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Lecture notes in computer science

Encryption Schemes Using Random Oracles: From Classical to Post-Quantum Security

Juliane Krämer, Patrick Struck

Published Jan 1, 2020• 9 citations
Publisher
DOI
10.1007/978-3-030-44223-1_29

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Modern cryptographic transformations require provable security against a relatively wide class of threats. Typically, such evidentiary security is achieved through formal analysis within the chosen security model. The development of quantum computers led to the emergence of new attack vectors to which classical cryptography was vulnerable. However, there are cryptographic systems that are considered resistant to quantum attacks and some of them are even standardized. The formal analysis of such systems has faced difficulties for a long time, which were associated with the impossibility of applying classical methods of proof to formal models that take into account quantum effects. However, in recent years, many new results have appeared that allow obtaining formal security proofs for quite complex cryptographic transformations, and most of the existing post-quantum asymmetric encryption and key encapsulation schemes currently have corresponding formal proofs within the quantum random oracle model, the most widespread security model for of post-quantum cryptography. DSTU 8961:2019 is the Ukrainian post-quantum standard for asymmetric encryption and key encapsulation. However, security proofs in the quantum random oracle model have not yet been published for it. As part of this work, security evidence was obtained for the design of the key encapsulation mechanism described in DSTU 8961:209. The obtained result is generalized for an arbitrary asymmetric encryption scheme, which may contain decryption errors and can be used to assess the security of not only DSTU 8961:2019, but also other similar asymmetric transformations.

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Proxy re-encryption (PRE) allows a semi-trusted party (called a proxy) to transform ciphertexts under one user's public key into ciphertexts under another user's public key, without revealing any information about the underlying messages or the secret keys. On the other hand, post-quantum cryptography (PQC) is one of the most active areas, and in particular, many lattice-based PRE schemes have been proposed so far in the area of PQC. However, there is no post-quantum PRE satisfying security against adaptive chosen ciphertext attacks ( C C A 2 security), while C C A 2 security is one of the most desirable security notions for PRE. In this paper, we propose a group-action-based PRE scheme that achieves C C A 2 security for the first time. We give security proofs for our PRE in the random oracle model (ROM) and the quantum random oracle model (QROM). In particular, we can obtain the CSIDH-instantiation of this PRE scheme as the first post-quantum PRE. Our result contributes to the developments of post-quantum cryptography, and shows further potential of isogeny-based cryptography.

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Strong Multiple-CCA Security in the Quantum Random Oracle Model for an FO-like PKE Combiner

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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.

IACR Communications in Cryptology•Oct 6, 2025•1 citation
Modern cryptographic transformations require provable security against a relatively wide class of threats. Typically, such evidentiary security is achieved through formal analysis within the chosen security model. The development of quantum computers led to the emergence of new attack vectors to which classical cryptography was vulnerable. However, there are cryptographic systems that are considered resistant to quantum attacks and some of them are even standardized. The formal analysis of such systems has faced difficulties for a long time, which were associated with the impossibility of applying classical methods of proof to formal models that take into account quantum effects. However, in recent years, many new results have appeared that allow obtaining formal security proofs for quite complex cryptographic transformations, and most of the existing post-quantum asymmetric encryption and key encapsulation schemes currently have corresponding formal proofs within the quantum random oracle model, the most widespread security model for of post-quantum cryptography. DSTU 8961:2019 is the Ukrainian post-quantum standard for asymmetric encryption and key encapsulation. However, security proofs in the quantum random oracle model have not yet been published for it. As part of this work, security evidence was obtained for the design of the key encapsulation mechanism described in DSTU 8961:209. The obtained result is generalized for an arbitrary asymmetric encryption scheme, which may contain decryption errors and can be used to assess the security of not only DSTU 8961:2019, but also other similar asymmetric transformations.
Proxy re-encryption (PRE) allows a semi-trusted party (called a proxy) to transform ciphertexts under one user's public key into ciphertexts under another user's public key, without revealing any information about the underlying messages or the secret keys. On the other hand, post-quantum cryptography (PQC) is one of the most active areas, and in particular, many lattice-based PRE schemes have been proposed so far in the area of PQC. However, there is no post-quantum PRE satisfying security against adaptive chosen ciphertext attacks ( C C A 2 security), while C C A 2 security is one of the most desirable security notions for PRE. In this paper, we propose a group-action-based PRE scheme that achieves C C A 2 security for the first time. We give security proofs for our PRE in the random oracle model (ROM) and the quantum random oracle model (QROM). In particular, we can obtain the CSIDH-instantiation of this PRE scheme as the first post-quantum PRE. Our result contributes to the developments of post-quantum cryptography, and shows further potential of isogeny-based cryptography.
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.