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A generalization of the class of hyper-bent Boolean
functions in binomial forms
Chunming Tang · Yu Lou · Yanfeng Qi · Baocheng Wang · Yixian Yang
Received: date / Accepted: date
Abstract Bent functions, which are maximally nonlinear Boolean functions with even numbers of variables and whose Hamming distance to the set of all affine functions equals 2n−1±2n
2−1, were introduced by Rothaus in 1976 when he considered problems in combinatorics. Bent functions have been ex-tensively studied due to their applications in cryptography, such as S-box, block cipher and stream cipher. Further, they have been applied to coding theory, spread spectrum and combinatorial design. Hyper-bent functions, as a special class of bent functions, were introduced by Youssef and Gong in 2001, which have stronger properties and rarer elements. Many research focus on the construction of bent and hyper-bent functions. In this paper, we consider functions defined over F2n by fa,b(r) := Trn1(axr(2
m−1)
) + Tr41(bx
2n−1
5 ), where
n = 2m, m ≡ 2 (mod 4), a ∈ F2m and b ∈ F16. When r ≡ 0 (mod 5), we
characterize the hyper-bentness of fa,b(r). When r 6≡ 0 (mod 5), a∈ F2m and
(b+ 1)(b4+b+ 1) = 0, with the help of Kloosterman sums and the factoriza-tion ofx5+x+a−1, we present a characterization of hyper-bentness off(r)
a,b. Further, we give all the hyper-bent functions offa,b(r)in the casea∈F2m
2.
Chunming Tang, Yu Lou and Yanfeng Qi acknowledge support from the Natural Science Foundation of China (Grant No.10990011 & No.60763009). Baocheng Wang and Yixian Yang acknowledge support from National Science Foundation of China Innovative Grant (70921061), the CAS/SAFEA International Partnership Program for Creative Research Teams.
Chunming Tang, Yu Lou and Yanfeng Qi
Laboratory of Mathematics and Applied Mathematics, School of Mathematical Sciences, Peking University, Beijing, 100871, China
Chunming Tang’s e-mail: [email protected]
Baocheng Wang and Yixian Yang
Keywords Boolean functions · bent functions · hyper-bent functions ·
Walsh-Hadamard transformation·Kloosterman sums
1 Introduction
Bent functions are maximally nonlinear Boolean functions with even numbers of variables whose Hamming distance to the set of all affine functions equals 2n−1±2n2−1. These functions introduced by Rothaus [30] as interesting combi-natorial objects have been extensively studied for their applications not only in cryptography, but also in coding theory [4, 27] and combinatorial design. Some basic knowledge and recent results on bent functions can be found in [3, 12, 27]. A bent function can be considered as a Boolean function defined over Fn
2, F2m ×F2m (n = 2m) or F2n. Thanks to the different structures
of the vector space Fn
2 and the Galois field F2n, bent functions can be well
studied. Although some algebraic properties of bent functions are well known, the general structure of bent functions on F2n is not clear yet. As a result,
much research on bent functions on F2n can be found in [2, 7, 8, 10, 11, 13, 14,
21, 22, 25–29, 32]. Youssef and Gong [31] introduced a class of bent functions called hyper-bent functions, which achieve the maximal minimum distance to all the coordinate functions of all bijective monomials (i.e., functions of the form Trn1(axi) +, gcd(i,2n−1) = 1). However, the definition of hyper-bent functions was given by Gong and Golomb [15] by a property of the extend Hadamard transform of Boolean functions. Hyper-bent functions as special bent functions with strong properties are hard to characterize and many re-lated problems are open. Much research give the precise characterization of hyper-bent functions in certain forms.
The complete classification of bent and hyper-bent functions is not yet achieved. The monomial bent functions in the form Trn1(axs) are considered in [2, 21]. Leander [21] described the necessary conditions forssuch that Trn1(axs) is a bent function. In particular, when s = r(2m−1) and (r,2m+ 1) = 1, the monomial functions Trn1(axs) (i.e., the Dillon functions) were extensively studied in [7, 10, 21]. A class of quadratic functions overF2nin polynomial form
n
2−1
P
i=1
aiTrn1(x1+2
i
)+an
2Tr
n
2
1(x
n
2+1) (ai ∈F2) was described and studied in [9, 17– 19, 23, 32]. Dobbertin et al. [13] constructed a class of binomial bent functions of the form Trn1(a1xs1+a2xs2),(a1, a2)∈(
F∗2n)2 with Niho power functions.
