crypto/tls: add LocalCertificate to ConnectionState

Adds a new field to ConnectionState, LocalCertificate, which contains
the certificate chain which was presented to the connection peer, if one
was.

Fixes #24673

Change-Id: Iae758fef4a2808e3295324890f4b2f55c71fcc4c
Reviewed-on: https://go-review.googlesource.com/c/go/+/788866
Auto-Submit: Roland Shoemaker <roland@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Bypass: Roland Shoemaker <roland@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>

common.go

@@ -325,6 +325,11 @@ type ConnectionState struct {
 	// are a server, or if we received a HelloRetryRequest if we are a client.
 	HelloRetryRequest bool
 
+	// LocalCertificate is the certificate chain presented to the peer, if any,
+	// during the handshake. This field is only populated for connections which
+	// are not resumed (DidResume is false).
+	LocalCertificate [][]byte
+
 	// ekm is a closure exposed via ExportKeyingMaterial.
 	ekm func(label string, context []byte, length int) ([]byte, error)
 

conn.go

@@ -60,6 +60,7 @@ type Conn struct {
 	ocspResponse     []byte   // stapled OCSP response
 	scts             [][]byte // signed certificate timestamps from server
 	peerCertificates []*x509.Certificate
+	localCertificate [][]byte
 	// verifiedChains contains the certificate chains that we built, as
 	// opposed to the ones presented by the server.
 	verifiedChains [][]*x509.Certificate
@@ -1690,6 +1691,7 @@ func (c *Conn) connectionStateLocked() ConnectionState {
 	state.ServerName = c.serverName
 	state.CipherSuite = c.cipherSuite
 	state.PeerCertificates = c.peerCertificates
+	state.LocalCertificate = c.localCertificate
 	state.VerifiedChains = c.verifiedChains
 	state.SignedCertificateTimestamps = c.scts
 	state.OCSPResponse = c.ocspResponse

handshake_client.go

@@ -730,6 +730,10 @@ func (hs *clientHandshakeState) doFullHandshake() error {
 		}
 	}
 
+	if chainToSend != nil {
+		hs.c.localCertificate = chainToSend.Certificate
+	}
+
 	shd, ok := msg.(*serverHelloDoneMsg)
 	if !ok {
 		c.sendAlert(alertUnexpectedMessage)

handshake_client_tls13.go

@@ -756,6 +756,10 @@ func (hs *clientHandshakeStateTLS13) sendClientCertificate() error {
 		return err
 	}
 
+	if cert != nil {
+		hs.c.localCertificate = cert.Certificate
+	}
+
 	certMsg := new(certificateMsgTLS13)
 
 	certMsg.certificate = *cert

handshake_server.go

@@ -280,6 +280,11 @@ func (hs *serverHandshakeState) processClientHello() error {
 		}
 		return err
 	}
+
+	if hs.cert != nil {
+		hs.c.localCertificate = hs.cert.Certificate
+	}
+
 	if hs.clientHello.scts {
 		hs.hello.scts = hs.cert.SignedCertificateTimestamps
 	}

handshake_server_tls13.go

@@ -587,6 +587,9 @@ func (hs *serverHandshakeStateTLS13) pickCertificate() error {
 		}
 		return err
 	}
+	if certificate != nil {
+		hs.c.localCertificate = certificate.Certificate
+	}
 	hs.sigAlg, err = selectSignatureScheme(c.vers, certificate, hs.clientHello.supportedSignatureAlgorithms)
 	if err != nil {
 		// getCertificate returned a certificate that is unsupported or