Post-Quantum Provable Security in Symmetric-Key Cryptography

光倫 細山田, Akinori Hosoyamada

Fang Song

Teemu Manninen

The development of quantum computers presents a threat to the security of modern communication systems; with Shor’s algorithm, all currently used asymmetric cryptography schemes can be broken. Fortunately, secure communication is still possible in the post-quantum era using symmetric encryption algorithms, such as the Advanced Encryption Standard (AES) or the provably secure one-time pad (OTP). However, the main problem of symmetric cryptography, the key distribution, has to be solved. Quantum key distribution (QKD) uses the fundamental properties of quantum mechanics to distribute encryption keys in a provably secure manner. In this thesis, the practical applicability of QKD is examined. This is achieved by testing the performance, stability, and usability of one of the few commercially available QKD platforms, ID Quantique’s Clavis 2. The tested system was stable for multiple months in normal operation and did not require any intervention from the user. The maximum distance with the test platform was 54 km. However, at these distances, the secret key rate was only tens of bits per second. For distances below 25 km, secret key rates in the kilobits per second range were achieved. Thus, if high data rates or communication distances close to 50 km are desired, ciphers less secure than OTP have to be used. Additionally, two flaws were discovered in the tested system, one making the system unstable, and the other preventing the key distribution process from starting. Fortunately, neither issue reduces the security of the system, and both could be fixed by modifying the software. Our results show that QKD can be considered as a mature technology that is ready for practical applications and even for commercial use.

Syed Ismail Abdul Lathif, Priya Saxena, Priyanshi Singh

Gorjan Alagic, Chen Bai, Christian Majenz +1 more

Block ciphers are versatile cryptographic ingredients that are used in a wide range of applications ranging from secure Internet communications to disk encryption. While post-quantum security of public-key cryptography has received significant attention, the case of symmetric-key cryptography (and block ciphers in particular) remains a largely unexplored topic. In this work, we set the foundations for a theory of post-quantum security for block ciphers and associated constructions. Leveraging our new techniques, we provide the first post-quantum security proofs for the key-length extension scheme FX, the tweakable block ciphers LRW and XEX, and most block cipher encryption and authentication modes. Our techniques can be used for security proofs in both the plain model and the quantum ideal cipher model. Our work takes significant initial steps in establishing a rigorous understanding of the post-quantum security of practical symmetric-key cryptography.

Jung Hee Cheon, Jean–Pierre Tillich