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_client.h

@@ -0,0 +1,58 @@
+/*
+  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 client
+  */
+
+#ifndef GOT_NTS_KE_CLIENT_H
+#define GOT_NTS_KE_CLIENT_H
+
+#include "addressing.h"
+#include "nts_ke.h"
+
+typedef struct NKC_Instance_Record *NKC_Instance;
+
+/* Init and fini functions */
+extern void NKC_Initialise(void);
+extern void NKC_Finalise(void);
+
+/* Create a client NTS-KE instance */
+extern NKC_Instance NKC_CreateInstance(IPSockAddr *address, const char *name);
+
+/* Destroy an instance */
+extern void NKC_DestroyInstance(NKC_Instance inst);
+
+/* Connect to the server, start an NTS-KE session, send an NTS-KE request, and
+   process the response (asynchronously) */
+extern int NKC_Start(NKC_Instance inst);
+
+/* Check if the client is still running */
+extern int NKC_IsActive(NKC_Instance inst);
+
+/* Get the NTS data if the session was successful */
+extern int NKC_GetNtsData(NKC_Instance inst,
+                          SIV_Algorithm *siv_algorithm, NKE_Key *c2s, NKE_Key *s2c,
+                          NKE_Cookie *cookies, int *num_cookies, int max_cookies,
+                          IPSockAddr *ntp_address);
+
+#endif