diff --git a/library/constant_time.h b/library/constant_time.h
index 0d6c0fd..3cbabe1 100644
--- a/library/constant_time.h
+++ b/library/constant_time.h
@@ -31,3 +31,5 @@
 
 
 unsigned mbedtls_cf_uint_mask( unsigned value );
+
+size_t mbedtls_cf_size_mask( size_t bit );