start, now time.Time) bool {
+	end := start.Add(dc.ValidTime)
+	return !now.Before(end)
+}
+
+// InvalidTTL returns true if the credential's validity period is longer than the
+// maximum permitted. This is defined by the certificate's notBefore field
+// (`start`) plus the ValidTime, minus the current time (`now`).
+func (dc *DelegatedCredential) InvalidTTL(start, now time.Time) bool {
+	return dc.ValidTime > (now.Sub(start) + dcMaxTTL).Round(time.Second)
+}
+
+// Validate checks that that the signature is valid, that the credential hasn't
+// expired, and that the TTL is valid. It also checks that certificate can be
+// used for delegation.
+func (dc *DelegatedCredential) Validate(cert *x509.Certificate, vers uint16, now time.Time) (bool, error) {
+	// Check that the cert can delegate.
+	if !canDelegate(cert) {
+		return false, errNoDelegationUsage
+	}
+
+	if dc.IsExpired(cert.NotBefore, now) {
+		return false, errors.New("credential has expired")
+	}
+
+	if dc.InvalidTTL(cert.NotBefore, now) {
+		return false, errors.New("credential TTL is invalid")
+	}
+
+	// Prepare the credential for verification.
+	hash := getHash(dc.Scheme)
+	cred := &credential{dc.ValidTime, dc.PublicKey, dc.publicKeyScheme}
+	in, err := prepareDelegation(hash, cred, cert.Raw, dc.Scheme, vers)
+	if err != nil {
+		return false, err
+	}
+
+	// TODO(any) This code overlaps signficantly with verifyHandshakeSignature()
+	// in ../auth.go. This should be refactored.
+	switch dc.Scheme {
+	case ECDSAWithP256AndSHA256,
+		ECDSAWithP384AndSHA384,
+		ECDSAWithP521AndSHA512:
+		pk, ok := cert.PublicKey.(*ecdsa.PublicKey)
+		if !ok {
+			return false, errors.New("expected ECDSA public key")
+		}
+		sig := new(ecdsaSignature)
+		if _, err = asn1.Unmarshal(dc.Signature, sig); err != nil {
+			return false, err
+		}
+		return ecdsa.Verify(pk, in, sig.R, sig.S), nil
+
+	default:
+		return false, fmt.Errorf(
+			"unsupported signature scheme: 0x%04x", dc.Scheme)
+	}
+}
+
+// Marshal encodes a DelegatedCredential structure per the spec. It also sets
+// dc.Raw to the output as a side effect.
+func (dc *DelegatedCredential) Marshal() ([]byte, error) {
+	// The credential.
+	cred := &credential{dc.ValidTime, dc.PublicKey, dc.publicKeyScheme}
+	serialized, err := cred.marshal()
+	if err != nil {
+		return nil, err
+	}
+
+	// The scheme.
+	serializedScheme := make([]byte, 2)
+	binary.BigEndian.PutUint16(serializedScheme, uint16(dc.Scheme))
+	serialized = append(serialized, serializedScheme...)
+
+	// The signature.
+	if len(dc.Signature) > dcMaxSignatureLen {
+		return nil, errors.New("signature is too long")
+	}
+	serializedSignature := make([]byte, 2)
+	binary.BigEndian.PutUint16(serializedSignature, uint16(len(dc.Signature)))
+	serializedSignature = append(serializedSignature, dc.Signature...)
+	serialized = append(serialized, serializedSignature...)
+
+	dc.Raw = serialized
+	return serialized, nil
+}
+
+// UnmarshalDelegatedCredential decodes a DelegatedCredential structure.
+func UnmarshalDelegatedCredential(serialized []byte) (*DelegatedCredential, error) {
+	// Get the length of the serialized credential that begins at the start of
+	// the input slice.
+	serializedCredentialLen, err := getCredentialLen(serialized)
+	if err != nil {
+		return nil, err
+	}
+
+	// Parse the credential.
+	cred, err := unmarshalCredential(serialized[:serializedCredentialLen])
+	if err != nil {
+		return nil, err
+	}
+
+	// Parse the signature scheme.
+	serialized = serialized[serializedCredentialLen:]
+	if len(serialized) < 4 {
+		return nil, errors.New("delegated credential is too short")
+	}
+	scheme := SignatureScheme(binary.BigEndian.Uint16(serialized))
+
+	// Parse the signature length.
