Notes on Lattice-Based Cryptography

Asymmetrisk kryptering er avhengig av antakelsen om at noen beregningsproblemer er vanskelige å løse. I 1994 viste Peter Shor at de to mest brukte beregningsproblemene, nemlig det diskrete logaritmeproblemet og primtallsfaktorisering, ikke lenger er vanskelige å løse når man bruker en kvantedatamaskin. Siden den gang har forskere jobbet med å finne nye beregningsproblemer som er motstandsdyktige mot kvanteangrep for å erstatte disse to. Gitterbasert kryptografi er forskningsfeltet som bruker kryptografiske primitiver som involverer vanskelige problemer definert på gitter, for eksempel det korteste vektorproblemet og det nærmeste vektorproblemet. NTRU-kryptosystemet, publisert i 1998, var et av de første som ble introdusert på dette feltet. Problemet Learning With Error (LWE) ble introdusert i 2005 av Regev, og det regnes nå som et av de mest lovende beregningsproblemene som snart tas i bruk i stor skala. Å studere vanskelighetsgraden og å finne nye og raskere algoritmer som løser den, ble et ledende forskningstema innen kryptografi. Denne oppgaven inkluderer følgende bidrag til feltet: - En ikke-triviell reduksjon av Mersenne Low Hamming Combination Search Problem, det underliggende problemet med et NTRU-lignende kryptosystem, til Integer Linear Programming (ILP). Særlig finner vi en familie av svake nøkler. - En konkret sikkerhetsanalyse av Integer-RLWE, en vanskelig beregningsproblemvariant av LWE, introdusert av Gu Chunsheng. Vi formaliserer et meet-in-the-middle og et gitterbasert angrep for denne saken, og vi utnytter en svakhet ved parametervalget gitt av Gu, for å bygge et forbedret gitterbasert angrep. - En forbedring av Blum-Kalai-Wasserman-algoritmen for å løse LWE. Mer spesifikt, introduserer vi et nytt reduksjonstrinn og en ny gjetteprosedyre til algoritmen. Disse tillot oss å utvikle to implementeringer av algoritmen, som er i stand til å løse relativt store LWE-forekomster. Mens den første effektivt bare bruker RAM-minne og er fullt parallelliserbar, utnytter den andre en kombinasjon av RAM og disklagring for å overvinne minnebegrensningene gitt av RAM. - Vi fyller et tomrom i paringsbasert kryptografi. Dette ved å gi konkrete formler for å beregne hash-funksjon til G2, den andre gruppen i paringsdomenet, for Barreto-Lynn-Scott-familien av paringsvennlige elliptiske kurver.Public-key Cryptography relies on the assumption that some computational problems are hard to solve. In 1994, Peter Shor showed that the two most used computational problems, namely the Discrete Logarithm Problem and the Integer Factoring Problem, are not hard to solve anymore when using a quantum computer. Since then, researchers have worked on finding new computational problems that are resistant to quantum attacks to replace these two. Lattice-based Cryptography is the research field that employs cryptographic primitives involving hard problems defined on lattices, such as the Shortest Vector Problem and the Closest Vector Problem. The NTRU cryptosystem, published in 1998, was one of the first to be introduced in this field. The Learning With Error (LWE) problem was introduced in 2005 by Regev, and it is now considered one of the most promising computational problems to be employed on a large scale in the near future. Studying its hardness and finding new and faster algorithms that solve it became a leading research topic in Cryptology. This thesis includes the following contributions to the field: - A non-trivial reduction of the Mersenne Low Hamming Combination Search Problem, the underlying problem of an NTRU-like cryptosystem, to Integer Linear Programming (ILP). In particular, we find a family of weak keys. - A concrete security analysis of the Integer-RLWE, a hard computational problem variant of LWE introduced by Gu Chunsheng. We formalize a meet-in-the-middle attack and a lattice-based attack for this case, and we exploit a weakness of the parameters choice given by Gu to build an improved lattice-based attack. - An improvement of the Blum-Kalai-Wasserman algorithm to solve LWE. In particular, we introduce a new reduction step and a new guessing procedure to the algorithm. These allowed us to develop two implementations of the algorithm that are able to solve relatively large LWE instances. While the first one efficiently uses only RAM memory and is fully parallelizable, the second one exploits a combination of RAM and disk storage to overcome the memory limitations given by the RAM. - We fill a gap in Pairing-based Cryptography by providing concrete formulas to compute hash-maps to G2, the second group in the pairing domain, for the Barreto-Lynn-Scott family of pairing-friendly elliptic curves.Doktorgradsavhandlin

