When a practical quantum computer is realized, most of the number theory-based cryptosystems are broken, but some error-correcting code-based (code-based) cryptosystems are thought to withstand the attacks of quantum computers. Gaborit et al. have proposed code-based HQC public key cryptography [1]. In this research, we apply their public key cryptosystem and propose a secret linear function calculation and a secret size comparison protocol, which is characterized by not using lost communication.