diff --git a/library/constant_time.c b/library/constant_time.c
index d73f4fe..7da4046 100644
--- a/library/constant_time.c
+++ b/library/constant_time.c
@@ -213,3 +213,19 @@
 
     return( 1 ^ diff1 );
 }
+
+/** Check whether a size is out of bounds, without branches.
+ *
+ * This is equivalent to `size > max`, but is likely to be compiled to
+ * to code using bitwise operation rather than a branch.
+ *
+ * \param size      Size to check.
+ * \param max       Maximum desired value for \p size.
+ * \return          \c 0 if `size <= max`.
+ * \return          \c 1 if `size > max`.
+ */
+unsigned mbedtls_cf_size_gt( size_t size, size_t max )
+{
+    /* Return the sign bit (1 for negative) of (max - size). */
+    return( ( max - size ) >> ( sizeof( size_t ) * 8 - 1 ) );
+}