Authenticated Key Exchange (AKE) establishes shared ('symmetric') cryptographic keys which are essential for secure online communication. AKE protocols can be constructed from public-key cryptography like Key Encapsulation Mechanisms (KEMs). Another approach is to use Quantum Key Distribution (QKD) to establish a symmetric key, which uses quantum communication. Combining post-quantum AKE and QKD appropriately may provide security against quantum attacks even if only one of the two approaches turns out to be secure. We provide an extensive review of existing security analyses for combined AKE and their formal security models, and identify some gaps in their treatment of QKD key IDs. In particular, improper handling of QKD key IDs leads to Dependent-Key attacks on AKE. As our main conceptual contribution, we model QKD as an oracle that closely resembles the standard ETSI 014 QKD interface. We demonstrate the usability of our QKD oracle for cryptographic security analyses by integrating it into a prominent security model for AKE, called CK+ model, thereby obtaining a security model for combined AKE that catches Dependent-Key attacks. In this model, we formally prove security of a new protocol that combines QKD with a triple-KEM handshake. This is the first provably secure hybrid protocol that maintains information-theoretic security of QKD.
Authenticated Key Exchange (AKE) establishes shared ('symmetric') cryptographic keys which are essential for secure online communication. AKE protocols can be constructed from public-key cryptography like Key Encapsulation Mechanisms (KEMs). Another approach is to use Quantum Key Distribution (QKD) to establish a symmetric key, which uses quantum communication. Combining post-quantum AKE and QKD appropriately may provide security against quantum attacks even if only one of the two approaches turns out to be secure. We provide an extensive review of existing security analyses for combined AKE and their formal security models, and identify some gaps in their treatment of QKD key IDs. In particular, improper handling of QKD key IDs leads to Dependent-Key attacks on AKE. As our main conceptual contribution, we model QKD as an oracle that closely resembles the standard ETSI 014 QKD interface. We demonstrate the usability of our QKD oracle for cryptographic security analyses by integrating it into a prominent security model for AKE, called CK+ model, thereby obtaining a security model for combined AKE that catches Dependent-Key attacks. In this model, we formally prove security of a new protocol that combines QKD with a triple-KEM handshake. This is the first provably secure hybrid protocol that maintains information-theoretic security of QKD.
With the advent of quantum computing, which threatens the very foundations of classical cryptography, several authenticated key exchange (AKE) protocols have been proposed, combining classical and post-quantum cryptographic algorithms, and a quantum key distribution (QKD) sub-protocol. The goal being to associate the claimed information theoretic security of QKD, and the security based upon computational assumptions of classical and post-quantum cryptography. To our knowledge, in existing security proofs of such hybrid AKE protocols, the QKD phase is treated as a black box and the impact of establishing the output quantum key appears similar as setting a symmetric key beforehand at the two communicating parties. In this paper, we describe an hybrid AKE protocol and a security model that captures multiple security properties. Our security analysis integrates the security loss induced by the QKD phase as well as that of implied by the classical and post-quantum cryptographic algorithms involved in the protocol.
Post-quantum cryptography (PQC) and quantum key distribution (QKD) address the quantum threat from opposite directions: PQC replaces vulnerable public-key algorithms with schemes believed to resist quantum cryptanalysis, while QKD derives keys whose secrecy rests on physical assumptions rather than computational ones. Because each approach has a distinct failure mode, a growing body of work combines them so that a session key remains secure if either component holds. We first establish the deployed baseline, hybrid classical-plus-PQC key exchange, against which QKD-augmented designs must be judged. We then propose a taxonomy of hybrid PQC-QKD designs along eight dimensions (integration layer, combiner construction, authentication scope and source, QKD topology, trusted entities, key-delivery interface, classical component, and evidence type), and use it to compare published protocols, including Muckle and its successors, MACsec and TLS integrations, and IKEv2 mechanisms standardised in RFCs 8784, 9370, and 9867. We examine key-combiner constructions in light of the KEM-combiner literature and of NIST, ETSI, and ITU-T guidance, and map the security arguments for hybrid PQC-QKD protocols onto a common grid of security model, assumptions, permitted compromise, established guarantee, and limitations. We synthesise deployment evidence from metropolitan and national QKD networks, distinguishing the few testbeds that have run PQC in the key path from pure-QKD deployments, and summarise the standards and policy landscape, including the positions of national security agencies that recommend against relying on QKD alone. We close with open problems: authentication of the QKD classical channel, shared-infrastructure threat models, device certification versus formal proofs, formal verification of hybrid handshakes, and the lack of peer-reviewed deployment economics.
