# Quantum-Secure Symmetric-Key Cryptography Based on Hidden Shifts

### Abstract

Recent results of Kaplan et al., building on work by Kuwakado and Morii, have shown that a wide variety of classically-secure symmetric-key cryptosystems can be completely broken by quantum chosen-plaintext attacks (qCPA). In such an attack, the quantum adversary has the ability to query the cryptographic functionality in superposition. The vulnerable cryptosystems include the Even-Mansour block cipher, the three-round Feistel network, the Encrypted-CBC-MAC, and many others. In this article, we study simple algebraic adaptations of such schemes that replace $(Z/2)^n$ addition with operations over alternate finite groups--such as $(Z/2)^n$--and provide evidence that these adaptations are qCPA-secure. These adaptations furthermore retain the classical security properties and basic structural features enjoyed by the original schemes. We establish security by treating the (quantum) hardness of the well-studied Hidden Shift problem as a cryptographic assumption. We observe that this problem has a number of attractive features in this cryptographic context, including random self-reducibility, hardness amplification, and--in many cases of interest--a reduction from the search version'' to the decisional version.'' We then establish, under this assumption, the qCPA-security of several such Hidden Shift adaptations of symmetric-key constructions. We show that a Hidden Shift version of the Even-Mansour block cipher yields a quantum-secure pseudorandom function, and that a Hidden Shift version of the Encrypted CBC-MAC yields a collision-resistant hash function. Finally, we observe that such adaptations frustrate the direct Simon's algorithm-based attacks in more general circumstances, e.g., Feistel networks and slide attacks.

Publication
36th Annual International Conference on the Theory and Applications of Cryptographic Techniques (EUROCRYPT 2017)
Date