Proof-Carrying Code (PCC) allows a code producer to provide to a host a program along with its formal safety proof. The proof attests a certain safety policy enforced by the code, and can be mechanically checked by the host. While this language-based approach to code certification is very general in principle, existing PCC systems have only focused on programs whose safety proofs can be automatically generated. As a result, many low-level system libraries (e.g., memory management) have not yet been handled. In this paper, we explore a complementary approach in which general properties and program correctness are semiautomatically certified. In particular, we introduce a low-level language CAP for building certified programs and present a certified library for dynamic storage allocation.