Garlet and Mesanager [6] studied the duals of the Niho bent functions in [13]. In [25, 26, 29], Mesnager considered the binomial functions of the form Trn1(axr(2m−1)) + Tr2
1(bx
2n−1
3 ), where a ∈ F∗
2n and b ∈ F∗4. Then he gave
not of the Niho type. Charpin and Gong [7] presented a characterization of bentness of Boolean functions overF2nof the form P
r∈R
Trn1(arxr(2
m−1)
), where
R is a subset of the set of representatives of the cyclotomic cosets modulo 2m+ 1 of maximal size n. These functions include the well-known monomial functions with the Dillon exponent as a special case. Then they described the bentness of these functions with the Dickson polynomials. Mesnager et al. [27, 28] generalized the results of Charpin and Gong [7] and considered the bentness of Boolean functions overF2nof the form P
r∈R
Trn1(arxr(2
m−1)
) + Tr21(bx2n3−1),
wheren= 2m,ar∈F2m andb∈F4. Further, they presented the link between
the bentness of such functions and some exponential sums (involving Dickson polynomials).
In this paper, we consider a class of Boolean functions defined over F2n
by the form: fa,b(r) := Trn1(axr(2m−1)) + Tr41(bx2
n−1
5 ), where n = 2m, m ≡ 2 (mod 4), a∈ F2m and b ∈F16. When r= 1, this class of Boolean functions
is studied in [1]. Generally, it is elusive to give a characterization of bentness and hyper-bentness of Boolean functions. When r ≡ 0 (mod 5), the hyper-bentness of fa,b(r) is characterized in this paper. When r 6≡ 0 (mod 5) and (b+ 1)(b4+b+ 1 = 0) = 0, this paper presents the hyper-bentness off(r)
a,b by the factorization ofx5+x+a−1and Kloosterman sums. Fora∈
F2m2 , we give all the hyper-bent functionsfa,b(r).
The rest of paper is organized as follows. In Section 2, we give some notations and recall some basic knowledge for this paper. In Section 3, we study the hyper-bentness of the Boolean functions fa,b(r) for two cases (1) (b+ 1)(b4+b+ 1) = 0; (2)a∈F2m
2. Finally, Section 4 makes a conclusion.
2 Preliminaries
2.1 Boolean functions
Let n be a positive integer. Fn2 is a n-dimensional vector space defined over
finite fieldF2. Take two vectorsx= (x1,· · ·, xn) andy = (y1,· · · , xn) in Fn2.
Their dot product is defined by
hx, yi:= n
X
i=1 xiyi.
F2n is a finite field with 2n elements and F∗2n is the multiplicative group of
F2n. LetF2kbe a subfield ofF2n. The trace function fromF2ntoF2k, denoted
by Trnk, is a map defined as
Trnk(x) :=x+x2
k
Whenk= 1, Trn1 is called the absolute trace. The trace function Trnk satisfies the following properties.
Trnk(ax+by) =aTrnk(x) +bTrnk(y), a, b∈F2k, x, y∈F2n.
Trnk(x2k) = Trkn(x), x∈F2n.
WhenF2k ⊆F2r ⊆F2n, the trace function Trnk satisfies the following
transi-tivity property.
Trnk(x) = Trrk(Trrn(x)), x∈F2n.
A Boolean function overFn
2 orF2nis anF2-valued function. The absolute trace
function is a useful tool in constructing Boolean functions overF2n. From the
absolute trace function, a dot product overF2n is defined by
hx, yi:= Trn1(xy), x, y∈F2n.
A Boolean function overF2nis often represented by the algebraic normal form
(ANF):
f(x1,· · · , xn) =
X
I⊆{1,···,n} aI(
Y
i∈I
xi), aI ∈F2.
WhenI=∅, let Q
i∈I
= 1. The terms Q
i∈I
xiare called monomials. The algebraic
degree of a Boolean functionfis the globe degree of its ANF, that is, deg(f) := max{#(I)|aI = 06 }, where #(I) is the order ofI and #(∅) = 0.
Another representation of a Boolean function is of the form
f(x) =
2n−1 X
j=0 ajxj.
In order to make f a Boolean function, we should require a0, a2n−1 ∈ F2
and a2j =a2j, where 2j is taken modulo 2n−1. This makes that f can be represented by a trace expansion of the form
f(x) = X j∈Γn
Tro1(j)(ajxj) +(1 +x2
n−1
)
called its polynomial form, where
– Γn is the set of integers obtained by choosing one element in each cyclo-tomic class of 2 module 2n−1 (j is often chosen as the smallest element in its cyclotomic class, called the coset leader of the class);
– o(j) is the size of the cyclotomic coset of 2 modulo 2n−1 containingj;
– aj ∈F2o(j);
– =wt(f) (mod 2), where wt(f) := #{x∈F2n|f(x) = 1}.