Efficient hash maps to G2 on BLS curves

When a pairing e:G1×G2→GT, on an elliptic curve E defined over a finite field Fq, is exploited for an identity-based protocol, there is often the need to hash binary strings into G1 and G2. Traditionally, if E admits a twist E~ of order d, then G1=E(Fq)∩E[r], where r is a prime integer, and G2=E~(Fqk/d)∩E~[r], where k is the embedding degree of E w.r.t. r. The standard approach for hashing into G2 is to map to a general point P∈E~(Fqk/d) and then multiply it by the cofactor c=#E~(Fqk/d)/r. Usually, the multiplication by c is computationally expensive. In order to speed up such a computation, two different methods—by Scott et al. (International conference on pairing-based cryptography. Springer, Berlin, pp 102–113, 2009) and by Fuentes-Castaneda et al. (International workshop on selected areas in cryptography)—have been proposed. In this paper we consider these two methods for BLS pairing-friendly curves having k∈{12,24,30,42,48}, providing efficiency comparisons. When k=42,48, the application of Fuentes et al. method requires expensive computations which were infeasible for the computational power at our disposal. For these cases, we propose hashing maps that we obtained following Fuentes et al. idea.publishedVersio

Improved Estimation of Key Enumeration with Applications to Solving LWE

In post-quantum cryptography (PQC), Learning With Errors (LWE) is one of the dominant underlying mathematical problems. For example, in NIST\u27s PQC standardization process, the Key Encapsulation Mechanism (KEM) protocol chosen for standardization was Kyber, an LWE-based scheme. Recently the dual attack surpassed the primal attack in terms of concrete complexity for solving the underlying LWE problem for multiple cryptographic schemes, including Kyber. The dual attack consists of a reduction part and a distinguishing part. When estimating the cost of the distinguishing part, one has to estimate the expected cost of enumerating over a certain number of positions of the secret key. Our contribution consists of giving a polynomial-time approach for calculating the expected complexity of such an enumeration procedure. This allows us to revise the complexity of the dual attack on the LWE-based protocols Kyber, Saber and TFHE. For all these schemes we improve upon the total bit-complexity in both the classical and the quantum setting. As our method of calculating the expected cost of enumeration is fairly general, it might be of independent interest in other areas of cryptography or even in other research areas

Further Improvements of the Estimation of Key Enumeration with Applications to Solving LWE

In post-quantum cryptography, Learning With Errors (LWE) is one of the dominant underlying mathematical problems. The dual attack is one of the main strategies for solving the LWE problem, and it has recently gathered significant attention within the research community. A series of studies initially suggested that it might be more efficient than the other main strategy, the primal attack. Then, a subsequent work by Ducas and Pulles (Crypto’23) raised doubts on the estimated complexity of such an approach. The dual attack consists of a reduction part and a distinguishing part. When estimating the cost of the distinguishing part, one has to estimate the expected cost of enumerating over a certain number of positions of the secret key. Our contribution consists of giving a polynomial-time approach for calculating the expected complexity of such an enumeration procedure. This allows us to decrease the estimated cost of this procedure and, hence, of the whole attack both classically and quantumly. In addition, we explore different enumeration strategies to achieve some further improvements. Our work is independent from the questions raised by Ducas and Pulles, which do not concern the estimation of the enumeration procedure in the dual attack. As our method of calculating the expected cost of enumeration is fairly general, it might be of independent interest in other areas of cryptanalysis or even in other research areas