Abstract. An unresolved problem in research on authenticated key exchange (AKE) is to construct a secure protocol against advanced attacks such as key compromise impersonation and maximal exposure attacks without relying on random oracles. HMQV, a state of the art AKE protocol, achieves both efficiency and the strong security proposed by Krawczyk (we call it the CK + model), which includes resistance to advanced attacks. However, the security proof is given under the random oracle model. We propose a generic construction of AKE from a key encapsulation mechanism (KEM). The construction is based on a chosen-ciphertext secure KEM, and the resultant AKE protocol is CK + secure in the standard model. The construction gives the first CK + secure AKE protocols based on the hardness of integer factorization problem, code-based problems, or learning problems with errors. In addition, instantiations under the Diffie-Hellman assumption or its variant can be proved to have strong security without non-standard assumptions such as πPRF and KEA1. Furthermore, we extend the CK + model to identity-based (called the id-CK + model), and propose a generic construction of identity-based AKE (ID-AKE) based on identity-based KEM, which satisfies id-CK + security. The construction leads first strongly secure ID-AKE protocols under the hardness of integer factorization problem, or learning problems with errors.
Authenticated Key Exchange (AKE) is a cryptographic protocol to share a common session key among multiple parties. Usually, PKI-based AKE schemes are designed to guarantee secrecy of the session key and mutual authentication. However, in practice, there are many cases where mutual authentication is undesirable such as in anonymous networks like Tor and Riffle, or difficult to achieve due to the certificate management at the user level such as the Internet. Goldberg et al. formulated a model of anonymous one-sided AKE which guarantees the anonymity of the client by allowing only the client to authenticate the server, and proposed a concrete scheme. However, existing anonymous one-sided AKE schemes are only known to be secure in the random oracle model. In this paper, we propose generic constructions of anonymous one-sided AKE in the random oracle model and in the standard model, respectively. Our constructions allow us to construct the first post-quantum anonymous one-sided AKE scheme from isogenies in the standard model.
Key establishment is a crucial primitive for building secure channels: in a multi-party setting, it allows two parties using only public authenticated communication to establish a secret session key which can be used to encrypt messages. But if the session key is compromised, the confidentiality of encrypted messages is typically compromised as well. Without quantum mechanics, key establishment can only be done under the assumption that some computational problem is hard. Since digital communication can be easily eavesdropped and recorded, it is important to consider the secrecy of information anticipating future algorithmic and computational discoveries which could break the secrecy of past keys, violating the secrecy of the confidential channel. Quantum key distribution (QKD) can be used generate secret keys that are secure against any future algorithmic or computational improvements. QKD protocols still require authentication of classical communication, however, which is most easily achieved using computationally secure digital signature schemes. It is generally considered folklore that QKD when used with computationally secure authentication is still secure against an unbounded adversary, provided the adversary did not break the authentication during the run of the protocol. We describe a security model for quantum key distribution based on traditional classical authenticated key exchange (AKE) security models. Using our model, we characterize the long-term security of the BB84 QKD protocol with computationally secure authentication against an eventually unbounded adversary. By basing our model on traditional AKE models, we can more readily compare the relative merits of various forms of QKD and existing classical AKE protocols. This comparison illustrates in which types of adversarial environments different quantum and classical key agreement protocols can be secure.