Improved key-reconciliation method

At PQ Crypto 2014, Peikert proposed efficient and practical lattice-based protocols for key transport, encryption and authenticated key exchange. One of the main technical innovations of this work is a reconciliation technique that allows two parties who approximately agree on a secret value to reach exact agreement, a setting common to essentially all lattice-based encryption schemes. Peikert\u27s reconciliation technique has been extended in the Frodo key exchange scheme, allowing for agreement on more than one bit. In both cases, only one reconciliation bit is required to reach exact agreement. As symmetric keys typically require many bits, say 128 or more, the parties compute multiple secret values, and reach exact agreement on each of those values individually. In this paper, we propose a reconciliation method that sends more than one reconciliation bit. In this way, the parties can agree on the same number of bits as with Peikert\u27s method with less stringent conditions on how approximate the approximate agreement must be. An instance of our method allows the two parties on a secret value that is one bit longer than with the previous methods, with virtually the same approximation requirements (i.e., with virtually the same security guarantees) as before. We numerically illustrate the advantages of our method with the impact to the instantiations of the Frodo scheme

Achieving secure and efficient lattice-based public-key encryption: the impact of the secret-key distribution

Lattice-based public-key encryption has a large number of design choices that can be combined in diverse ways to obtain different tradeoffs. One of these choices is the distribution from which secret keys are sampled. Numerous secret-key distributions exist in the state of the art, including (discrete) Gaussian, binomial, ternary, and fixed-weight ternary. Although the secret-key distribution impacts both the concrete security and the performance of the schemes, it has not been compared in a detailed way how the choice of secret-key distribution affects this tradeoff. In this paper, we compare different aspects of secret-key distributions from submissions to the NIST post-quantum standardization effort. We consider their impact on concrete security (influenced by the entropy and variance of the distribution), and on decryption failures and IND-CCA2 security (influenced by the probability of sampling keys with ``non average, large\u27\u27 norm). Next, we select concrete parameters of an encryption scheme instantiated with the above distributions %optimized for key sizes, to identify which distribution(s) offer the best tradeoffs between security and key sizes. The conclusions of the paper are: first, the above optimization shows that fixed-weight ternary secret keys result in the smallest key sizes in the analyzed scheme. The reason is that such secret keys reduce the decryption failure rate and hence allow for a higher noise-to-modulus ratio, alleviating the slight increase in lattice dimension required for countering specialized attacks that apply in this case. Second, compared to secret keys with independently sampled components, secret keys with a fixed composition (i.e., the number of secret key components equal to any possible value is fixed) result in the scheme becoming more secure against active attacks based on decryption failures

Results on polynomial interpolation with mixed modular operations and unknown moduli

Motivated by a recently introduced HIMMO key predistribution scheme, we investigate the limits of various attacks on the polynomial interpolation problem with mixedmodular operations and hidden moduli. We firstly review the classical attack and consider itin a quantum-setting. Then, we introduce new techniques for finding out the secret moduli and consider quantum speed-ups

spKEX: An optimized lattice-based key exchange

The advent of large-scale quantum computers has resulted in significant interest in quantum-safe cryptographic primitives. Lattice-based cryptography is one of the most attractive post-quantum cryptographic families due to its well-understood security, efficient operation and versatility. However, LWE-based schemes are still relatively bulky and slow. In this work, we present spKEX, a forward-secret, post-quantum, unauthenticated lattice-based key-exchange scheme that combines four techniques to optimize performance. spKEX relies on Learning with Rounding (LWR) to reduce bandwidth; it uses sparse and ternary secrets to speed up computations and reduce failure probability; it applies an improved key reconciliation scheme to reduce bandwidth and failure probability; and computes the public matrix A by means of a permutation to improve performance while allowing for a fresh A in each key exchange. For a quantum security level of 128 bits, our scheme requires 30% lesser bandwidth than the LWE-based key-exchange proposal Frodo [9] and allows for a fast implementation of the key exchange