+	serialized = serialized[2:]
+	serializedSignatureLen := binary.BigEndian.Uint16(serialized)
+
+	// Prase the signature.
+	serialized = serialized[2:]
+	if len(serialized) < int(serializedSignatureLen) {
+		return nil, errors.New("signature of delegated credential is too short")
+	}
+	sig := serialized[:serializedSignatureLen]
+
+	return &DelegatedCredential{
+		ValidTime:       cred.validTime,
+		PublicKey:       cred.publicKey,
+		publicKeyScheme: cred.scheme,
+		Scheme:          scheme,
+		Signature:       sig,
+	}, nil
+}
+
+// getCurve maps the SignatureScheme to its corresponding elliptic.Curve.
+func getCurve(scheme SignatureScheme) elliptic.Curve {
+	switch scheme {
+	case ECDSAWithP256AndSHA256:
+		return elliptic.P256()
+	case ECDSAWithP384AndSHA384:
+		return elliptic.P384()
+	case ECDSAWithP521AndSHA512:
+		return elliptic.P521()
+	default:
+		return nil
+	}
+}
+
+// getHash maps the SignatureScheme to its corresponding hash function.
+//
+// TODO(any) This function overlaps with hashForSignatureScheme in 13.go.
+func getHash(scheme SignatureScheme) crypto.Hash {
+	switch scheme {
+	case ECDSAWithP256AndSHA256:
+		return crypto.SHA256
+	case ECDSAWithP384AndSHA384:
+		return crypto.SHA384
+	case ECDSAWithP521AndSHA512:
+		return crypto.SHA512
+	default:
+		return 0 // Unknown hash function
+	}
+}
+
+// prepareDelegation returns a hash of the message that the delegator is to
+// sign. The inputs are the credential (cred), the DER-encoded delegator
+// certificate (`delegatorCert`), the signature scheme of the delegator
+// (`delegatorScheme`), and the protocol version (`vers`) in which the credential
+// is to be used.
+func prepareDelegation(hash crypto.Hash, cred *credential, delegatorCert []byte, delegatorScheme SignatureScheme, vers uint16) ([]byte, error) {
+	h := hash.New()
+
+	// The header.
+	h.Write(bytes.Repeat([]byte{0x20}, 64))
+	h.Write([]byte("TLS, server delegated credentials"))
+	h.Write([]byte{0x00})
+
+	// The protocol version.
+	var serializedVers [2]byte
+	binary.BigEndian.PutUint16(serializedVers[:], uint16(vers))
+	h.Write(serializedVers[:])
+
+	// The delegation certificate.
+	h.Write(delegatorCert)
+
+	// The delegator signature scheme.
+	var serializedScheme [2]byte
+	binary.BigEndian.PutUint16(serializedScheme[:], uint16(delegatorScheme))
+	h.Write(serializedScheme[:])
+
+	// The credential.
+	serializedCred, err := cred.marshal()
+	if err != nil {
+		return nil, err
+	}
+	h.Write(serializedCred)
+
+	return h.Sum(nil), nil
+}

subcerts_test.go

@@ -0,0 +1,500 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package tls
+
+import (
+	"bytes"
+	"crypto"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/x509"
+	"errors"
+	"fmt"
+	"testing"
+	"time"
+)
+
+// dcWithPrivateKey stores a delegated credential and its corresponding private
+// key.
+type dcWithPrivateKey struct {
+	*DelegatedCredential
+	privateKey crypto.PrivateKey
+}
+
+// These test keys were generated with the following program, available in the
+// crypto/tls directory:
+//
+//		go run generate_cert.go -ecdsa-curve P256 -host 127.0.0.1 -dc
+//
+// To get a certificate without the DelegationUsage extension, remove the `-dc`
+// parameter.