Let wt2(j) be the number of 1’s in the binary expansion ofj. Then
deg(f) =
2.2 Bent and hyper-bent functions
The ”sign” function of a Boolean functionf is defined by
χ(f) := (−1)f.
Whenf is a Boolean function overFn2, the Walsh Hadamard transform off
is the discrete Fourier transform ofχ(f), whose value at w∈Fn2 is defined by
b
χf(w) :=
X
x∈Fn2
(−1)f(x)+hw,xi.
Whenf is a Boolean function overF2n, the Walsh Hadamard transform off
is defined by
b
χf(w) :=
X
x∈F2n
(−1)f(x)+Trn1(wx),
wherew∈F2n. Then we can define the bent functions.
Definition 1 A Boolean function f : F2n →F2 is called a bent function, if b
χf(w) =±2
n
2 (∀w∈F2n).
If f is a bent function, n must be even. Further, deg(f) ≤ n
2 [3].
Hyper-bent functions are an important subclass of Hyper-bent functions. The definition of hyper-bent functions is given below.
Definition 2 A bent function f :F2n →F2 is called a hyper-bent function,
if, for anyisatisfying (i,2n−1) = 1,f(xi) is also a bent function.
[4] and [31] proved that iff is a hyper-bent function, then deg(f) =n2. For a bent functionf, wt(f) is even. Then= 0, that is,
f(x) = X j∈Γn
Tro1(j)(ajxj).
If a Boolean function f is defined over F
2n2 ×F2n2, then we have a class of bent functions[10, 24].
Definition 3 The Maiorana-McFarland classMis the set of all the Boolean functionsf defined onF2n
2 ×F2n2 of the formf(x, y) =hx, π(y)i+g(y), where
x, y ∈ F2n
2, π is a permutation of F2n2 and g(x) is a Boolean function over
F2n
2.
For Boolean functions overF2n
2 ×F2n2, we have a class of hyper-bent functions
PSap[4].
Definition 4 Letn= 2m, thePSap class is the set of all the Boolean func-tions of the formf(x, y) =g(yx), wherex, y∈F2m andgis a balanced Boolean
functions (i.e., wt(f) = 2m−1) andg(0) = 0. Wheny= 0, let x y =xy
2n−2
= 0.
Each Boolean functionf inPSap satisfiesf(βz) =f(z) and f(0) = 0, where
β∈F∗mandz∈Fm×Fm. Youssef and Gong [31] studied these functions over
Proposition 1 Let n = 2m, α be a primitive element in F2n and f be a
Boolean function overF2n such thatf(α2 m+1
x) =f(x)(∀x∈F2n)andf(0) =
0, then f is a hyper-bent function if and only if the weight of (f(1),f(α), f(α2),· · ·,f(α2m))is2m−1.
Further, [4] proved the following result.
Proposition 2 Letf be a Boolean function defined in Proposition 1. Iff(1) = 0, then f is in PSap. If f(1) = 1, then there exists a Boolean functiong in
PSap andδ∈F∗2n satisfyingf(x) =g(δx).
LetPS#apbe the set of hyper-bent functions in the form ofg(δx), whereg(x)∈ PSap,δ∈F∗2n andg(δ) = 1. Charpin and Gong expressed Proposition 2 in a
different version below.
Proposition 3 Let n = 2m, α be a primitive element of F2n and f be a
Boolean function overF2nsatisfyingf(α2 m+1
x) =f(x) (∀x∈F2n)andf(0) =
0. Let ξbe a primitive2m+ 1-th root in
F∗2n. Then f is a hyper-bent function
if and only if the cardinality of the set {i|f(ξi) = 1,0≤i≤2m} is2m−1.
In fact, Dillon [10] introduced the Partial Spreads classPS−, which is a bigger class of bent functions thanPSap andPS#ap.
Theorem 1 Let Ei(i = 1,2,· · · , N) be N subspaces in F2n of dimension m
such that Ei ∩Ej = {0} for all i, j ∈ {1,· · · , N} with i 6= j. Let f be a
Boolean function over F2n. If the support off is given by supp(f) =
N
S
i=1 Ei∗,
whereEi∗=Ei\{0}, thenf is a bent function if and only ifN = 2m−1.
The set of all the functions in Theorem 1 is defined byPS−.
2.3 Kloosterman sums and Weil sums
The Kloosterman sums onF2n are:
Km(a) :=
X
x∈F2m
χ(Trm1(ax+1
x)), a∈F2m.
Some properties of Kloosterman sums are given by the following proposi-tion[16, 20].
Proposition 4 Let a∈F2m. ThenKm(a)∈[1−2(m+2)/2,1 + 2(m+2)/2]and
4|Km(a).
