{"title":"Algebraic properties of the maps $$\\chi _n$$","authors":"Jan Schoone, Joan Daemen","doi":"10.1007/s10623-024-01395-w","DOIUrl":null,"url":null,"abstract":"<p>The Boolean map <span>\\(\\chi _n :\\mathbb {F}_2^n \\rightarrow \\mathbb {F}_2^n,\\ x \\mapsto y\\)</span> defined by <span>\\(y_i = x_i + (x_{i+1}+1)x_{i+2}\\)</span> (where <span>\\(i\\in \\mathbb {Z}/n\\mathbb {Z}\\)</span>) is used in various permutations that are part of cryptographic schemes, e.g., <span>Keccak</span>-f (the SHA-3-permutation), ASCON (the winner of the NIST Lightweight competition), Xoodoo, Rasta and Subterranean (2.0). In this paper, we study various algebraic properties of this map. We consider <span>\\(\\chi _n\\)</span> (through vectorial isomorphism) as a univariate polynomial. We show that it is a power function if and only if <span>\\(n=1,3\\)</span>. We furthermore compute bounds on the sparsity and degree of these univariate polynomials, and the number of different univariate representations. Secondly, we compute the number of monomials of given degree in the inverse of <span>\\(\\chi _n\\)</span> (if it exists). This number coincides with binomial coefficients. Lastly, we consider <span>\\(\\chi _n\\)</span> as a polynomial map, to study whether the same rule (<span>\\(y_i = x_i + (x_{i+1}+1)x_{i+2}\\)</span>) gives a bijection on field extensions of <span>\\(\\mathbb {F}_2\\)</span>. We show that this is not the case for extensions whose degree is divisible by two or three. Based on these results, we conjecture that this rule does not give a bijection on any extension field of <span>\\(\\mathbb {F}_2\\)</span>.</p>","PeriodicalId":11130,"journal":{"name":"Designs, Codes and Cryptography","volume":null,"pages":null},"PeriodicalIF":1.4000,"publicationDate":"2024-04-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Designs, Codes and Cryptography","FirstCategoryId":"100","ListUrlMain":"https://doi.org/10.1007/s10623-024-01395-w","RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q3","JCRName":"COMPUTER SCIENCE, THEORY & METHODS","Score":null,"Total":0}
引用次数: 0
Abstract
The Boolean map \(\chi _n :\mathbb {F}_2^n \rightarrow \mathbb {F}_2^n,\ x \mapsto y\) defined by \(y_i = x_i + (x_{i+1}+1)x_{i+2}\) (where \(i\in \mathbb {Z}/n\mathbb {Z}\)) is used in various permutations that are part of cryptographic schemes, e.g., Keccak-f (the SHA-3-permutation), ASCON (the winner of the NIST Lightweight competition), Xoodoo, Rasta and Subterranean (2.0). In this paper, we study various algebraic properties of this map. We consider \(\chi _n\) (through vectorial isomorphism) as a univariate polynomial. We show that it is a power function if and only if \(n=1,3\). We furthermore compute bounds on the sparsity and degree of these univariate polynomials, and the number of different univariate representations. Secondly, we compute the number of monomials of given degree in the inverse of \(\chi _n\) (if it exists). This number coincides with binomial coefficients. Lastly, we consider \(\chi _n\) as a polynomial map, to study whether the same rule (\(y_i = x_i + (x_{i+1}+1)x_{i+2}\)) gives a bijection on field extensions of \(\mathbb {F}_2\). We show that this is not the case for extensions whose degree is divisible by two or three. Based on these results, we conjecture that this rule does not give a bijection on any extension field of \(\mathbb {F}_2\).
期刊介绍:
Designs, Codes and Cryptography is an archival peer-reviewed technical journal publishing original research papers in the designated areas. There is a great deal of activity in design theory, coding theory and cryptography, including a substantial amount of research which brings together more than one of the subjects. While many journals exist for each of the individual areas, few encourage the interaction of the disciplines.
The journal was founded to meet the needs of mathematicians, engineers and computer scientists working in these areas, whose interests extend beyond the bounds of any one of the individual disciplines. The journal provides a forum for high quality research in its three areas, with papers touching more than one of the areas especially welcome.
The journal also considers high quality submissions in the closely related areas of finite fields and finite geometries, which provide important tools for both the construction and the actual application of designs, codes and cryptographic systems. In particular, it includes (mostly theoretical) papers on computational aspects of finite fields. It also considers topics in sequence design, which frequently admit equivalent formulations in the journal’s main areas.
Designs, Codes and Cryptography is mathematically oriented, emphasizing the algebraic and geometric aspects of the areas it covers. The journal considers high quality papers of both a theoretical and a practical nature, provided they contain a substantial amount of mathematics.