Anamorphic Encryption with CCA Security: A Standard Model Construction

TL;DR

This paper introduces a new, standard-model construction for anamorphic encryption that achieves CCA security, enabling more secure covert communication even under coercion.

Contribution

It formalizes the Anamorphic Key Encapsulation Mechanism (AKEM) and provides generic, CCA-secure constructions based on KEMs with randomness recovery, with rigorous proofs.

Findings

Achieves strong IND-CCA security for covert channels.

Provides formal proofs in the standard model.

Ensures security based on the injective property of KEMs.

Abstract

Anamorphic encryption serves as a vital tool for covert communication, maintaining secrecy even during post-compromise scenarios. Particularly in the receiver-anamorphic setting, a user can shield hidden messages even when coerced into surrendering their secret keys. However, a major bottleneck in existing research is the reliance on CPA-security, leaving the construction of a generic, CCA-secure anamorphic scheme in the standard model as a persistent open challenge. To bridge this gap, we formalize the Anamorphic Key Encapsulation Mechanism (AKEM), encompassing both Public-Key (PKAKEM) and Symmetric-Key (SKAKEM) variants. We propose generic constructions for these primitives, which can be instantiated using any KEM that facilitates randomness recovery. Notably, our framework achieves strong IND-CCA (sIND-CCA) security for the covert channel. We provide a rigorous formal proof in the standard model, demonstrating resilience against a "dictator" who controls the decapsulation key. The security of our approach is anchored in the injective property of the base KEM, which ensures a unique mapping between ciphertexts and randomness. By integrating anamorphism into the KEM-DEM paradigm, our work significantly enhances the practical utility of covert channels within modern cryptographic infrastructures.