HIMMO - A lightweight collusion-resistant key predistribution scheme

In this paper we introduce HIMMO as a truly practical and lightweight collusion-resistant key predistribution scheme. The scheme is reminiscent ofBlundo et al\u27s elegant key predistribution scheme, in which the master key is a symmetric bivariate polynomial over a finite field, and a unique common key is defined for every pair of nodes as the evaluation of the polynomial at the finite field elements associated with the nodes. Unlike Blundo et al\u27s scheme, however, which completely breaks down once the number of colluding nodes exceeds the degree of the polynomial, the new scheme is designed to tolerateany number of colluding nodes. Key establishment in HIMMO amounts to the evaluation of a single low-degree univariate polynomial involving reasonably sized numbers, thus exhibiting excellent performance even for constrained devices such as 8-bit CPUs, as we demonstrate. On top of this, the scheme is very versatile, as it not only supports implicit authentication of the nodes like any key predistribution scheme, but also supports identity-based key predistribution in a natural and efficient way. The latter property derives from the fact that HIMMO supports long node identifiers at a reasonable cost, allowing outputs of a collision-resistant hash function to be used as node identifiers. Moreover, HIMMO allows for a transparent way to split the master key between multiple parties. The new scheme is superior to any of the existing alternatives due to the intricate way it combines the use of multiple symmetric bivariate polynomials evaluated over ``different\u27\u27 finite rings. We have extensively analyzed the security of HIMMO against two attacks. For these attacks, we have identified the Hiding Information (HI) problem and the Mixing Modular Operations (MMO) problem as the underlying problems. These problems are closely related to some well-defined lattice problems, and therefore the best attacks on HIMMO are dependent on lattice-basis reduction. Based on these connections, we propose concrete values for all relevant parameters, for which we conjecture that the scheme is secure

Shorter Messages and Faster Post-Quantum Encryption with Round5 on Cortex M

Round5 is a Public Key Encryption and Key Encapsulation Mechanism (KEM) based on General Learning with Rounding (GLWR), a lattice problem. We argue that the ring variant of GLWR is better suited for embedded targets than the more common RLWE (Ring Learning With Errors) due to significantly shorter keys and messages. Round5 incorporates GLWR with error correction, building on design features from NIST Post-Quantum Standardization candidates Round2 and Hila5. The proposal avoids Number Theoretic Transforms (NTT), allowing more flexibility in parameter selection and making it simpler to implement. We discuss implementation techniques of Round5 ring variants and compare them to other NIST PQC candidates on lightweight Cortex M4 platform. We show that the current development version of Round5 offers not only the shortest key and ciphertext sizes among Lattice-based candidates, but also has leading performance and implementation size characteristics

Round2: KEM and PKE based on GLWR

Cryptographic primitives that are secure against quantum computing are receiving growing attention with recent, steady advances in quantum computing and standardization initiatives in post-quantum cryptography by NIST and ETSI. Lattice-based cryptography is one of the families in post-quantum cryptography, demonstrating desirable features such as well-understood security, efficient performance, and versatility. In this work, we present Round2 that consists of a key-encapsulation mechanism and a public-key encryption scheme. Round2 is based on the General Learning with Rounding problem, that unifies the Learning with Rounding and Ring Learning with Rounding problems. Round2\u27s construction using the above problem allows for a unified description and implementation. The key-encapsulation mechanism and public-key encryption scheme furthermore share common building blocks, simplifying (security and operational) analysis and code review. Round2\u27s reliance on prime cyclotomic rings offers a large design space that allows fine-tuning of parameters to required security levels. The use of rounding reduces bandwidth requirements and the use of sparse-trinary secrets improves CPU performance and decryption success rates. Finally, Round2 includes various approaches of refreshing the system public parameter A, allowing efficient ways of preventing precomputation and back-door attacks