An IND-CCA2 secure post-quantum encryption scheme and a secure cloud storage use case

Code-based public key encryption (PKE) is a popular choice to achieve post-quantum security, partly due to its capability to achieve fast encryption/decryption. However, code-based PKE has larger ciphertext and public key sizes in comparison to conventional PKE schemes (e.g., those based on RSA). In 2018, Lau and Tan proposed a new rank metric code-based PKE scheme, which has smaller public key and ciphertext sizes compared to other code-based PKE schemes. They also proved that their scheme achieves IND-CPA security, assuming the intractability of the decisional rank syndrome decoding problem. It is known that IND-CCA2 security is the strongest and most popular security assurance for PKE schemes. Therefore, in this paper, we obtain a new code-based PKE scheme from Lau and Tan’s scheme, in order to inherit the underlying small public key and ciphertext sizes. However, our new scheme is shown to achieve IND-CCA2 security, instead of the weaker IND-CPA security. Specifically, the respective public key size and ciphertext size in our new scheme are 15.06 KB and 1.37 KB under 141-bit security level, and 16.76 KB and 1.76 KB under 154-bit security level. We then present a use case for the proposed scheme, that is for secure cloud storage.