dh-gex: Add client implementation

Signed-off-by: Aris Adamantiadis <aris@0xbadc0de.be>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

include/libssh/crypto.h

@@ -58,6 +58,9 @@ enum ssh_key_exchange_e {
   SSH_KEX_DH_GROUP1_SHA1=1,
   /* diffie-hellman-group14-sha1 */
   SSH_KEX_DH_GROUP14_SHA1,
+  /* diffie-hellman-group-exchange-sha1 */
+  SSH_KEX_DH_GEX_SHA1,
+  SSH_KEX_DH_GEX_SHA256,
   /* ecdh-sha2-nistp256 */
   SSH_KEX_ECDH_SHA2_NISTP256,
   /* ecdh-sha2-nistp384 */

include/libssh/dh-gex.h

@@ -0,0 +1,37 @@
+/*
+ * This file is part of the SSH Library
+ *
+ * Copyright (c) 2016 by Aris Adamantiadis <aris@0xbadc0de.be>
+ *
+ * The SSH Library is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation; either version 2.1 of the License, or (at your
+ * option) any later version.
+ *
+ * The SSH Library 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 Lesser General Public
+ * License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with the SSH Library; see the file COPYING.  If not, write to
+ * the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
+ * MA 02111-1307, USA.
+ */
+
+
+#ifndef SRC_DH_GEX_H_
+#define SRC_DH_GEX_H_
+
+/* Minimum, recommanded and maximum size of DH group */
+#define DH_PMIN 2048
+#define DH_PREQ 2048
+#define DH_PMAX 8192
+
+int ssh_client_dhgex_init(ssh_session session);
+
+#ifdef WITH_SERVER
+void ssh_server_dhgex_init(ssh_session session);
+#endif /* WITH_SERVER */
+
+#endif /* SRC_DH_GEX_H_ */

include/libssh/session.h

@@ -49,6 +49,7 @@ enum ssh_session_state_e {
 
 enum ssh_dh_state_e {
   DH_STATE_INIT=0,
+  DH_STATE_REQUEST_SENT,
   DH_STATE_INIT_SENT,
   DH_STATE_NEWKEYS_SENT,
   DH_STATE_FINISHED