nts: add NTS-KE server and client

Add a client and server implementing the Network Time Security (NTS) Key
Establishment. Use the GnuTLS library for TLS.

nts_ke_session.h

@@ -0,0 +1,83 @@
+/*
+  chronyd/chronyc - Programs for keeping computer clocks accurate.
+
+ **********************************************************************
+ * Copyright (C) Miroslav Lichvar  2020
+ * 
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of version 2 of the GNU General Public License as
+ * published by the Free Software Foundation.
+ * 
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ * 
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+ * 
+ **********************************************************************
+
+  =======================================================================
+
+  Header file for the NTS-KE session
+  */
+
+#ifndef GOT_NTS_KE_SESSION_H
+#define GOT_NTS_KE_SESSION_H
+
+#include "nts_ke.h"
+#include "siv.h"
+
+typedef struct NKSN_Instance_Record *NKSN_Instance;
+
+/* Handler for received NTS-KE messages.  A non-zero return code stops
+   the session. */
+typedef int (*NKSN_MessageHandler)(void *arg);
+
+/* Get client or server credentials using certificates of trusted CAs,
+   or a server certificate and key.  The credentials may be shared between
+   different clients or servers. */
+extern void *NKSN_CreateCertCredentials(char *cert, char *key, char *trusted_certs);
+
+/* Destroy the credentials */
+extern void NKSN_DestroyCertCredentials(void *credentials);
+
+/* Create an instance */
+extern NKSN_Instance NKSN_CreateInstance(int server_mode, const char *name,
+                                         NKSN_MessageHandler handler, void *handler_arg);
+
+/* Destroy an instance */
+extern void NKSN_DestroyInstance(NKSN_Instance inst);
+
+/* Start a new NTS-KE session */
+extern int NKSN_StartSession(NKSN_Instance inst, int sock_fd, void *credentials,
+                             double timeout);
+
+/* Begin an NTS-KE message.  A request should be made right after starting
+   the session and response should be made in the message handler. */
+extern void NKSN_BeginMessage(NKSN_Instance inst);
+
+/* Add a record to the message */
+extern int NKSN_AddRecord(NKSN_Instance inst, int critical, int type,
+                          const void *body, int body_length);
+
+/* Terminate the message */
+extern int NKSN_EndMessage(NKSN_Instance inst);
+
+/* Get the next record from the received message.  This function should be
+   called from the message handler. */
+extern int NKSN_GetRecord(NKSN_Instance inst, int *critical, int *type, int *body_length,
+                          void *body, int buffer_length);
+
+/* Export NTS keys for a specified algorithm */
+extern int NKSN_GetKeys(NKSN_Instance inst, SIV_Algorithm siv, NKE_Key *c2s, NKE_Key *s2c);
+
+/* Check if the session has stopped */
+extern int NKSN_IsStopped(NKSN_Instance inst);
+
+/* Stop the session */
+extern void NKSN_StopSession(NKSN_Instance inst);
+
+#endif