+var delegatorCertPEM = `-----BEGIN CERTIFICATE-----
+MIIBdzCCAR2gAwIBAgIQLVIvEpo0/0TzRja4ImvB1TAKBggqhkjOPQQDAjASMRAw
+DgYDVQQKEwdBY21lIENvMB4XDTE4MDcwMzE2NTE1M1oXDTE5MDcwMzE2NTE1M1ow
+EjEQMA4GA1UEChMHQWNtZSBDbzBZMBMGByqGSM49AgEGCCqGSM49AwEHA0IABOhB
+U6adaAgliLaFc1PAo9HBO4Wish1G4df3IK5EXLy+ooYfmkfzT1FxqbNLZufNYzve
+25fmpal/1VJAjpVyKq2jVTBTMA4GA1UdDwEB/wQEAwIFoDATBgNVHSUEDDAKBggr
+BgEFBQcDATAMBgNVHRMBAf8EAjAAMA8GA1UdEQQIMAaHBH8AAAEwDQYJKwYBBAGC
+2kssBAAwCgYIKoZIzj0EAwIDSAAwRQIhAPNwRk6cygm6zO5rjOzohKYWS+1KuWCM
+OetDIvU4mdyoAiAGN97y3GJccYn9ZOJS4UOqhr9oO8PuZMLgdq4OrMRiiA==
+-----END CERTIFICATE-----
+`
+
+var delegatorKeyPEM = `-----BEGIN EC PRIVATE KEY-----
+MHcCAQEEIJDVlo+sJolMcNjMkfCGDUjMJcE4UgclcXGCrOtbJAi2oAoGCCqGSM49
+AwEHoUQDQgAE6EFTpp1oCCWItoVzU8Cj0cE7haKyHUbh1/cgrkRcvL6ihh+aR/NP
+UXGps0tm581jO97bl+alqX/VUkCOlXIqrQ==
+-----END EC PRIVATE KEY-----
+`
+
+var nonDelegatorCertPEM = `-----BEGIN CERTIFICATE-----
+MIIBaTCCAQ6gAwIBAgIQSUo+9uaip3qCW+1EPeHZgDAKBggqhkjOPQQDAjASMRAw
+DgYDVQQKEwdBY21lIENvMB4XDTE4MDYxMjIzNDAyNloXDTE5MDYxMjIzNDAyNlow
+EjEQMA4GA1UEChMHQWNtZSBDbzBZMBMGByqGSM49AgEGCCqGSM49AwEHA0IABLf7
+fiznPVdc3V5mM3ymswU2/IoJaq/deA6dgdj50ozdYyRiAPjxzcz9zRsZw1apTF/h
+yNfiLhV4EE1VrwXcT5OjRjBEMA4GA1UdDwEB/wQEAwIFoDATBgNVHSUEDDAKBggr
+BgEFBQcDATAMBgNVHRMBAf8EAjAAMA8GA1UdEQQIMAaHBH8AAAEwCgYIKoZIzj0E
+AwIDSQAwRgIhANXG0zmrVtQBK0TNZZoEGMOtSwxmiZzXNe+IjdpxO3TiAiEA5VYx
+0CWJq5zqpVXbJMeKVMASo2nrXZoA6NhJvFQ97hw=
+-----END CERTIFICATE-----
+`
+
+var nonDelegatorKeyPEM = `-----BEGIN EC PRIVATE KEY-----
+MHcCAQEEIMw9DiOfGI1E/XZrrW2huZSjYi0EKwvVjAe+dYtyFsSloAoGCCqGSM49
+AwEHoUQDQgAEt/t+LOc9V1zdXmYzfKazBTb8iglqr914Dp2B2PnSjN1jJGIA+PHN
+zP3NGxnDVqlMX+HI1+IuFXgQTVWvBdxPkw==
+-----END EC PRIVATE KEY-----
+`
+
+// Invalid TLS versions used for testing purposes.
+const (
+	versionInvalidDC     uint16 = 0xff00
+	versionMalformedDC12 uint16 = 0xff12
+	versionMalformedDC13 uint16 = 0xff13
+)
+
+var dcTestConfig *Config
+var dcTestCerts map[string]*Certificate
+var dcTestDCs map[uint16]dcWithPrivateKey
+var dcNow time.Time
+var dcTestDCScheme = ECDSAWithP521AndSHA512
+var dcTestDCVersions = []uint16{
+	VersionTLS12,
+	VersionTLS13,
+	VersionTLS13Draft23,
+	versionInvalidDC,
+}
+
+func init() {
+
+	// Use a static time for testing at whcih time the test certificates are
+	// valid.
+	dcNow = time.Date(2018, 07, 03, 18, 0, 0, 234234, time.UTC)
+
+	dcTestConfig = &Config{
+		Time: func() time.Time {
+			return dcNow
+		},
+		Rand:         zeroSource{},
+		Certificates: nil,
+		MinVersion:   VersionTLS10,
+		MaxVersion:   VersionTLS13Draft22,
+		CipherSuites: allCipherSuites(),
+	}
+
+	// The certificates of the server.
