Claw Finding Algorithms Using Quantum Walk

The claw finding problem has been studied in terms of query complexity as one of the problems closely connected to cryptography. For given two functions, f and g, as an oracle which have domains of size N and M (N<=M), respectively, and the same range, the goal of the problem is to find x and y such that f(x)=g(y). This paper describes an optimal algorithm using quantum walk that solves this problem. Our algorithm can be generalized to find a claw of k functions for any constant integer k>1, where the domains of the functions may have different size.Comment: 12 pages. Introduction revised. A reference added. Weak lower bound delete

A Note About Claw Function with a Small Range

In the claw detection problem we are given two functions f:D ? R and g:D ? R (|D| = n, |R| = k), and we have to determine if there is exist x,y ? D such that f(x) = g(y). We show that the quantum query complexity of this problem is between ?(n^{1/2}k^{1/6}) and O(n^{1/2+?}k^{1/4}) when 2 ? k < n

Quantum All-Subkeys-Recovery Attacks on 6-round Feistel-2* Structure Based on Multi-Equations Quantum Claw Finding

Exploiting quantum mechanisms, quantum attacks have the potential ability to break the cipher structure. Recently, Ito et al. proposed a quantum attack on Feistel-2* structure (Ito et al.'s attack) based onthe Q2 model. However, it is not realistic since the quantum oracle needs to be accessed by the adversary, and the data complexityis high. To solve this problem, a quantum all-subkeys-recovery (ASR) attack based on multi-equations quantum claw-finding is proposed, which takes a more realistic model, the Q1 model, as the scenario, and only requires 3 plain-ciphertext pairs to quickly crack the 6-round Feistel-2* structure. First, we proposed a multi-equations quantum claw-finding algorithm to solve the claw problem of finding multiple equations. In addition, Grover's algorithm is used to speedup the rest subkeys recovery. Compared with Ito et al.'s attack, the data complexity of our attack is reduced from O(2^n) to O(1), while the time complexity and memory complexity are also significantly reduced.Comment: 18 pages, 4 figure

Faster Cryptographic Hash Function From Supersingular Isogeny Graphs

We propose a variant of the CGL hash, Charles et al. 2009, that is significantly faster than the original algorithm, and prove that it is preimage and collision resistant. For $n = \log p$ where $p$ is the characteristic of the finite field, the performance ratio between CGL and the new proposal is $(5.7n + 110) / (13.5\log n + 46.4)$. This gives an exponential speed up as the size of $p$ increases. Assuming the best quantum preimage attack on the hash has complexity $O(p^{\frac{1}{4}})$, we attain a concrete speed-up for a 256-bit quantum preimage security level by a factor 33.5. For a 384-bit quantum preimage security level, the speed-up is by a factor 47.8

Computational problems in supersingular elliptic curve isogenies

We present an overview of supersingular isogeny cryptography and how it fits into the broad theme of post-quantum public key crypto. The paper also gives a brief tutorial of elliptic curve isogenies and the computational problems relevant for supersingular isogeny crypto. Supersingular isogeny crypto is attracting attention due to the fact that the best attacks, both classical and quantum, require exponential time. However, the underlying computational problems have not been sufficiently studied by quantum algorithm researchers, especially since there are significant mathematical preliminaries needed to fully understand isogeny crypto. The main goal of the paper is to advertise various related computational problems, and to explain the relationships between them, in a way that is accessible to experts in quantum algorithms. This is a post-peer-review, pre-copyedit version of an article to be published as a perspective paper in the journal Quantum Information Processing

A post-quantum digital signature scheme based on supersingular isogenies

We present the first general-purpose digital signature scheme based on supersingular elliptic curve isogenies secure against quantum adversaries in the quantum random oracle model with small key sizes. This scheme is an application of Unruh’s construction of non-interactive zero-knowledge proofs to an interactive zero-knowledge proof proposed by De Feo, Jao, and Plut. We implement our proposed scheme on an x86- 64 PC platform as well as an ARM-powered device. We exploit the stateof-the-art techniques to speed up the computations for general C and assembly. Finally, we provide timing results for real world applications

