diff --git a/library/constant_time.h b/library/constant_time.h
index e14232b..0d6c0fd 100644
--- a/library/constant_time.h
+++ b/library/constant_time.h
@@ -28,3 +28,6 @@
 unsigned char mbedtls_nist_kw_safer_memcmp( const void *a, const void *b, size_t n );
 
 int mbedtls_safer_memcmp( const void *a, const void *b, size_t n );
+
+
+unsigned mbedtls_cf_uint_mask( unsigned value );