crypto/tls: don't assume an RSA private key in the API.

We still very much assume it in the code, but with this change in
place we can implement other things later without changing and users
of the package.

Fixes #2319.

R=golang-dev, bradfitz, r
CC=golang-dev
https://golang.org/cl/5489073

common.go

@@ -5,8 +5,8 @@
 package tls
 
 import (
+	"crypto"
 	"crypto/rand"
-	"crypto/rsa"
 	"crypto/x509"
 	"io"
 	"strings"
@@ -255,7 +255,7 @@ func (c *Config) BuildNameToCertificate() {
 // A Certificate is a chain of one or more certificates, leaf first.
 type Certificate struct {
 	Certificate [][]byte
-	PrivateKey  *rsa.PrivateKey
+	PrivateKey  crypto.PrivateKey // supported types: *rsa.PrivateKey
 	// OCSPStaple contains an optional OCSP response which will be served
 	// to clients that request it.
 	OCSPStaple []byte

handshake_client.go

@@ -234,7 +234,7 @@ func (c *Conn) clientHandshake() error {
 		digest := make([]byte, 0, 36)
 		digest = finishedHash.serverMD5.Sum(digest)
 		digest = finishedHash.serverSHA1.Sum(digest)
-		signed, err := rsa.SignPKCS1v15(c.config.rand(), c.config.Certificates[0].PrivateKey, crypto.MD5SHA1, digest)
+		signed, err := rsa.SignPKCS1v15(c.config.rand(), c.config.Certificates[0].PrivateKey.(*rsa.PrivateKey), crypto.MD5SHA1, digest)
 		if err != nil {
 			return c.sendAlert(alertInternalError)
 		}

key_agreement.go

@@ -44,7 +44,7 @@ func (ka rsaKeyAgreement) processClientKeyExchange(config *Config, ckx *clientKe
 		ciphertext = ckx.ciphertext[2:]
 	}
 
-	err = rsa.DecryptPKCS1v15SessionKey(config.rand(), config.Certificates[0].PrivateKey, ciphertext, preMasterSecret)
+	err = rsa.DecryptPKCS1v15SessionKey(config.rand(), config.Certificates[0].PrivateKey.(*rsa.PrivateKey), ciphertext, preMasterSecret)
 	if err != nil {
 		return nil, err
 	}
@@ -147,7 +147,7 @@ Curve:
 	copy(serverECDHParams[4:], ecdhePublic)
 
 	md5sha1 := md5SHA1Hash(clientHello.random, hello.random, serverECDHParams)
-	sig, err := rsa.SignPKCS1v15(config.rand(), config.Certificates[0].PrivateKey, crypto.MD5SHA1, md5sha1)
+	sig, err := rsa.SignPKCS1v15(config.rand(), config.Certificates[0].PrivateKey.(*rsa.PrivateKey), crypto.MD5SHA1, md5sha1)
 	if err != nil {
 		return nil, errors.New("failed to sign ECDHE parameters: " + err.Error())
 	}