Quintic Weil sums onF2m are:
Qm(a) :=
X
x∈F2m
χ(Trm1 (a(x5+x3+x))), a∈F2m.
To determine the value of Qm(a), we should consider the factorization of the polynomialP(x) =x5+x+a−1. We write thatP(x) = (n1)r1(n2)r2· · ·(n
to indicate that ri of the irreducible factors of P(x) have degree ni. When
P(x) =x5+x+a−1is irreducible over F2m, the value ofQm(a) is related to
the parity of the quadratic formq(x) = Trm1 (x(ax4+ax2+a2x)).q(x) is the
quadratic form associated to the simplectic form:
< x, y >q:= Trm1 (x(ay
4+ay2+a2y) +y(ax4+ax2+a2x)),
which is non-degenerate. Then there exists a normal simplectic basise1,em1+1,
· · ·, em1, e2m1 (2m1 = m). If i 6≡ j( modm1), < ei, ej >q= 0. For any
i(1≤i≤m1),< ei, em1+i>q= 1. If #{i|q(ei) =q(em1+i) = 1,1≤i≤m1}
is even, then the quadratic form q(x) is called an even quadratic form and
Qm(a) = 2m1. If #{i|q(ei) = q(em1+i) = 1,1 ≤ i ≤ m1} is odd, then the
quadratic formq(x) is called a odd quadratic form andQm(a) =−2m1.
3 A generalization of the class of hyper-bent functions in binomial forms
In this section, we will discuss the hyper-bentness of fa,b(r)(x). We introduce some notations on character sums in [1]. Letξ=α2m−1
, thenU =< ξ >. Let
V =< ξ5>. Since 5|(2m+ 1),V is the subgroup of U and #V = 2m+1
5 . Let β=α2
n−1
5 .
For anyi∈F2m and an integeri, we define
Si=
X
v∈V
χ(Tr(aξi(2m−1)v))
=X
v∈V
χ(Tr(aξi(2m+1)−5i+3iv))
=X
v∈V
χ(Tr(aξ3iv)). (F rom ξ−5i∈V)
From the definition of Si, Si = Sj when i ≡ j (mod 5). Further, Si =
S−i(Lemma 1 [1]).
3.1 The hyper-bentness of Boolean functionsfa,b(5)(x)
In this subsection, we consider the hyper-bentness offa,b(r)(x) withr= 5 of the form
fa,b(5)(x) := Trn1(ax5(2m−1)) + Tr41(bx2
n−1
5 ), (1)
wheren= 2m,m≡2 (mod 4),a∈F2m andb∈F16.
Since m≡2 (mod 4), 2m+ 1≡0 (mod 5). For anyy ∈
F2m, y2 m−1
= 1. Then
fa,b(5)(α2m+1x) =fa,b(5)(x), x∈F2n,
whereαis a primitive element ofF2n. Further,f(5)
Proposition 5 Letfa,b(5)be the Boolean function defined by(1), wherea∈F2m
andb∈F16. Define the following character sum
Λ5(a, b) :=X u∈U
χ(fa,b(5)(u)) (2)
whereU is the group of all the2m+1-th root of unity inF2n, that is,U ={x∈
F2n|x2 m+1
= 1}. Thenfa,b(5)is a hyper-bent function if and only ifΛ5(a, b) = 1. Further, the hyper-bent functionfa,b(5) lies inPSap if and only if Tr41(b) = 0.
Proof Similar to the proof of Proposition 9 in [1], this proposition follows.
Proposition 6 Let n= 2m andm≡ ±2,±6 (mod 20), Ifb∈ {0}S {βi|i= 0,1,2,3,4}, then the Boolean functionfa,b(5) in(1)is not a hyper-bent function. Further, ifb∈F∗
16\{βi|0≤i≤4},f (5)
a,b is a hyper-bent function if and only if
X
v∈V
χ(Trn1(av)) = 1.
Proof From (2),
Λ5(a, b) =X u∈U
χ(fa,b(5)(u))
=X
u∈U
χ(Trn1(au5(2m−1)) + Tr41(bu2
n−1
5 ))
=X
u∈U
χ(Trn1(au5(2m−1)))χ(Tr41(bu2
n−1 5 )).
