diff --git a/library/constant_time.h b/library/constant_time.h
index 8e8992d..50108d7 100644
--- a/library/constant_time.h
+++ b/library/constant_time.h
@@ -37,3 +37,5 @@
 size_t mbedtls_cf_size_mask_lt( size_t x, size_t y );
 
 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 );