crypto/tls: client certificate support.

This changeset implements client certificate support in crypto/tls
for both handshake_server.go and handshake_client.go

The updated server implementation sends an empty CertificateAuthorities
field in the CertificateRequest, thus allowing clients to send any
certificates they wish. Likewise, the client code will only respond
with its certificate when the server requests a certificate with this
field empty.

R=agl, rsc, agl1
CC=golang-dev
https://golang.org/cl/1975042

src/pkg/crypto/tls/handshake_server.go

@@ -18,6 +18,7 @@ import (
 	"crypto/rsa"
 	"crypto/sha1"
 	"crypto/subtle"
+	"crypto/x509"
 	"io"
 	"os"
 )
@@ -112,10 +113,62 @@ func (c *Conn) serverHandshake() os.Error {
 	finishedHash.Write(certMsg.marshal())
 	c.writeRecord(recordTypeHandshake, certMsg.marshal())
 
+	if config.AuthenticateClient {
+		// Request a client certificate
+		certReq := new(certificateRequestMsg)
+		certReq.certificateTypes = []byte{certTypeRSASign}
+		// An empty list of certificateAuthorities signals to
+		// the client that it may send any certificate in response
+		// to our request.
+
+		finishedHash.Write(certReq.marshal())
+		c.writeRecord(recordTypeHandshake, certReq.marshal())
+	}
+
 	helloDone := new(serverHelloDoneMsg)
 	finishedHash.Write(helloDone.marshal())
 	c.writeRecord(recordTypeHandshake, helloDone.marshal())
 
+	var pub *rsa.PublicKey
+	if config.AuthenticateClient {
+		// Get client certificate
+		msg, err = c.readHandshake()
+		if err != nil {
+			return err
+		}
+		certMsg, ok = msg.(*certificateMsg)
+		if !ok {
+			return c.sendAlert(alertUnexpectedMessage)
+		}
+		finishedHash.Write(certMsg.marshal())
+
+		certs := make([]*x509.Certificate, len(certMsg.certificates))
+		for i, asn1Data := range certMsg.certificates {
+			cert, err := x509.ParseCertificate(asn1Data)
+			if err != nil {
+				return c.sendAlert(alertBadCertificate)
+			}
+			certs[i] = cert
+		}
+
+		// TODO(agl): do better validation of certs: max path length, name restrictions etc.
+		for i := 1; i < len(certs); i++ {
+			if err := certs[i-1].CheckSignatureFrom(certs[i]); err != nil {
+				return c.sendAlert(alertBadCertificate)
+			}
+		}
+
+		if len(certs) > 0 {
+			key, ok := certs[0].PublicKey.(*rsa.PublicKey)
+			if !ok {
+				return c.sendAlert(alertUnsupportedCertificate)
+			}
+			pub = key
+			c.peerCertificates = certs
+		}
+	}
+
+	// Get client key exchange
 	msg, err = c.readHandshake()
 	if err != nil {
 		return err
@@ -126,6 +179,33 @@ func (c *Conn) serverHandshake() os.Error {
 	}
 	finishedHash.Write(ckx.marshal())
 
+	// If we received a client cert in response to our certificate request message,
+	// the client will send us a certificateVerifyMsg immediately after the
+	// clientKeyExchangeMsg.  This message is a MD5SHA1 digest of all preceeding
+	// handshake-layer messages that is signed using the private key corresponding
+	// to the client's certificate. This allows us to verify that the client is in
+	// posession of the private key of the certificate.
+	if len(c.peerCertificates) > 0 {
+		msg, err = c.readHandshake()
+		if err != nil {
+			return err
+		}
+		certVerify, ok := msg.(*certificateVerifyMsg)
+		if !ok {
+			return c.sendAlert(alertUnexpectedMessage)
+		}
+
+		digest := make([]byte, 36)
+		copy(digest[0:16], finishedHash.serverMD5.Sum())
+		copy(digest[16:36], finishedHash.serverSHA1.Sum())
+		err = rsa.VerifyPKCS1v15(pub, rsa.HashMD5SHA1, digest, certVerify.signature)
+		if err != nil {
+			return c.sendAlert(alertBadCertificate)
+		}
+
+		finishedHash.Write(certVerify.marshal())
+	}
+
 	preMasterSecret := make([]byte, 48)
 	_, err = io.ReadFull(config.Rand, preMasterSecret[2:])
 	if err != nil {