The Mersenne Low Hamming Combination Search Problem can be reduced to an ILP Problem

In 2017, Aggarwal, Joux, Prakash, and Santha proposed an innovative NTRU-like public-key cryptosystem that was believed to be quantum resistant, based on Mersenne prime numbers q = 2^N-1. After a successful attack designed by Beunardeau, Connolly, Geraud, and Naccache, the authors revised the protocol which was accepted for Round 1 of the Post-Quantum Cryptography Standardization Process organized by NIST. The security of this protocol is based on the assumption that a so-called Mersenne Low Hamming Combination Search Problem (MLHCombSP) is hard to solve. In this work, we present a reduction of MLHCombSP to Integer Linear Programming (ILP). This opens new research directions for assessing the concrete robustness of such cryptosystem. In particular, we uncover a new family of weak keys, for whose our attack runs in polynomial time

New Public-Key Crypto-System EHT

In this note, an LWE problem with a hidden trapdoor is introduced. It is used to construct an efficient public-key crypto-system EHT. The new system is significantly different from LWE based NIST candidates like FrodoKEM. The performance of EHT compares favorably with FrodoKEM

Improvements on making BKW practical for solving LWE

The learning with errors (LWE) problem is one of the main mathematical foundations of post-quantum cryptography. One of the main groups of algorithms for solving LWE is the Blum–Kalai–Wasserman (BKW) algorithm. This paper presents new improvements of BKW-style algorithms for solving LWE instances. We target minimum concrete complexity, and we introduce a new reduction step where we partially reduce the last position in an iteration and finish the reduction in the next iteration, allowing non-integer step sizes. We also introduce a new procedure in the secret recovery by mapping the problem to binary problems and applying the fast Walsh Hadamard transform. The complexity of the resulting algorithm compares favorably with all other previous approaches, including lattice sieving. We additionally show the steps of implementing the approach for large LWE problem instances. We provide two implementations of the algorithm, one RAM-based approach that is optimized for speed, and one file-based approach which overcomes RAM limitations by using file-based storage.publishedVersio

Lattice Isomorphism as a Group Action and Hard Problems on Quadratic Forms

Group actions have been used as a foundation in Public-key Cryptography to provide a framework for hard problems and assumptions. In this work we formalize the Lattice Isomorphism Problem (LIP) within the context of cryptographic group actions. We show that a quadratic number of queries to a randomized oracle outputting LIP instances sharing the same secret is enough for inverting the group action in polynomial time. We use this result to uncover a family of weak isomorphisms and to derive two new hard problems equivalent to LIP for quadratic forms with trivial automorphism group

SoK: Methods for Sampling Random Permutations in Post-Quantum Cryptography

In post-quantum cryptography, permutations are frequently employed to construct cryptographic primitives. Careful design and implementation of sampling random unbiased permutations is essential for efficiency and protection against side-channel attacks. Nevertheless, there is a lack of systematic research on this topic. Our work seeks to fill this gap by studying the most prominent permutation sampling algorithms and assessing their advantages and limitations. We combine theoretical and experimental comparisons and provide a C library with the implementations of the algorithms discussed. Furthermore, we introduce a new sampling algorithm tailored for cryptographic applications

Enabling PERK on Resource-Constrained Devices

PERK is a digital signature scheme submitted to the recent NIST Post-Quantum Cryptography Standardization Process for Additional Digital Signature Schemes. For NIST security level I, PERK features sizes ranging from 6kB to 8.5kB, encompassing both the signature and public key, depending on the parameter set. Given its inherent characteristics, PERK\u27s signing and verification algorithms involve the computation of numerous large objects, resulting in substantial stack-memory consumption ranging from 300kB to 1.5MB for NIST security level I and from 1.1MB to 5.7MB for NIST security level V. In this paper, we present a memory-versus-performance trade-off strategy that significantly reduces PERK\u27s memory consumption to a maximum of approximately 82kB for any security level, enabling PERK to be executed on resource-constrained devices. Additionally, we explore various optimizations tailored to the Cortex M4 and introduce the first implementation of PERK designed for this platform