Tightly CCA-secure encryption scheme in a multi-user setting with corruptions

The security of public-key encryption (PKE) schemes in a multi-user setting is aimed at capturing real-world scenarios in which an adversary could attack multiple users and multiple ciphertexts of its choice. However, the fact that a real-world adversary can also mount key-exposure attacks for a set of multiple public keys requires us to consider a more realistic notion of security in multi-user settings. In this study, we establish the security notion of PKE in a multi-user setting with corruptions, where an adversary is able to issue (adaptive) encryption, decryption, and corruption (i.e., private key) queries. We then propose the first practical PKE scheme whose security is proven in a multi-user setting with corruptions. The security of our scheme is based on the computational Diffie–Hellman (CDH) assumption and is proven to be tightly chosen-ciphertext secure in a random oracle model. Our scheme essentially follows the recently proposed modular approach of combining KEM and augmented DEM in a multi-user setting, but we show that this modular approach works well in a multi-user setting with corruptions.