tls-server: Add flag that makes client authentication optional

This allows clients to send an empty certificate payload if the server
sent a certificate request.  If an identity was set previously, it will
be reset so get_peer_id() may be used to check if the client was
authenticated.

src/libtls/tls.h

@@ -207,6 +207,8 @@ enum tls_name_type_t {
 enum tls_flag_t {
 	/** set if cipher suites with null encryption are acceptable */
 	TLS_FLAG_ENCRYPTION_OPTIONAL = 1,
+	/** set if client authentication is optional even if cert req sent */
+	TLS_FLAG_CLIENT_AUTH_OPTIONAL = 2,
 };
 
 /**

src/libtls/tls_server.c

@@ -705,9 +705,18 @@ static status_t process_certificate(private_tls_server_t *this,
 	certs = bio_reader_create(data);
 	if (!certs->remaining(certs))
 	{
-		DBG1(DBG_TLS, "no certificate sent by peer");
-		this->alert->add(this->alert, TLS_FATAL, TLS_DECODE_ERROR);
-		return NEED_MORE;
+		if (this->tls->get_flags(this->tls) & TLS_FLAG_CLIENT_AUTH_OPTIONAL)
+		{
+			/* client authentication is not required so we clear the identity */
+			DESTROY_IF(this->peer);
+			this->peer = NULL;
+		}
+		else
+		{
+			DBG1(DBG_TLS, "no certificate sent by peer");
+			this->alert->add(this->alert, TLS_FATAL, TLS_DECODE_ERROR);
+			return NEED_MORE;
+		}
 	}
 	while (certs->remaining(certs))
 	{