diff --git a/library/constant_time.h b/library/constant_time.h
index 3cbabe1..0b75900 100644
--- a/library/constant_time.h
+++ b/library/constant_time.h
@@ -33,3 +33,5 @@
 unsigned mbedtls_cf_uint_mask( unsigned value );
 
 size_t mbedtls_cf_size_mask( size_t bit );
+
+size_t mbedtls_cf_size_mask_lt( size_t x, size_t y );