Quantum Demiric-Selçuk Meet-in-the-Middle Attacks: Applications to 6-Round Generic Feistel Constructions

This paper shows that quantum computers can significantly speed-up a type of meet-in-the-middle attacks initiated by Demiric and Selçuk (DS-MITM attacks), which is currently one of the most powerful cryptanalytic approaches in the classical setting against symmetric-key schemes. The quantum DS-MITM attacks are demonstrated against 6 rounds of the generic Feistel construction supporting an $n$-bit key and an $n$-bit block, which was attacked by Guo et al. in the classical setting with data, time, and memory complexities of $O(2^{3n/4})$. The complexities of our quantum attacks depend on the adversary\u27s model and the number of qubits available. When the adversary has an access to quantum computers for offline computations but online queries are made in a classical manner (so called Q1 model), the attack complexities are $O(2^{n/2})$ classical queries, $O(2^n/q)$ quantum computations by using about $q$ qubits. Those are balanced at $\tilde{O}(2^{n/2})$, which significantly improves the classical attack. Technically, we convert the quantum claw finding algorithm to be suitable in the Q1 model. The attack is then extended to the case that the adversary can make superposition queries (so called Q2 model). The attack approach is drastically changed from the one in the Q1 model; the attack is based on 3-round distinguishers with Simon\u27s algorithm and then appends 3 rounds for key recovery. This can be solved by applying the combination of Simon\u27s and Grover\u27s algorithms recently proposed by Leander and May

Development of Cryptography since Shannon

This paper presents the development of cryptography since Shannon\u27s seminal paper ``Communication Theory of Secrecy Systems\u27\u27 in 1949

Efficient algorithms for supersingular isogeny Diffie-Hellman

We propose a new suite of algorithms that significantly improve the performance of supersingular isogeny Diffie-Hellman (SIDH) key exchange. Subsequently, we present a full-fledged implementation of SIDH that is geared towards the 128-bit quantum and 192-bit classical security levels. Our library is the first constant-time SIDH implementation and is up to 2.9 times faster than the previous best (non-constant-time) SIDH software. The high speeds in this paper are driven by compact, inversion-free point and isogeny arithmetic and fast SIDH-tailored field arithmetic: on an Intel Haswell processor, generating ephemeral public keys takes 46 million cycles for Alice and 54 million cycles for Bob, while computing the shared secret takes 44 million and 52 million cycles, respectively. The size of public keys is only 564 bytes, which is significantly smaller than most of the popular post-quantum key exchange alternatives. Ultimately, the size and speed of our software illustrates the strong potential of SIDH as a post-quantum key exchange candidate and we hope that these results encourage a wider cryptanalytic effort

Post-Quantum Elliptic Curve Cryptography

We propose and develop new schemes for post-quantum cryptography based on isogenies over elliptic curves. First we show that ordinary elliptic curves are have less than exponential security against quantum computers. These results were used as the motivation for De Feo, Jao and Pl\^ut's construction of public key cryptosystems using supersingular elliptic curve isogenies. We extend their construction and show that isogenies between supersingular elliptic curves can be used as the underlying hard mathematical problem for other quantum-resistant schemes. For our second contribution, we propose is an undeniable signature scheme based on elliptic curve isogenies. We prove its security under certain reasonable number-theoretic computational assumptions for which no efficient quantum algorithms are known. This proposal represents only the second known quantum-resistant undeniable signature scheme, and the first such scheme secure under a number-theoretic complexity assumption. Finally, we also propose a security model for evaluating the security of authenticated encryption schemes in the post-quantum setting. Our model is based on a combination of the classical Bellare-Namprempre security model for authenticated encryption together with modifications from Boneh and Zhandry to handle message authentication against quantum adversaries. We give a generic construction based on Bellare-Namprempre for producing an authenticated encryption protocol from any quantum-resistant symmetric-key encryption scheme together with any digital signature scheme or MAC admitting any classical security reduction to a quantum-computationally hard problem. We apply the results and show how we can explicitly construct authenticated encryption schemes based on isogenies