Post-Quantum Security of Block Cipher Constructions
Gorjan Alagic, Chen Bai, Christian Majenz, Kaiyan Shi
TL;DR
This work lays the foundations for post-quantum security of block cipher constructions and provides the first formal PQ security proofs for FX, LRW, XEX2, and common block-cipher modes. By developing a quantum ideal cipher model (QICM) and a robust resampling technique, the authors establish tight bounds that quantify quantum advantages in key-extension, tweakable ciphers, and mode-based constructions. The results demonstrate that many classical security proofs extend to the post-quantum setting with carefully adjusted bounds, enabling secure deployment guidelines for symmetric-key primitives under quantum adversaries. The methodology also yields practical implications, including PQ-security assurances for lightweight ciphers like PRINCE and widely used disk-encryption and authentication modes. Overall, the paper advances a rigorous framework for evaluating and proving the post-quantum security of practical symmetric-key cryptography.
Abstract
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.