+	dcTestCerts = make(map[string]*Certificate)
+	var err error
+
+	// The delegation certificate.
+	dcCert := new(Certificate)
+	*dcCert, err = X509KeyPair([]byte(delegatorCertPEM), []byte(delegatorKeyPEM))
+	if err != nil {
+		panic(err)
+	}
+	dcCert.Leaf, err = x509.ParseCertificate(dcCert.Certificate[0])
+	if err != nil {
+		panic(err)
+	}
+	dcTestCerts["dc"] = dcCert
+
+	// The standard certificate.
+	ndcCert := new(Certificate)
+	*ndcCert, err = X509KeyPair([]byte(nonDelegatorCertPEM), []byte(nonDelegatorKeyPEM))
+	if err != nil {
+		panic(err)
+	}
+	ndcCert.Leaf, err = x509.ParseCertificate(ndcCert.Certificate[0])
+	if err != nil {
+		panic(err)
+	}
+	dcTestCerts["no dc"] = ndcCert
+
+	// The root certificates for the client.
+	dcTestConfig.RootCAs = x509.NewCertPool()
+
+	dcRoot, err := x509.ParseCertificate(dcCert.Certificate[len(dcCert.Certificate)-1])
+	if err != nil {
+		panic(err)
+	}
+	dcTestConfig.RootCAs.AddCert(dcRoot)
+
+	ndcRoot, err := x509.ParseCertificate(ndcCert.Certificate[len(ndcCert.Certificate)-1])
+	if err != nil {
+		panic(err)
+	}
+	dcTestConfig.RootCAs.AddCert(ndcRoot)
+
+	// A pool of DCs.
+	dcTestDCs = make(map[uint16]dcWithPrivateKey)
+	for _, vers := range dcTestDCVersions {
+		dc, sk, err := NewDelegatedCredential(dcCert, dcTestDCScheme, dcNow.Sub(dcCert.Leaf.NotBefore)+dcMaxTTL, vers)
+		if err != nil {
+			panic(err)
+		}
+		dcTestDCs[vers] = dcWithPrivateKey{dc, sk}
+	}
+	// Add two DCs with invalid private keys, one for TLS 1.2 and another for
+	// 1.3.
+	malformedDC12 := new(DelegatedCredential)
+	*malformedDC12 = *dcTestDCs[VersionTLS12].DelegatedCredential
+	dcTestDCs[versionMalformedDC12] = dcWithPrivateKey{
+		malformedDC12,
+		dcTestDCs[versionInvalidDC].privateKey,
+	}
+	malformedDC13 := new(DelegatedCredential)
+	*malformedDC13 = *dcTestDCs[VersionTLS13].DelegatedCredential
+	dcTestDCs[versionMalformedDC13] = dcWithPrivateKey{
+		malformedDC13,
+		dcTestDCs[versionInvalidDC].privateKey,
+	}
+}
+
+func checkECDSAPublicKeysEqual(
+	publicKey, publicKey2 crypto.PublicKey, scheme SignatureScheme) error {
+
+	curve := getCurve(scheme)
+	pk := publicKey.(*ecdsa.PublicKey)
+	pk2 := publicKey2.(*ecdsa.PublicKey)
+	serializedPublicKey := elliptic.Marshal(curve, pk.X, pk.Y)
+	serializedPublicKey2 := elliptic.Marshal(curve, pk2.X, pk2.Y)
+	if !bytes.Equal(serializedPublicKey2, serializedPublicKey) {
+		return errors.New("PublicKey mismatch")
+	}
+
+	return nil
+}
+
+// Test that cred and cred2 are equal.
+func checkCredentialsEqual(dc, dc2 *DelegatedCredential) error {
+	if dc2.ValidTime != dc.ValidTime {
+		return fmt.Errorf("ValidTime mismatch: got %d; want %d", dc2.ValidTime, dc.ValidTime)
+	}
+	if dc2.publicKeyScheme != dc.publicKeyScheme {
+		return fmt.Errorf("scheme mismatch: got %04x; want %04x", dc2.publicKeyScheme, dc.publicKeyScheme)
+	}
+
+	return checkECDSAPublicKeysEqual(dc.PublicKey, dc2.PublicKey, dc.publicKeyScheme)
+}
+
+// Test delegation and validation of credentials.
