PARALLEL MULTIPLICATION IN F
2
n
USING CONDENSED
MATRIX REPRESENTATION
Christophe Negre
´
Equipe DALI, LP2A, Universit
´
e de Perpignan
avenue P. Alduy, 66 000 Perpignan, France
Keywords:
Finite field, multiplication, matrix representation, irreducible trinomial.
Abstract:
In this paper we explore a matrix representation of binary fields F
2
n
defined by an irreducible trinomial
P = X
n
+ X
k
+ 1. We obtain a multiplier with time complexity of T
A
+ (⌈log
2
(n)⌉)T
X
and space
complexity of (2n − 1)n AND and (2n − 1)(n − 1) XOR . This multiplier reaches the lower bound on time
complexity. Until now this was possible only for binary field defined by AOP (Silverman, 1999), which are
quite few. The interest of this multiplier remains theoretical since the size of the architecture is roughly two
times bigger than usual polynomial basis multiplier (Mastrovito, 1991; Koc and Sunar, 1999).
1 INTRODUCTION
A binary field F
2
n
= F
2
[X]/(P ) is a set of 2
n
ele-
ments in which we can do all the basic arithmetic op-
eration like addition, subtraction, multiplication and
inversion modulo an irreducible binary polynomial P .
Finite field arithmetic is widely used in cryptographic
applications (Miller, 1985) and error-correcting code
(Berlekamp, 1982). For these applications, the most
important finite field operation is the multiplication.
The representation of binary field elements have a
big influence on the efficiency of field arithmetic. Un-
til now, field elements were represented as sum of ba-
sis elements: the basis is composed by n elements
B
1
, . . . , B
n
∈ F
2
n
, in this situation an element U
in F
2
n
is written as U = u
1
B
1
, + · · · + u
n
B
n
with
u
i
∈ {0, 1}.
The most used bases are polynomial bases (Mas-
trovito, 1991; Koc and Sunar, 1999; Chang et al.,
2005) and normal bases (Wu and Hasan, 1998; Koc
and Sunar, 2001).
Our purpose here is to investigate a new represen-
tation: the matrix representation. We will focus on
field defined by a trinomial F
2
n
= F
2
[X]/(P ) with
P = X
n
+ X
k
+ 1. In the matrix representation an
element U of F
2
n
is represented by the n
2
coefficients
of a n × n matrix M
U
. The additions of two elements
U and V simply consists to add the two matrices M
U
and M
V
and to multiply U and V it consists to multi-
ply the matrix product M
U
· M
V
.
This gives a faster multiplication than multiplica-
tion using basis representation: a parallel multiplier
associated to a matrix representation has a time com-
plexity of T
A
+ (⌈log
2
(n)⌉)T
X
, whereas in basis rep-
resentation, for field defined by a trinomial (Koc and
Sunar, 1999; Mastrovito, 1991), the time complexity
is generally equal to T
A
+ (2 + ⌈log
2
(n)⌉)T
X
. The
major drawback of this method is due to the length of
the representation which requires n
2
coefficients, and
provides parallel multiplier with a cubic space com-
plexity in n. But if we carefully select a subset of the
matrix coefficients, the number of distinct coefficients
in each matrix M
U
becomes small: in our situation it
is equal to (2n − 1). In other words we condense the
matrix representation in (2n − 1) distinct coefficients
to decrease the space complexity.
The paper is organized as follows : in the first sec-
tion we recall the method of Koc and Sunar (Koc and
Sunar, 1999) for finite field multiplication modulo tri-
nomial. They perform the reduction modulo the trino-
mial on a matrix and then compute a matrix-vector to
get the product of two elements. In the second section
we study the possibility to use the matrix constructed
with Koc and Sunar’s method to represent finite field
elements. After that we evaluate the complexity of a
parallel multiplier in this matrix representation. Next,
we study a condensed matrix representation and the
associated multiplier. We finally give a small exam-
ple of our matrix multiplier and finish by a complexity
comparison and a brief conclusion.
254
Negre C. (2006).
PARALLEL MULTIPLICATION IN F2n USING CONDENSED MATRIX REPRESENTATION.
In Proceedings of the International Conference on Security and Cryptography, pages 254-259
DOI: 10.5220/0002096402540259
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