Combiners for cryptographic schemes are a common way to increase security using redundancy. The security notions for public key encryption (PKE) combiners can be extended beyond the standard IND-CCA security to achieve even stronger security notions. In the indistinguishability under adaptive strong multiple chosen-ciphertext attack (IND-sMCCA) security notion, the adversary has additional oracle access to the underlying cipher components of the combiner. Recently, combiners have received more attention because of the possibility of combining classical and post-quantum (PQ) cryptography. This allows for the use of novel PQ algorithms while still having the security guarantees of the classical algorithms. In order to examine the security against quantum adversaries, the quantum random oracle model (QROM) has become the most relevant security model. However, there are no PKE combiners that achieve IND-sMCCA security in the QROM, even though this security notion describes much better the current state where classical, PQ, and combined schemes are deployed at the same time. In this paper, we close this gap by providing a new PKE combiner that is IND-sMCCA secure in the QROM. Our construction is more efficient and lean than the existing PKE combiners, considering the primitives used and the ciphertext size. We accomplish this by applying the Fujisaki-Okamoto (FO) transformation to a PKE combiner of Asmuth and Blakley. To achieve IND-sMCCA security for the combiner, the PKE components must be OW-CCA secure. However, when using weaker (OW-CPA secure) PKE components, we still achieve standard IND-CCA security for the combiner. The security reductions are given in the ROM and the QROM, achieving bounds of different tightness.
Combiners for cryptographic schemes are a common way to increase security using redundancy. The security notions for public key encryption (PKE) combiners can be extended beyond the standard IND-CCA security to achieve even stronger security notions. In the indistinguishability under adaptive strong multiple chosen-ciphertext attack (IND-sMCCA) security notion, the adversary has additional oracle access to the underlying cipher components of the combiner. Recently, combiners have received more attention because of the possibility of combining classical and post-quantum (PQ) cryptography. This allows for the use of novel PQ algorithms while still having the security guarantees of the classical algorithms. In order to examine the security against quantum adversaries, the quantum random oracle model (QROM) has become the most relevant security model. However, there are no PKE combiners that achieve IND-sMCCA security in the QROM, even though this security notion describes much better the current state where classical, PQ, and combined schemes are deployed at the same time. In this paper, we close this gap by providing a new PKE combiner that is IND-sMCCA secure in the QROM. Our construction is more efficient and lean than the existing PKE combiners, considering the primitives used and the ciphertext size. We accomplish this by applying the Fujisaki-Okamoto (FO) transformation to a PKE combiner of Asmuth and Blakley. To achieve IND-sMCCA security for the combiner, the PKE components must be OW-CCA secure. However, when using weaker (OW-CPA secure) PKE components, we still achieve standard IND-CCA security for the combiner. The security reductions are given in the ROM and the QROM, achieving bounds of different tightness.
Practical deployments of key encapsulation mechanisms (KEMs) may entail large servers each using their public keys to communicate with potentially millions of clients simultaneously. While the standard IND-CCA security definition for KEMs considers only a single challenge public key and single challenge ciphertext, it can be relevant to consider multi-target scenarios where the adversary aims to break one of many challenge ciphertexts, for one of many challenge public keys. Many post-quantum KEMs have been built by applying the Fujisaki-Okamoto (FO) transform to a public key encryption (PKE) scheme. Although the FO transform incurs only a few bits of security loss for the standard, single-challenge IND-CCA property, this does not hold in the multi-target setting. Attacks have been identified against standards-track FO-based KEMs with 128-bit message spaces (FrodoKEM-640 and HQC-128) which become feasible if the adversary is given many challenge ciphertexts (say, 2^64). These attacks exploit the deterministic encryption induced by the FO transform which allows the IND-CCA experiment to be reduced to a search problem on the message space, which in some cases may not be large enough to avoid collisions between pre-computation and challenge values. A cost effective way to amplify the hardness of this search problem is to add a random but public salt during encapsulation. While revised versions of FrodoKEM and HQC have used salts, there has been no proof showing that salting provides multi-ciphertext security. In this work, we formally analyze a salted variant of the Fujisaki-Okamoto transform, in the classical and quantum random oracle model (ROM); for the classical ROM, we show that multi-target IND-CCA security of the resulting KEM tightly reduces to the multi-target IND-CPA security of the underlying PKE. Our results imply that, for FrodoKEM and HQC at the 128-bit security level, replacing the FO transform with the salted variant can recover 62 bits of multi-target security, at the cost of a very small overhead increase.
