We present a cryptographic string commitment scheme that is computationally hiding and binding based on (modular) subset sum problems. It is believed that these NP-complete problems provide post-quantum security contrary to the number theory assumptions currently used in cryptography. Using techniques recently introduced by Feneuil, Maire, Rivain, and Vergnaud, this simple commitment scheme enables an efficient zero-knowledge proof of knowledge for committed values as well as proofs showing Boolean relations amongst the committed bits. In particular, one can prove that committed bits $m_0, m_1, ..., m_\ell$ satisfy $m_0 = C(m_1, ..., m_\ell)$ for any Boolean circuit $C$ (without revealing any information on those bits). The proof system achieves good communication and computational complexity since for a security parameter $\lambda$, the protocol's communication complexity is $\tilde{O}(|C| \lambda + \lambda^2)$ (compared to $\tilde{O}(|C| \lambda^2)$ for the best code-based protocol due to Jain, Krenn, Pietrzak and Tentes).