+func TestDelegateValidate(t *testing.T) {
+	ver := uint16(VersionTLS12)
+	cert := dcTestCerts["dc"]
+
+	validTime := dcNow.Sub(cert.Leaf.NotBefore) + dcMaxTTL
+	shortValidTime := dcNow.Sub(cert.Leaf.NotBefore) + time.Second
+
+	delegatedCred, _, err := NewDelegatedCredential(cert, ECDSAWithP256AndSHA256, validTime, ver)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Test validation of good DC.
+	if v, err := delegatedCred.Validate(cert.Leaf, ver, dcNow); err != nil {
+		t.Error(err)
+	} else if !v {
+		t.Error("good DC is invalid; want valid")
+	}
+
+	// Test validation of expired DC.
+	tooLate := dcNow.Add(dcMaxTTL).Add(time.Nanosecond)
+	if v, err := delegatedCred.Validate(cert.Leaf, ver, tooLate); err == nil {
+		t.Error("expired DC validation succeeded; want failure")
+	} else if v {
+		t.Error("expired DC is valid; want invalid")
+	}
+
+	// Test protocol binding.
+	if v, err := delegatedCred.Validate(cert.Leaf, VersionSSL30, dcNow); err != nil {
+		t.Fatal(err)
+	} else if v {
+		t.Error("DC with wrong version is valid; want invalid")
+	}
+
+	// Test signature algorithm binding.
+	delegatedCred.Scheme = ECDSAWithP521AndSHA512
+	if v, err := delegatedCred.Validate(cert.Leaf, ver, dcNow); err != nil {
+		t.Fatal(err)
+	} else if v {
+		t.Error("DC with wrong scheme is valid; want invalid")
+	}
+	delegatedCred.Scheme = ECDSAWithP256AndSHA256
+
+	// Test delegation cedrtificate binding.
+	cert.Leaf.Raw[0] ^= byte(42)
+	if v, err := delegatedCred.Validate(cert.Leaf, ver, dcNow); err != nil {
+		t.Fatal(err)
+	} else if v {
+		t.Error("DC with wrong cert is valid; want invalid")
+	}
+	cert.Leaf.Raw[0] ^= byte(42)
+
+	// Test validation of DC who's TTL is too long.
+	delegatedCred2, _, err := NewDelegatedCredential(cert, ECDSAWithP256AndSHA256, validTime+time.Second, ver)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if v, err := delegatedCred2.Validate(cert.Leaf, ver, dcNow); err == nil {
+		t.Error("DC validation with long TTL succeeded; want failure")
+	} else if v {
+		t.Error("DC with long TTL is valid; want invalid")
+	}
+
+	// Test validation of DC who's TTL is short.
+	delegatedCred3, _, err := NewDelegatedCredential(cert, ECDSAWithP256AndSHA256, shortValidTime, ver)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if v, err := delegatedCred3.Validate(cert.Leaf, ver, dcNow); err != nil {
+		t.Error(err)
+	} else if !v {
+		t.Error("good DC is invalid; want valid")
+	}
+
+	// Test validation of DC using a certificate that can't delegate.
+	if v, err := delegatedCred.Validate(
+		dcTestCerts["no dc"].Leaf, ver, dcNow); err != errNoDelegationUsage {
+		t.Error("DC validation with non-delegation cert succeeded; want failure")
+	} else if v {
+		t.Error("DC with non-delegation cert is valid; want invalid")
+	}
+}
+
+// Test encoding/decoding of delegated credentials.
+func TestDelegatedCredentialMarshalUnmarshal(t *testing.T) {
+	cert := dcTestCerts["dc"]
+	delegatedCred, _, err := NewDelegatedCredential(cert,
+		ECDSAWithP256AndSHA256,
+		dcNow.Sub(cert.Leaf.NotBefore)+dcMaxTTL,
+		VersionTLS12)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	serialized, err := delegatedCred.Marshal()
+	if err != nil {
+		t.Error(err)
+	}
+
+	delegatedCred2, err := UnmarshalDelegatedCredential(serialized)
+	if err != nil {
+		t.Error(err)
+	}
+
+	err = checkCredentialsEqual(delegatedCred, delegatedCred2)
+	if err != nil {
+		t.Error(err)
+	}
+
+	if delegatedCred.Scheme != delegatedCred2.Scheme {
+		t.Errorf("scheme mismatch: got %04x; want %04x",
+			delegatedCred2.Scheme, delegatedCred.Scheme)
+	}
+
+	if !bytes.Equal(delegatedCred2.Signature, delegatedCred.Signature) {
+		t.Error("Signature mismatch")
+	}
+}
+
+// Tests the handshake and one round of application data. Returns true if the
+// connection used a DC.