Note thatU =< ξ >,V =< ξ5>and
U =ξ0V [ξ1V [ξ2V[ξ3V [ξ4V. (3)
Then,
Λ5(a, b) =
4
X
i=0
X
v∈V
χ(Tr41(b(ξ
i
v)2
n−1
5 ))χ(Trn
1(a(ξ
i
v)5(2m−1)))
=
4
X
i=0
X
v∈V
χ(Tr41(b(ξ
i
v)2
n−1
5 ))χ(Trn
1(a(ξ 5i
Since (ξ5i)2m−1∈V andm≡ ±2,±6 (mod 20), (5(2m−1),#V) = (5,2m5+1) = 1. Thenv7−→(ξ5i)2m−1v5(2m−1)is a permutation of V. Hence,
Λ5(a, b) = 4
X
i=0
X
v∈V
χ(Tr41(b(ξiv)2
n−1
5 ))χ(Trn
1(av))
=
4
X
i=0
X
v∈V
χ(Tr41(bξ
i2n−1
5 ))χ(Trn
1(av))
=(
4
X
i=0
χ(Tr41(bξ
i2n−1 5 )))(X
v∈V
χ(Trn1(av))).
Sinceξ2
n−1 5 = (α2
m−1
)(2
m−1)(2m+1)
5 =β2
m−1
=β2m+1−2=β3, then
Λ5(a, b) =(
4
X
i=0
χ(Tr41(bβ 3i
))(X v∈V
χ(Trn1(av)))
=(
4
X
i=0
χ(Tr41(bβi))(
X
v∈V
χ(Trn1(av))). (5)
From (5), whenb= 0,Λ5(a,0) = 5 P
v∈V
χ(Trn1(av)). Hence,Λ5(a,0)6= 1. From
Proposition 5,fa,(5)0 is not a hyper-bent function.
Whenb6= 0,bcan be represented byb=ωβj, whereω3= 1 and 0≤j≤4.
Then
4
X
i=0
χ(Tr41(bβi)) =
4
X
i=0
χ(Tr41(ωβi+j)) =
4
X
i=0
χ(Tr41(ωβi))). (6)
Sinceω3= 1 andω4=ω,
Tr41(ωβi) = Tr41(ω4β4i) = Tr41(ωβ4i).
In particular, we takei= 1,2. Then
Tr41(ωβ) = Tr41(ωβ4), (7)
Tr41(ωβ2) = Tr41(ωβ3). (8)
Ifω= 1,
4
P
i=0
χ(Tr41(bβi) = P4
i=0
χ(Tr41(βi)). Sinceβsatisfiesβ4+β3+β2+β+1 =
0, Tr41(βi) = 1. Then 4
P
i=0
χ(Tr41(bβi) =−3. Therefore,
Λ5(a, b) =−3X v∈V
From Propsition 5,fa,β(5)j is not a hyper-bent function. When ω6= 1, we have
Tr41(ωβ)+Tr41(ωβ2) = Tr41(ω(β+β2)) =ω(β+β2+β3+β4)+ω2(β+β2+β3+β4) = 1.
Thenχ(Tr41(ωβ)) +χ(Tr41(ωβ2)) = 0. Similarly,χ(Tr4
1(ωβ3)) +χ(Tr 4
1(ωβ4)) =
0. Therefore,
Λ5(a, b) =X v∈V
χ(Trn1(av)), b=ωβj,0≤j≤4, ω3= 1, ω6= 1.
From Proposition 5, the second part of this proposition follows.
In Proposition 6, we consider the hyper-bentness of the Boolean function
fa,b(5) for m ≡ ±2,±6 (mod 20). The proposition below discusses the hyper-bentness offa,b(5) form≡10 (mod 20).
Proposition 7 Let n= 2m,m ≡10 (mod 20), a∈F2m, b ∈F16. then the
Boolean functionfa,b(5) in (1)is not a hyper-bent function.
Proof Note that
Λ5(a, b) =
4
X
i=0
X
v∈V
χ(Tr41(bξ
i2n−1
5 ))χ(Trn
1(a(ξ 5i
)2m−1v5(2m−1))).
Sincem≡10 (mod 20), 25|(2m+ 1) and (5(2m−1),2m+1
5 ) = 5. Thenv7−→ v5(2m−1)is 5 to 1 fromV toV5:={v5|v∈V}. Therefore,
Λ5(a, b) = 5
4
X
i=0
X
v∈V5
χ(Tr41(bξi2
n−1
5 ))χ(Trn
1(a(ξ
5i)2m−1v)).
Hence, 5|Λ5(a, b) and Λ5(a, b) is not equal to 1, From Proposition 5, fa,b(5) is not a hyper-bent function.
From Proposition 6,
X
v∈V
χ(Trn1(av)) = X v∈V
χ(Trn1(av2m−1)).
Note that P
v∈V
χ(Trn1(av)) =S0 in [1]. From Proposition 15 in [1],
X
v∈V
χ(Trn1(av)) =
1
5[1−Km(a) + 2Qm(a)]. (9)
Further, from Proposition 16 and 18 in [1], we have the following results.