The Fujisaki-Okamoto (FO) conversion is a generic conversion to enhance a public key encryption scheme with security of one-way against chosen plaintext attacks (OW-CPA) to security of indistinguishable against adaptive chosen ciphertext attacks (IND-CCA) in the random oracle model. Existing works have shown that the FO conversion also can generically upgrade the security of the identity-based encryption (IBE) schemes or certificateless public key encryption (CL-PKE) schemes. However, it is still unknown if the same holds for certificate-based encryption (CBE) schemes. In this paper, we investigate the generic security of the CBE scheme obtained by applying the FO conversion to an arbitrary underlying OW-CBE-CPA secure CBE scheme and confirm that the FO conversion can generically convert any OW-CBE-CPA secure CBE into an IND-CBE-CCA secure CBE. We also note that the straightforward application of the FO conversion only leads to a CBE scheme with a loose reduction. To solve this problem, we propose an enhanced FO conversion which just introduces minor extra computation overhead, but results in considerably more efficient security reduction.
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).
Receiver selective opening (RSO) attack for public key encryption (PKE) captures a situation where one sender sends messages to multiple receivers, an adversary can corrupt a set of receivers and get their messages and secret keys. Security against RSO attack for a PKE scheme ensures confidentiality of other uncorrupted receivers' ciphertexts. Among all of the RSO security notions, simulation-based RSO security against chosen ciphertext attack (SIM-RSO-CCA security) is the strongest notion. In this paper, we explore constructions of SIM-RSO-CCA secure PKE from various computational assumptions. Toward this goal, we show that a SIM-RSO-CCA secure PKE scheme can be constructed based on an IND-CPA secure PKE scheme and a designated-verifier non-interactive zero-knowledge (DV-NIZK) argument satisfying one-time simulation soundness. Moreover, we give the first construction of DV-NIZK argument satisfying one-time simulation soundness. Consequently, through our generic construction, we obtain the first SIM-RSO-CCA secure PKE scheme under the computational Diffie-Hellman (CDH) or learning parity with noise (LPN) assumption.
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.
TLS 1.3 is a crucial protocol for securing modern internet communications. To facilitate a smooth transition to post-quantum security, hybrid key exchange, which combines classical key exchange algorithms with post-quantum key encapsulation mechanisms (KEMs), is proposed to enhance the security of the current TLS 1.3 handshake. However, existing drafts and implementations of hybrid key exchange for TLS 1.3 primarily rely on CCA-secure KEMs (i.e., secure against chosen-ciphertext attacks) based on the Fujisaki-Okamoto (FO) transform. The re-encryption step in their decapsulation algorithms not only introduces additional performance overhead but also raises the risk of side-channel attacks. Although Huguenin-Dumittan and Vaudenay (Eurocrypt 2022) and Zhou et al. (Asiacrypt 2024) demonstrated that the weaker CPA-secure KEMs (i.e., secure against chosen-plaintext attacks) suffice for constructing a secure TLS 1.3 handshake, their analyses were limited to single-KEM settings and did not consider the hybrid key exchange scenario. This work challenges the necessity of CCA security by proving that CPA-secure KEMs are sufficient for the TLS 1.3 handshake even in the hybrid key exchange setting. We provide the first formal security proofs for this claim, covering both the classical random oracle model (ROM) and the quantum random oracle model (QROM), thereby ensuring security against quantum adversaries. To validate the practical benefits, we conduct an extensive performance evaluation based on the latest OpenSSL implementation. Our results show that using CPA-secure KEMs yields up to 44.8% performance improvement at the key exchange layer and up to approximately 9% acceleration for the full TLS 1.3 handshake. Beyond performance gains, this approach reduces the codebase's attack surface by eliminating the re-encryption step, thereby mitigating a class of side-channel vulnerabilities. Our work positions CPA-secure KEMs as a secure, efficient, and practical alternative for standardizing and deploying post-quantum TLS 1.3 even with hybrid key exchange.