+func testConnWithDC(t *testing.T,
+	clientMsg, serverMsg string,
+	clientConfig, serverConfig *Config) (bool, error) {
+
+	ln := newLocalListener(t)
+	defer ln.Close()
+
+	srvCh := make(chan *Conn, 1)
+	var serr error
+	go func() {
+		sconn, err := ln.Accept()
+		if err != nil {
+			serr = err
+			srvCh <- nil
+			return
+		}
+		srv := Server(sconn, serverConfig)
+		if err := srv.Handshake(); err != nil {
+			serr = fmt.Errorf("handshake: %v", err)
+			srvCh <- nil
+			return
+		}
+		srvCh <- srv
+	}()
+
+	cli, err := Dial("tcp", ln.Addr().String(), clientConfig)
+	if err != nil {
+		return false, err
+	}
+	defer cli.Close()
+
+	srv := <-srvCh
+	if srv == nil {
+		return false, serr
+	}
+
+	bufLen := len(clientMsg)
+	if len(serverMsg) > len(clientMsg) {
+		bufLen = len(serverMsg)
+	}
+	buf := make([]byte, bufLen)
+
+	cli.Write([]byte(clientMsg))
+	n, err := srv.Read(buf)
+	if n != len(clientMsg) || string(buf[:n]) != clientMsg {
+		return false, fmt.Errorf("Server read = %d, buf= %q; want %d, %s", n, buf, len(clientMsg), clientMsg)
+	}
+
+	srv.Write([]byte(serverMsg))
+	n, err = cli.Read(buf)
+	if n != len(serverMsg) || err != nil || string(buf[:n]) != serverMsg {
+		return false, fmt.Errorf("Client read = %d, %v, data %q; want %d, nil, %s", n, err, buf, len(serverMsg), serverMsg)
+	}
+
+	// Return true if the client's conn.dc structure was instantiated.
+	return (cli.verifiedDc != nil), nil
+}
+
+// Checks that the client suppports a version >= 1.2 and accepts delegated
+// credentials. If so, it returns the delegation certificate; otherwise it
+// returns a plain certificate.
+func testServerGetCertificate(ch *ClientHelloInfo) (*Certificate, error) {
+	versOk := false
+	for _, vers := range ch.SupportedVersions {
+		versOk = versOk || (vers >= uint16(VersionTLS12))
+	}
+
+	if versOk && ch.AcceptsDelegatedCredential {
+		return dcTestCerts["dc"], nil
+	}
+	return dcTestCerts["no dc"], nil
+}
+
+// Checks that the ciient supports the signature algorithm supported by the test
+// server, and that the server has a DC for the selected protocol version.
+func testServerGetDC(ch *ClientHelloInfo, vers uint16) (*DelegatedCredential, crypto.PrivateKey, error) {
+	schemeOk := false
+	for _, scheme := range ch.SignatureSchemes {
+		schemeOk = schemeOk || (scheme == dcTestDCScheme)
+	}
+
+	versOk := false
+	for _, testVers := range dcTestDCVersions {
+		versOk = versOk || (vers == testVers)
+	}
+
+	if schemeOk && versOk && ch.AcceptsDelegatedCredential {
+		d := dcTestDCs[vers]
+		return d.DelegatedCredential, d.privateKey, nil
+	}
+	return nil, nil, nil
+}
+
+// Returns a DC signed with a bad version number.
+func testServerGetInvalidDC(ch *ClientHelloInfo, vers uint16) (*DelegatedCredential, crypto.PrivateKey, error) {
+	d := dcTestDCs[versionInvalidDC]
+	return d.DelegatedCredential, d.privateKey, nil
+}
+
+// Returns a DC with the wrong private key.
