Refactor the keyAgreement interface

It's sufficient to pass in the *tls.Certificate (resp.
*x509.Certificate) to the server functions (resp. client funcctions),
but not necessary; the existing keyAgreement implementations only makes
use of the private key (resp. public key). Moreover, this change is
necessary for implementing the delegated credentials extension, which
replaces the private key (resp. public key) used in the handshake.

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)
-	processClientKeyExchange(*Config, *Certificate, *clientKeyExchangeMsg, uint16) ([]byte, error)
+	generateServerKeyExchange(*Config, crypto.PrivateKey, *clientHelloMsg, *serverHelloMsg) (*serverKeyExchangeMsg, 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
-	generateClientKeyExchange(*Config, *clientHelloMsg, *x509.Certificate) ([]byte, *clientKeyExchangeMsg, error)
+	processServerKeyExchange(*Config, *clientHelloMsg, *serverHelloMsg, crypto.PublicKey, *serverKeyExchangeMsg) error
+	generateClientKeyExchange(*Config, *clientHelloMsg, crypto.PublicKey) ([]byte, *clientKeyExchangeMsg, error)
 }
 
 const (

handshake_client.go

@@ -480,7 +480,7 @@ 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)
+		err = keyAgreement.processServerKeyExchange(c.config, hs.hello, hs.serverHello, c.peerCertificates[0].PublicKey, skx)
 		if err != nil {
 			c.sendAlert(alertUnexpectedMessage)
 			return err
@@ -529,7 +529,7 @@ 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, c.peerCertificates[0].PublicKey)
 	if err != nil {
 		c.sendAlert(alertInternalError)
 		return err

handshake_server.go

@@ -479,7 +479,7 @@ 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.cert.PrivateKey, hs.clientHello, hs.hello)
 	if err != nil {
 		c.sendAlert(alertHandshakeFailure)
 		return err
@@ -572,7 +572,7 @@ 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.cert.PrivateKey, ckx, c.vers)
 	if err != nil {
 		if err == errClientKeyExchange {
 			c.sendAlert(alertDecodeError)

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")
 	}