util: add function for constant-time memory comparison

Add a function to check if two buffers of the same length contain the
same data, but do the comparison in a constant time with respect to the
returned value to avoid creating a timing side channel, i.e. the time
depends only on the buffer length, not on the content.

Use the gnutls_memcmp() or nettle_memeql_sec() functions if available,
otherwise use the same algorithm as nettle - bitwise ORing XORed data.

util.c

@@ -29,6 +29,12 @@
 
 #include "sysincl.h"
 
+#if defined(HAVE_NETTLE)
+#include <nettle/memops.h>
+#elif defined(HAVE_GNUTLS)
+#include <gnutls/gnutls.h>
+#endif
+
 #include "logging.h"
 #include "memory.h"
 #include "util.h"
@@ -1648,3 +1654,22 @@ UTI_SplitString(char *string, char **words, int max_saved_words)
 
   return i;
 }
+
+/* ================================================== */
+
+int
+UTI_IsMemoryEqual(const void *s1, const void *s2, unsigned int len)
+{
+#if defined(HAVE_NETTLE)
+  return nettle_memeql_sec(s1, s2, len);
+#elif defined(HAVE_GNUTLS)
+  return gnutls_memcmp(s1, s2, len) == 0;
+#else
+  unsigned int i, x;
+
+  for (i = 0, x = 0; i < len; i++)
+    x |= ((const unsigned char *)s1)[i] ^ ((const unsigned char *)s2)[i];
+
+  return x == 0;
+#endif
+}