Re-randomization Attack on the Certificateless Encryption Scheme

Abstract

Guo, Li, and Qin proposed a lightweight certificateless encryption (CLE) scheme designed for IoT environments (Discover Computing, 2025). We demonstrate that the proposed scheme does not achieve Chosen-Ciphertext Attack (CCA) security, contrary to the authors' claim. Specifically, we identify two critical points. First, even a scaling of the ciphertext leads to a predictable transformation of the decrypted plaintext. Second, since the ciphertext retains a multiplicative ElGamal-like structure, it can always be re-randomized using randomness. Based on these properties, an adversary can transform a challenge ciphertext into another valid ciphertext of the same plaintext, and then query the decryption oracle with the transformed ciphertext to recover the challenge plaintext. This attack exploits a definitional gap in the CCA game, where only direct decryption queries on the challenge ciphertext are prohibited. In this work, we formalize the attack procedure and verify its validity based on implementation.