Provide a RNG for libgfshare

The gfshare_ctx_free function of Libgfshare takes care of
randomizing the memory contents of its context before freeing it.
In order to do that, the gfshare_fill_rand global variable must
be initialized with a randomizing function.

README.md

@@ -45,7 +45,7 @@ Gfsecret depends on the following libraries at compile-time:
 - [libgfshare](http://www.digital-scurf.org/software/libgfshare), which
   implements the secret sharing scheme proper (mandatory);
 - [libgcrypt](http://www.gnupg.org/), for the SHA-256 implementation
-  (mandatory);
+  and random number generation (mandatory);
 - [GIO](http://developer.gnome.org/gio/), to access shares stored on
   external volumes (typically USB storage) (optional);
 - [libmtp](http://libmtp.sourceforge.net/), to access shares stored on

src/secret.c

@@ -22,6 +22,7 @@
 
 #include "secret.h"
 
+#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 
@@ -91,6 +92,19 @@
  *
  */
 
+/**
+ * Fills a buffer with random data.
+ * This function is used internally by libgfshare.
+ *
+ * @param buffer The buffer to fill.
+ * @param len    The buffer size.
+ */
+static void
+gfsec_fill_rand(unsigned char *buffer, unsigned int len)
+{
+    gcry_randomize(buffer, len, GCRY_STRONG_RANDOM);
+}
+
 /**
  * Initializes a new share object.
  *
@@ -445,6 +459,9 @@ gfsec_secret_combine(gfsec_secret_t *secret)
     if ( ! (secret->data = malloc(secret->len)) )
         return GFSEC_ERR_SYSTEM_ERROR;
 
+    if ( gfshare_fill_rand == NULL )
+        gfshare_fill_rand = gfsec_fill_rand;
+
     if ( secret->full_share )
         memcpy(secret->data, secret->full_share->data, secret->len);
     else {
@@ -520,6 +537,9 @@ gfsec_secret_split(gfsec_secret_t *secret, unsigned char threshold)
     if ( threshold >= n )
         return GFSEC_ERR_NOT_ENOUGH_SHARES;
 
+    if ( gfshare_fill_rand == NULL )
+        gfshare_fill_rand = gfsec_fill_rand;
+
     for ( u = v = rc = 0; u < secret->n_shares && rc == 0; u++ ) {
         share = secret->shares[u];