Proposition 8 Letn= 2m,m≡ ±2,±6 (mod 20),m≥6andb∈F∗16\{βi|0≤ i≤4}, then fa,b(5) is a hyper-bent function if and only if one of the assertions
(1)and(2) holds.
(1) Qm(a) = 0,Km(a) =−4.
From Theorem 3 in [1], we have the following theorem.
Theorem 2 Letn= 2m,m≡ ±2,±6 (mod 20),m≥6andb∈F∗16\{βi|0≤ i ≤4}, thenfa,b(5) is a hyper-bent function if and only if one of the following assertions(1) and(2)holds.
(1) p(x) =x5+x+a−1 over
F2m is(1)(2)2 and Km(a) =−4. (2)p(x) =x5+x+a−1is irreducible over
F2m. The quadratic formq(x) =
Trm1 (x(ax4+ax2+a2x))over
F2m is even.Km(a) = 2·2m1−4.
3.2 The hyper-bentness offa,b(r)(x)
In the rest of the paper, we consider the Boolean function
fa,b(r)(x) := Trn1(axr(2
m−1)
) + Tr41(bx
2n−1
5 ), (10)
where n = 2m, m ≡ 2 (mod 4), a ∈ F2m and b ∈ F16. Then we define the
character sum
Λr(a, b) :=
X
u∈U
χ(fa,b(r)(u)). (11)
Similarly,fa,b(r)(x) is a hyper-bent function if and only ifΛr(a, b) = 1.
Theorem 3 Letn= 2m,m≡2 (mod 4),a∈F2mandb∈F16. If(r,2 m+1
5 )>
1, then fa,b(r) is not a hyper-bent function. Further, if(r,2m5+1) = 1, then
(1) If r≡0 (mod 5), then fa,b(r) andfa,b(5) have the same hyper-bentness.
(2) If r≡ ±1 (mod 5), thenfa,b(r)andfa,b(1) have the same hyper-bentness.
(3) If r≡ ±2 (mod 5), thenfa,b(r)andfa,b(2) have the same hyper-bentness.
Proof Note that
Λr(a, b) =
4
X
i=0
X
v∈V
χ(Tr41(b(ξiv)2
n−1
5 ))χ(Trn
1(a(ξ
iv)r(2m−1)
))
=
4
X
i=0
X
v∈V
χ(Tr41(bξi2
n−1
5 ))χ(Trn
1(aξ
ri(2m−1)
vr(2m−1))).
Letd:= (r(2m−1),#V) = (r,2m5+1), then
Λr(a, b) =d
4
X
i=0
χ(Tr41(bξi2
n−1 5 )) X
v∈Vd
χ(Trn1(aξri(2m−1)vr(2m−1))), (12)
where Vd := {vd|v ∈V}. If d= (r,2m+1
5 )>1, d|Λr(a, b) and Λr(a, b)6= 1.
Whend= (r,2m5+1) = 1,
Λr(a, b) =
4
X
i=0
χ(Tr41(bξi2
n−1 5 ))X
v∈V
χ(Trn1(aξri(2m−1)v)). (13)
Ifr≡0 (mod 5), from ξ2
n−1
5 =β3, we have
Λr(a, b) =
4
X
i=0
χ(Tr41(bβ3i))X v∈V
χ(Trn1(aξri(2m−1)v))
=
4
X
i=0
χ(Tr41(bβ3i))X v∈V
χ(Trn1(av))
=
4
X
i=0
χ(Tr41(bβi))X v∈V
χ(Trn1(av)).
Then Λr(a, b) = Λ5(a, b). Therefore, f
(r)
a,b and f
(5)
a,b have the same hyper-bentness.
Ifr≡1 (mod 5), then
Λr(a, b) =
4
X
i=0
χ(Tr41(bξi2
n−1
5 ))X v∈V
χ(Trn1(aξi(2m−1)v)).
From Proposition 10 in [1],Λr(a, b) =Λ1(a, b). Hence,f (r)
a,b and f
(1)
a,b have the same hyper-bentness.
Ifr≡2 (mod 5), then
Λr(a, b) =
4
X
i=0
χ(Tr41(bξi2
n−1
5 ))
X
v∈V
χ(Trn1(aξ2i(2m−1)v))
=
4
X
i=0
χ(Tr41(bβ3i))S2i
=
4
X
i=0
χ(Tr41(bβ9i))S6i
=
4
X
i=0
χ(Tr41(bβ4i))Si.
From Lemma 1 in [1], then
Λr(a, b) =χ(Tr41(b))S0+ (χ(Tr 4
1(bβ)) +χ(Tr 4 1(bβ
4)))S1+ (χ(Tr4 1(bβ
2)) +χ(Tr4 1(bβ
3)))S2.
