diff --git a/include/ntfs/security.h b/include/ntfs/security.h
index 7a7d7458..1eac550a 100644
--- a/include/ntfs/security.h
+++ b/include/ntfs/security.h
@@ -31,6 +31,24 @@ extern const GUID *const zero_guid;
 extern BOOL ntfs_guid_is_zero(const GUID *guid);
 extern char *ntfs_guid_to_mbs(const GUID *guid, char *guid_str);
 
-extern char *ntfs_sid_to_mbs(const SID *sid, char *sid_str);
+/**
+ * ntfs_sid_is_valid - determine if a SID is valid
+ * @sid:	[IN]  SID for which to determine if it is valid
+ *
+ * Determine if the SID pointed to by @sid is valid.
+ *
+ * Return TRUE if it is valid and FALSE otherwise.
+ */
+static __inline__ BOOL ntfs_sid_is_valid(const SID *sid)
+{
+	if (!sid || sid->revision != SID_REVISION ||
+			sid->sub_authority_count > SID_MAX_SUB_AUTHORITIES)
+		return FALSE;
+	return TRUE;
+}
+
+extern int ntfs_sid_to_mbs_size(const SID *sid);
+extern char *ntfs_sid_to_mbs(const SID *sid, char *sid_str,
+		size_t sid_str_size);
 
 #endif /* defined _NTFS_SECURITY_H */