The multivariate method strikes again: New power functions with low differential uniformity in odd characteristic
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The multivariate method strikes again: New power functions with low differential uniformity in odd characteristic Patrick Felke1 Received: 19 September 2019 / Accepted: 3 May 2020 / © The Author(s) 2020
Abstract Let f (x) = x d be a power mapping over Fn and Ud the maximum number of solutions x ∈ Fn of f,c (x) := f (x + c) − f (x) = a, where c, a ∈ Fn and c = 0. f is said to be differentially k-uniform if Ud = k. The investigation of power functions with low differential uniformity over finite fields Fn of odd characteristic has attracted a lot of research interest since Helleseth, Rong and Sandberg started to conduct extensive computer search to identify such functions. These numerical results are well-known as the Helleseth-RongSandberg tables and are the basis of many infinite families of power mappings x dn , n ∈ N, of low uniformity (see e.g. Dobbertin et al. Discret. Math. 267, 95–112 2003; Helleseth et al. IEEE Trans. Inform Theory, 45, 475–485 1999; Helleseth and Sandberg AAECC, 8, 363–370 1997; Leducq Amer. J. Math. 1(3) 115–123 1878; Zha and Wang Sci. China Math. 53(8) 1931–1940 2010). Recently the crypto currency IOTA and Cybercrypt started to build computer chips around base-3 logic to employ their new ternary hash function Troika, which currently increases the cryptogrpahic interest in such families. Especially bijective power mappings are of interest, as they can also be employed in block- and stream ciphers. In this paper we contribute to this development and give a family of power mappings x dn with low uniformity over Fn , which is bijective for p ≡ 3 mod 4. For p = 3 this yields a family x dn with 3 ≤ Udn ≤ 4, where the family of inverses has a very simple description. These results explain “open entries” in the Helleseth-Rong-Sandberg tables. We apply the multivariate method to compute the uniformity and thereby give a self-contained introduction to this method. Moreover we will prove for a related family of low uniformity introduced in Helleseth and Sandberg (AAECC, 8 363–370 1997) that it yields permutations. Keywords Almost perfect nonlinear · Differential cryptanalysis · Differential uniformity · Differential spectrum · Perfect nonlinear · Power function · Exponential sums · Quadratic Character
This article belongs to the Topical Collection: Boolean Functions and Their Applications IV Guest Editors: Lilya Budaghyan and Tor Helleseth Patrick Felke
[email protected] 1
University of Applied Sciences Emden-Leer, Constantiaplatz 4, 26723 Emden, Germany
Cryptography and Communications
Mathematics Subject Classification (2010) 06E30 · 11T23 · 94A60 · 11L99 · 94A99
1 Introduction We assume that the reader is familiar with basic facts on finite fields. Lidl et al. [13] is a good reference. The finite field with pn elements is denoted by Fn . The cyclic group of invertible elements is denoted by F× n and a generator ω of this group is called a primitive element. Throughout this paper p denotes an odd prime. Definition 1.1 Let f be a mapping f : Fn → Fn . 1.
For c ∈ Fn the -map
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