packet_crypt: Move secure_memcmp() to a shared source

Move the secure_memcmp() function to a shared source to make it
available internally for other crypto implementations.

Signed-off-by: Anderson Toshiyuki Sasaki <ansasaki@redhat.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>

include/libssh/crypto.h

@@ -213,4 +213,6 @@ int sshkdf_derive_key(struct ssh_crypto_struct *crypto,
                       int key_type, unsigned char *output,
                       size_t requested_len);
 
+int secure_memcmp(const void *s1, const void *s2, size_t n);
+
 #endif /* _CRYPTO_H_ */

src/CMakeLists.txt

@@ -112,6 +112,7 @@ set(libssh_SRCS
   config.c
   connect.c
   connector.c
+  crypto_common.c
   curve25519.c
   dh.c
   ecdh.c

src/crypto_common.c

@@ -0,0 +1,34 @@
+/*
+ * This file is part of the SSH Library
+ *
+ * Copyright (c) 2020 by Anderson Toshiyuki Sasaki - Red Hat, Inc.
+ *
+ * This 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.
+ *
+ * This 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 this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include "config.h"
+#include "libssh/crypto.h"
+
+int secure_memcmp(const void *s1, const void *s2, size_t n)
+{
+    int rc = 0;
+    const unsigned char *p1 = s1;
+    const unsigned char *p2 = s2;
+    for (; n > 0; --n) {
+        rc |= *p1++ ^ *p2++;
+    }
+    return (rc != 0);
+}
+

src/packet_crypt.c

@@ -216,17 +216,6 @@ unsigned char *ssh_packet_encrypt(ssh_session session, void *data, uint32_t len)
   return crypto->hmacbuf;
 }
 
-static int secure_memcmp(const void *s1, const void *s2, size_t n)
-{
-    int rc = 0;
-    const unsigned char *p1 = s1;
-    const unsigned char *p2 = s2;
-    for (; n > 0; --n) {
-        rc |= *p1++ ^ *p2++;
-    }
-    return (rc != 0);
-}
-
 /**
  * @internal
  *