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_server.h

@@ -0,0 +1,42 @@
+/*
+  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 server
+  */
+
+#ifndef GOT_NTS_KE_SERVER_H
+#define GOT_NTS_KE_SERVER_H
+
+#include "nts_ke.h"
+
+/* Init and fini functions */
+extern void NKS_Initialise(int scfilter_level);
+extern void NKS_Finalise(void);
+
+/* Generate a new NTS cookie containing the C2S and S2C keys */
+extern int NKS_GenerateCookie(NKE_Key *c2s, NKE_Key *s2c, NKE_Cookie *cookie);
+
+/* Validate a cookie and extract the C2S and S2C keys */
+extern int NKS_DecodeCookie(NKE_Cookie *cookie, NKE_Key *c2s, NKE_Key *s2c);
+
+#endif