(14)
Hence,Λr(a, b) =Λ2(a, b).f
(r)
a,b andf
(2)
Ifr≡3 (mod 5),
Λr(a, b) =
4
X
i=0
χ(Tr41(bξi2
n−1 5 ))X
v∈V
χ(Trn1(aξ3i(2m−1)v))
=
4
X
i=0
χ(Tr41(bβ 3i
))S3i
=
4
X
i=0
χ(Tr41(bβ
i ))Si.
From Lemma 1 in [1],
Λr(a, b) =χ(Tr41(b))S0+ (χ(Tr41(bβ)) +χ(Tr 4 1(bβ
4)))S
1+ (χ(Tr41(bβ
2)) +χ(Tr4 1(bβ
3)))S 2.
(15)
Hence,Λr(a, b) =Λ3(a, b). From (14) and (15),
Λ2(a, b) =Λ3(a, b).
fa,b(r) andfa,b(2) have the same hyper-bentness. Similarly, if r≡4 (mod 5),
Λr(a, b) =Λ4(a, b) =Λ1(a, b).
fa,b(r) andfa,b(1) have the same hyper-bentness. Above all, this theorem follows.
From Theorem 3, to characterize the hyper-bentness of fa,b(r), we just con-sider the hyper-bentness offa,b(1), fa,b(2) and fa,b(5). The hyper-bentness of fa,b(1) is considered in [1]. And the hyper-bentness offa,b(5)is discussed before. Next, we just study the hyper-bentness offa,b(2).
Whenb= 0, the hyper-bentness offa,(2)0is given in [2]. Then we just consider the caseb6= 0. We first give properties ofΛ2(a, b) in the following proposition.
Proposition 9 Let a∈F2m andb∈F∗16, then
(1) If b= 1, then Λ2(a, b) =S0−2(S1+S2) = 2S0−Λ2(a,0).
(2)Ifb∈ {β+β2, β+β3, β2+β4, β3+β4}, that is,bis a primitive element satisfyingTr41(b) = 0, thenΛ2(a, b) =S0.
(3) If b=β or β4, thenΛ
2(a, b) =−S0−2S2.
(4) If b=β2 orβ3, then Λ2(a, b) =−S0−2S1.
(5) If b= 1 +β or1 +β4, thenΛ2(a, b) =−S0+ 2S2.
(6) If b= 1 +β2 or1 +β3, thenΛ2(a, b) =−S0+ 2S1.
(7) If b=β+β4, thenΛ2(a, b) =S0+ 2S2−2S1.
(8) If b=β2+β3, thenΛ2(a, b) =S0−2S2+ 2S1.
Corollary 1 Leta∈F2m andb∈F∗16, then
(1) fa,b(2) andfa,b(1)2 have the same hyper-bentness.
(2) If b satisfies(b+ 1)(b4+b+ 1) = 0, thenfa,b(2) andfa,b(1) have the same hyper-bentness.
Proof (1) From Proposition 13 in [1] and Proposition 9,
Λ2(a, b2) =Λ1(a, b).
Hence,fa,b(2) andfa,b(1)2 have the same hyper-bentness. (2) Similarly, if bsatisfies (b+ 1)(b4+b+ 1) = 0,
Λ2(a, b) =Λ1(a, b).
Hence,fa,b(2) andfa,b(1) have the same hyper-bentness.
From the above discussion, we have the following result on fa,b(r).
Proposition 10 Leta∈F2m and(r,2 m+1
5 ) = 1, then
(1) If 15[1−Km(a) + 2Qm(a)] = 1, then the following Boolean functions (a) fa,b(r),b∈F∗16\{βi|i= 0,1,2,3,4},r≡0 (mod 5).
(b) fa,b(r),r6≡0 (mod 5),b4+b+ 1 = 0. are hyper-bent functions.
(2)If−1
5[3(1−Km(a))−4Qm(a)] = 1, then the Boolean functionf (r)
a,1 (r6≡0
(mod 5))is a hyper-bent function.
In fact, the converse proposition still holds.
Proof From Proposition 16 in [1] and Theorem 3, (9) and Proposition 6, this proposition follows.
We generalize Theorem 3 in [1] and get the following theorem.
Theorem 4 Letn= 2m,m= 2m1,m1≡1 (mod 2),m1≥3and(r,2m5+1) = 1, If one of two assertions (1)and(2) holds,
(1) p(x) =x5+x+a−1 overF2m is(1)(2)2 and Km(a) =−4.
(2)p(x) =x5+x+a−1is irreducible overF2m. The quadratic formq(x) =
Trm1 (x(ax4+ax2+a2x))over
F2m is even.Km(a) = 2·2m1−4.
