diff --git a/library/constant_time.h b/library/constant_time.h
index 50108d7..eff7f44 100644
--- a/library/constant_time.h
+++ b/library/constant_time.h
@@ -39,3 +39,5 @@
 size_t mbedtls_cf_size_mask_ge( size_t x, size_t y );
 
 size_t mbedtls_cf_size_bool_eq( size_t x, size_t y );
+
+unsigned mbedtls_cf_size_gt( size_t size, size_t max );