+func testServerGetMalformedDC(ch *ClientHelloInfo, vers uint16) (*DelegatedCredential, crypto.PrivateKey, error) {
+	if vers == VersionTLS12 {
+		d := dcTestDCs[versionMalformedDC12]
+		return d.DelegatedCredential, d.privateKey, nil
+	} else if vers == VersionTLS13 {
+		d := dcTestDCs[versionMalformedDC13]
+		return d.DelegatedCredential, d.privateKey, nil
+	} else {
+		return nil, nil, fmt.Errorf("testServerGetMalformedDC: unsupported version %x", vers)
+	}
+
+}
+
+var dcTests = []struct {
+	clientDC         bool
+	serverDC         bool
+	clientSkipVerify bool
+	clientMaxVers    uint16
+	serverMaxVers    uint16
+	useMalformedDC   bool
+	useInvalidDC     bool
+	expectSuccess    bool
+	expectDC         bool
+	name             string
+}{
+	{true, true, false, VersionTLS12, VersionTLS12, false, false, true, true, "tls12"},
+	{true, true, false, VersionTLS13, VersionTLS13, false, false, true, true, "tls13"},
+	{true, true, false, VersionTLS12, VersionTLS12, true, false, false, false, "tls12, malformed dc"},
+	{true, true, false, VersionTLS13, VersionTLS13, true, false, false, false, "tls13, malformed dc"},
+	{true, true, true, VersionTLS12, VersionTLS12, false, true, true, true, "tls12, invalid dc, skip verify"},
+	{true, true, true, VersionTLS13, VersionTLS13, false, true, true, true, "tls13, invalid dc, skip verify"},
+	{false, true, false, VersionTLS12, VersionTLS12, false, false, true, false, "client no dc"},
+	{true, false, false, VersionTLS12, VersionTLS12, false, false, true, false, "server no dc"},
+	{true, true, false, VersionTLS11, VersionTLS12, false, false, true, false, "client old"},
+	{true, true, false, VersionTLS12, VersionTLS11, false, false, true, false, "server old"},
+}
+
+// Tests the handshake with the delegated credential extension.
+func TestDCHandshake(t *testing.T) {
+	serverMsg := "hello"
+	clientMsg := "world"
+
+	clientConfig := dcTestConfig.Clone()
+	serverConfig := dcTestConfig.Clone()
+	serverConfig.GetCertificate = testServerGetCertificate
+
+	for i, test := range dcTests {
+		clientConfig.AcceptDelegatedCredential = test.clientDC
+		clientConfig.InsecureSkipVerify = test.clientSkipVerify
+
+		if test.serverDC {
+			if test.useInvalidDC {
+				serverConfig.GetDelegatedCredential = testServerGetInvalidDC
+			} else if test.useMalformedDC {
+				serverConfig.GetDelegatedCredential = testServerGetMalformedDC
+			} else {
+				serverConfig.GetDelegatedCredential = testServerGetDC
+			}
+		} else {
+			serverConfig.GetDelegatedCredential = nil
+		}
+
+		clientConfig.MaxVersion = test.clientMaxVers
+		serverConfig.MaxVersion = test.serverMaxVers
+
+		usedDC, err := testConnWithDC(t, clientMsg, serverMsg, clientConfig, serverConfig)
+		if err != nil && test.expectSuccess {
+			t.Errorf("test #%d (%s) fails: %s", i+1, test.name, err)
+		} else if err == nil && !test.expectSuccess {
+			t.Errorf("test #%d (%s) succeeds; expected failure", i+1, test.name)
+		}
+
+		if usedDC != test.expectDC {
+			t.Errorf("test #%d (%s) usedDC = %v; expected %v", i+1, test.name, usedDC, test.expectDC)
+		}
+	}
+}

tls_test.go

@@ -674,7 +674,7 @@ func TestCloneNonFuncFields(t *testing.T) {
 		switch fn := typ.Field(i).Name; fn {
 		case "Rand":
 			f.Set(reflect.ValueOf(io.Reader(os.Stdin)))
-		case "Time", "GetCertificate", "GetConfigForClient", "VerifyPeerCertificate", "GetClientCertificate":
+		case "Time", "GetCertificate", "GetConfigForClient", "VerifyPeerCertificate", "GetClientCertificate", "GetDelegatedCredential":
 			// DeepEqual can't compare functions. If you add a
 			// function field to this list, you must also change
 			// TestCloneFuncFields to ensure that the func field is
@@ -713,6 +713,8 @@ func TestCloneNonFuncFields(t *testing.T) {
 			f.Set(reflect.ValueOf(uint32(0)))
 		case "SessionTicketSealer":
 			// TODO
+		case "AcceptDelegatedCredential":
+			f.Set(reflect.ValueOf(false))
 		default:
 			t.Errorf("all fields must be accounted for, but saw unknown field %q", fn)
 		}