Then the Boolean functions
(a) fa,b(r),b∈F∗16\{βi|i= 0,1,2,3,4},r≡0 (mod 5).
(b) fa,b(r),r6≡0 (mod 5),b4+b+ 1 = 0. are hyper-bent functions
In fact, the converse theorem still holds.
Proof From Proposition 16 and Theorem 3 in [1] and Proposition 10, this theorem follows.
Theorem 5 Let n = 2m,m = 2m1,m1 ≡1 (mod 2),m1≥3,(r,2m5+1) = 1 and r 6≡ 0 (mod 5), then fa,(r1) is a hyper-bent function if and only if the following assertions holds.
(1) p(x) =x5+x+a−1 is irreducible over F2m.
(2) The quadratic formq(x) = Trm1 (x(ax4+ax2+a2x))over
F2m is even.
(3) Km(a) =43(2−2m1).
In fact, the converse theorem still holds.
Proof From Proposition 16 and Theorem 2 in [1] and Proposition 10, this theorem follows.
Ifa∈F2m
2 , we have the hyper-bentness off
(r)
a,b in the theorem below.
Theorem 6 Let n = 2m, m = 2m1, m1 ≡1 (mod 2) andm1 ≥ 3. If n6=
12,28, any Boolean function in
{fa,b(r)|a∈F2m
2, b∈F16} (16)
is not a hyper-bent function. Further, if n= 12, all the hyper-bent functions in(16)are
Tr121 (axr(26−1)) + Tr41(bx2125−1),
wherer6≡0 (mod 5),(r,2m5+1) = 1,(a+ 1)(a3+a2+ 1) = 0andb=βi, i= 1,2,3,4. Ifn= 28, all the hyper-bent functions in (16)are
Tr281 (axr(214−1)) + Tr41(bx2285−1),
wherer6≡0 (mod 5),(r,2m5+1) = 1,(a+ 1)(a7+a6+a5+a4+a3+a2+ 1) = 0 andb=βi, i= 1,2,3,4.
Proof Note that a∈F2m
2. From Theorem 3, iff
(r)
a,b is a hyper-bent function, (r,2m5+1) = 1.
Suppose (r,2m+1
5 ) = 1. we first prove thatf (r)
a,0is not a hyper-bent function
when r ≡0 (mod 5). From Theorem 3, fa,b(r) is a hyper-bent function if and only iffa,b(5) is a hyper-bent function. If b= 0,
Λ5(a,0) = X u∈U
χ(Trn1(au5(2m−1))) = 5X v∈V
χ(Trn1(av2m−1)).
Hence, 5|Λ5(a,0) and Λ5(a,0) 6= 1. Therefore,fa,(5)0 is not a hyper-bent func-tion. Thenfa,(r0) is not a hyper-bent function.
When b 6= 0, from Theorem 4, fa,b(r) is a hyper-bent function if and only if fa,b(1)0 (b04+b0 + 1 = 0) is a hyper-bent function. From Theorem 5 in [1],
fa,b(1)0 (b04 +b0 + 1 = 0) is not a hyper-bent function. Hence, f
(r)
Then we discuss the caser≡ ±1 (mod 5) and (r,2m5+1) = 1. From Theo-rem 3,fa,b(r)is a hyper-bent function if and only iffa,b(1)is a hyper-bent function. From Theorem 5 in [1], there are only two cases. The first case isn= 12, where
aandbsatisfy
(a+ 1)(a3+a2+ 1) = 0, b=βi, i= 1,2,3,4.
The second case isn= 28, whereaandbsatisfy
(a+ 1)(a7+a6+a5+a4+a3+a2+ 1) = 0, b=βi, i= 1,2,3,4.
Whenr≡ ±2 (mod 5) and (r,2m5+1) = 1, we have similar results. Above all, this theorem follows.
4 Conclusion
This paper considers the hyper-bentness of the Boolean functions fa,b(r) of the form fa,b(r) := Tr1n(axr(2m−1)) + Tr4
1(bx
2n−1
5 ), wheren = 2m, m= 2 (mod 4),
a ∈F2m and b ∈F16. When r ≡0 (mod 5), we give the characterization of
hyper-bentness offa,b(r). Ifr6≡0 (mod 5) andb= 1 or bis a primitive element in F16 such that Tr41(b) = 0, the hyper-bentness of fa,b(r) can be characterized by Kloosterman sums and the factorization ofx5+x+a−1. Ifa∈F2m
2, with the results of [1], we prove thatfa,b(r)is not a hyper-bent function unlessn= 12 orn= 28. Further, we give all the hyper-bent functions forn= 12 orn= 28.
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