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.
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.
Anamorphic encryption is a cryptographic paradigm that allows covert messages to be hidden inside ordinary ciphertexts of a public-key encryption scheme. The covert channel is enabled by a secret double-key shared between the sender and an authorized receiver. Even if an adversary coerces the receiver into revealing the secret-key, the hidden communication remains undetectable without the double-key. Existing formulations of asymmetric anamorphic encryption suffer from two limitations. First, key pairs of a public-key encryption scheme and double-keys are tightly coupled: once a public-key is deployed, it is no longer possible to associate fresh double-keys with it. Second, existing constructions of asymmetric anamorphic encryption either rely on additional cryptographic primitives or on stronger structural assumptions, or they are scheme-specific.In this work, we refine the syntax of asymmetric anamorphic encryption so that double-key generation is separated from key deployment, allowing multiple double-keys to be registered with the same public-key after setup. We then construct an asymmetric anamorphic encryption scheme from the group-action-based public-key encryption scheme $ \mathsf{gaPKE} $, which is built from a regular effective group action over a commutative group. The resulting scheme is secure in the standard model. Our construction combines a hybrid encryption scheme, built under the $ \textsf{GA-DDH}$ assumption and entropy smoothing of hash functions, with a second independent group-action instance that carries the covert message. The resulting scheme satisfies anamorphic security, sender indistinguishability under chosen-plaintext attack security and colluder indistinguishability under chosen-plaintext attack security. We instantiate our framework in both post-quantum and classical settings. Using $ \textsf{CSIDH}$ group actions, we obtain the first isogeny-based asymmetric anamorphic encryption scheme. Using $ \textsf{DDH}$-hard groups, we obtain a classical anamorphic encryption scheme based on hashed ElGamal.
This paper proposes a new generic Encrypt-thenMAC (EtM) transformation for constructing IND-CCA secure Key Encapsulation Mechanisms (KEMs) in the post-quantum setting. The transformation builds on a public-key encryption scheme satisfying one-wayness under plaintext-checking attacks (OW-PCA) and an existentially unforgeable MAC, and formally achieves tight IND-CCA security in the random oracle model via a rigorous security reduction. Unlike the widely used FujisakiOkamoto (FO) transformation, our approach eliminates the need for ciphertext re-encryption during decapsulation-a major source of computational overhead and side-channel vulnerability in lattice-based implementations. We instantiate the transformation using the ML-KEM (FIPS 203) subroutine, yielding the ML-KEM-EtM variant. Experimental results show that ML-KEMEtM reduces decapsulation cycle counts by72−79%across security levels, with only a modest increase in encapsulation cost(1.8−7.3%)and ciphertext size(1.0−2.1%). The scheme thus offers a practical and efficient a lternative to F O-based KEMs, especially in performance-critical scenarios such as post-quantum TLS handshakes, where decapsulation efficiency is paramount.
A hybrid encryption (HE) system is an efficient public key encryption system for arbitrarily long messages. An HE system consists of a public key component called key encapsulation mechanism (KEM), and a symmetric key component called data encapsulation mechanism (DEM). The HE encryption algorithm uses a KEM generated key k to encapsulate the message using DEM, and send the ciphertext together with the encapsulaton of k, to the decryptor who decapsulates k and uses it to decapsulate the message using the corresponding KEM and DEM components. The KEM/DEM composition theorem proves that if KEM and DEM satisfy well-defined security notions, then HE will be secure with well defined security. We introduce HE in correlated randomness model where the encryption and decryption algorithms have samples of correlated random variables that are partially leaked to the adversary. Security of the new KEM/DEM paradigm is defined against computationally unbounded or polynomially bounded adversaries. We define iKEM and cKEM with respective information theoretic computational security, and prove a composition theorem for them and a computationally secure DEM, resulting in secure HEs with proved computational security (CPA and CCA) and without any computational assumption. We construct two iKEMs that provably satisfy the required security notions of the composition theorem. The iKEMs are used to construct two efficient quantum-resistant HEs when used with an AES based DEM. We also define and construct combiners with proved security that combine the new KEM/DEM paradigm of HE with the traditional public key based paradigm of HE.
Key Encapsulation Mechanism (KEM) is a foundational cryptography primitive, which can provide secure symmetric cryptographic key material for transmission by using public key algorithms. Until now, many Chosen-Ciphertext (IND-CCA) secure KEM schemes are constructed from Chosen-Plaintext (IND-CPA) or One-Way (OW-CPA) secure PKE via the generic Fujisaki-Okamoto (FO) transformations (TCC 2017). However, the security relies on the Random Oracle Model (ROM). To the best of our knowledge, there are no IND-CCA secure KEM schemes based on Learning Parity with Noise (LPN) assumption that can against post quantum attacks in the standard model. In this work, we propose the first direct construction of LPN-based KEM, which is secure in the standard model. In particular, we use double-trapdoor technique to answer adversary’s decryption queries correctly and a Target Collision Resistant (TCR) hash function to check the validity of the ciphertext. The encapsulated key is determined by a special LPN problem (with no random oracle required). The scheme is IND-CCA secure against post-quantum attacks under the low-noise LPN assumptions by a series of games and the security reduction is tight. Compared with previous schemes on 128-bit security level, our CCA-secure scheme only holds 50.78MB public keys, 62.50MB secret keys and 4.54KB ciphertexts, which is more efficient than the schemes of Döttlinget al.(ASIACRYPT 2012), Kiltzet al.(PKC 2014) and Yuet al.(CRYPTO 2016) ((7.27GB, 7.24GB, 7.03KB), (80.89MB, 46.23MB, 6.80KB) and (70.95MB, 70.65MB, 86.50KB) respectively).
The concept of anamorphic encryption, first formally introduced by Persiano et al. in their influential 2022 paper titled ``Anamorphic Encryption: Private Communication Against a Dictator,'' enables embedding covert messages within ciphertexts. One of the key distinctions between a ciphertext embedding a covert message and an original ciphertext, compared to an anamorphic ciphertext, lies in the indistinguishability between the original ciphertext and the anamorphic ciphertext. This encryption procedure has been defined based on a public-key cryptosystem. Initially, we present a quantum analogue of the classical anamorphic encryption definition that is based on public-key encryption. Additionally, we introduce a definition of quantum anamorphic encryption that relies on symmetric key encryption. Furthermore, we provide a detailed generalized construction of quantum anamorphic symmetric key encryption within a general framework, which involves taking any two quantum density matrices of any different dimensions and constructing a single quantum density matrix, which is the quantum anamorphic ciphertext containing ciphertexts of both of them. Subsequently, we introduce a definition of computational anamorphic secret-sharing and extend the work of Çakan et al. on computational quantum secret-sharing to computational quantum anamorphic secret-sharing, specifically addressing scenarios with multiple messages, multiple keys, and a single share function. This proposed secret-sharing scheme demonstrates impeccable security measures against quantum adversaries.
Key encapsulation mechanism (KEM) is an important key distribution mechanism that not only allows both sender and receiver to safely share a random session key, but also can be mainly applied to construct a hybrid public key encryption scheme. In this paper, we give an positive answer to the question of if it is possible to build an efficient KEM over lattices. More precisely, we design an efficient KEM scheme in standard model based on ideal lattices. We prove that the proposed scheme captures indistinguishability against active chosen ciphertext attacks (IND-CCA) under the ring learning with errors problem, or more formally, IND-CCA security. Compared with the current CCA secure KEM schemes based on lattices in the standard model, our scheme has shorter public key, secret key and encapsulation ciphertext. In addition, our KEM scheme realizes IND-CCA security in the standard model.