Abstract
Continuous Key Agreement (CKA) is a two-party procedure used by Double Ratchet protocols (e. g., Signal). This is a continuous and synchronous protocol that generates a fresh key for every sent/received message. It guarantees forward secrecy and post-compromise security. Alwen et al. have recently proposed a new KEM-based CKA construction where every message contains a ciphertext and a fresh public key. This can be made quantum-safe by deploying a quantum-safe KEM. They mention that the bandwidth can be reduced when using an ElGamal KEM (which is not quantum-safe). In this paper, we generalized their approach by defining a new primitive, namely Merged KEM (MKEM). This primitive merges the key generation and the encapsulation steps of a KEM. This is not possible for every KEM and we discuss cases where a KEM can be converted to an MKEM. One example is the quantum-safe proposal BIKE1, where the BIKE-MKEM saves \(50\%\) of the communication bandwidth, compared to the original construction. In addition, we offer the notion and two constructions for hybrid CKA.
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Notes
- 1.
We use the terminology quantum-safe to cryptographic algorithms that rely on problems that are believed to be hard even in the presence of quantum computers. For example, cryptographic algorithms that rely on factorization (e. g., RSA) are not considered quantum-safe due to Shor’s algorithm [18]. On the other hand for some parameters cryptographic algorithms that rely on the Shortest Vector Problem over lattices are commonly considered quantum-safe.
- 2.
CKA uses ephemeral keys for both KEM and MKEM. This protects the scheme from attacks that may exploit decapsulation failures, such as [12] in the context of QC-MDPC codes. We note that CKA is aborted (and subsequently re-initialized) upon encountering a decapsulation failure.
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Acknowledgments
This research was supported by: The Israel Science Foundation (grant No. 1018/16); The BIU Center for Research in Applied Cryptography and Cyber Security, in conjunction with the Israel National Cyber Bureau in the Prime Minister’s Office; the Center for Cyber Law & Policy at the University of Haifa in conjunction with the Israel National Cyber Directorate in the Prime Minister’s Office.
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Drucker, N., Gueron, S. (2019). Continuous Key Agreement with Reduced Bandwidth. In: Dolev, S., Hendler, D., Lodha, S., Yung, M. (eds) Cyber Security Cryptography and Machine Learning. CSCML 2019. Lecture Notes in Computer Science(), vol 11527. Springer, Cham. https://doi.org/10.1007/978-3-030-20951-3_3
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