client: Handle the MSG_EXT_INFO packet signalling supported extensions

RFC 8308: The extension negotiation in Secure Shell (SSH) Protocol

RFC 8332: Use of RSA Keys with SHA-256 and SHA-512
          in the Secure Shell (SSH) Protocol

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
(cherry picked from commit 761225712a)

include/libssh/packet.h

@@ -51,6 +51,7 @@ SSH_PACKET_CALLBACK(ssh_packet_ignore_callback);
 SSH_PACKET_CALLBACK(ssh_packet_dh_reply);
 SSH_PACKET_CALLBACK(ssh_packet_newkeys);
 SSH_PACKET_CALLBACK(ssh_packet_service_accept);
+SSH_PACKET_CALLBACK(ssh_packet_ext_info);
 
 #ifdef WITH_SERVER
 SSH_PACKET_CALLBACK(ssh_packet_kexdh_init);

include/libssh/session.h

@@ -86,6 +86,11 @@ enum ssh_pending_call_e {
 #define SSH_OPT_FLAG_KBDINT_AUTH 0x4
 #define SSH_OPT_FLAG_GSSAPI_AUTH 0x8
 
+/* extensions flags */
+/* server-sig-algs extension */
+#define SSH_EXT_SIG_RSA_SHA256  0x01
+#define SSH_EXT_SIG_RSA_SHA512  0x02
+
 /* members that are common to ssh_session and ssh_bind */
 struct ssh_common_struct {
     struct error_struct error;
@@ -114,6 +119,9 @@ struct ssh_session_struct {
     /* session flags (SSH_SESSION_FLAG_*) */
     int flags;
 
+    /* Extensions negotiated using RFC 8308 */
+    uint32_t extensions;
+
     ssh_string banner; /* that's the issue banner from
                        the server */
     char *discon_msg; /* disconnect message from

include/libssh/ssh2.h

@@ -7,6 +7,7 @@
 #define SSH2_MSG_DEBUG	4
 #define SSH2_MSG_SERVICE_REQUEST	5
 #define SSH2_MSG_SERVICE_ACCEPT 6
+#define SSH2_MSG_EXT_INFO 7
 
 #define SSH2_MSG_KEXINIT	 20
 #define SSH2_MSG_NEWKEYS 21