Try to build a trustchain for all configured certificates before enforcing one

This enables the daemon to select from multiple configured certificates
by building trustchains against the received certificate requests.

src/libstrongswan/credentials/credential_manager.c

@@ -1049,6 +1049,7 @@ METHOD(credential_manager_t, get_private, private_key_t*,
 	certificate_t *cert;
 	private_key_t *private = NULL;
 	auth_cfg_t *trustchain;
+	auth_rule_t rule;
 
 	/* check if this is a lookup by key ID, and do it if so */
 	if (id && id->get_type(id) == ID_KEY_ID)
@@ -1062,7 +1063,34 @@ METHOD(credential_manager_t, get_private, private_key_t*,
 
 	if (auth)
 	{
-		/* if a specific certificate is preferred, check for a matching key */
+		/* try to find a trustchain with one of the configured subject certs */
+		enumerator = auth->create_enumerator(auth);
+		while (enumerator->enumerate(enumerator, &rule, &cert))
+		{
+			if (rule == AUTH_RULE_SUBJECT_CERT)
+			{
+				private = get_private_by_cert(this, cert, type);
+				if (private)
+				{
+					trustchain = build_trustchain(this, cert, auth);
+					if (trustchain)
+					{
+						auth->merge(auth, trustchain, FALSE);
+						trustchain->destroy(trustchain);
+						break;
+					}
+					private->destroy(private);
+					private = NULL;
+				}
+			}
+		}
+		enumerator->destroy(enumerator);
+		if (private)
+		{
+			return private;
+		}
+
+		/* if none yielded a trustchain, enforce the first configured cert */
 		cert = auth->get(auth, AUTH_RULE_SUBJECT_CERT);
 		if (cert)
 		{