Method for the Simultaneous Generation of Two Nonlinear Pseudo
Random Sequences: 5-ary and Binary
ZHANETA N. SAVOVA1, ANTONIYA T. TASHEVA2, ROSEN A. BOGDANOV3
1Computer Systems and Technologies Department,
Faculty of Artillery, Air Defense and Communication and Information Systems,
National Military University,
1 Karel Shkorpil Street, Shumen, 9700,
BULGARIA
2Computer Systems Department,
Faculty of Computer Systems and Technologies,
Technical University of Sofia, 8 Kliment Ohridski Blvd, Sofia, 1000,
BULGARIA
3Communication Networks and Systems Department
Faculty of Artillery, Air Defense and Communication and Information Systems,
National Military University,
1 Karel Shkorpil Street, Shumen, 9700,
BULGARIA
Abstract: - Multi-level signals and sequences have become a significant aspect of modern high-speed
communication systems. Hence, to ensure the confidentiality and integrity of the transmitted information,
advanced methods and devices are necessary to produce strong cryptographic properties for not only binary but
also for nonbinary keystreams, which can be used in resource-constrained microcontrollers. The proposed
method and apparatus generate both a balanced nonlinear 5-ary pseudo-random sequence and a binary
keystream sequence. The nonlinearity is determined by applying shrinking and multiplexing techniques to the
generated linear 5-ary pseudo-random sequence and the running zero in its conversion to a binary balanced
sequence.
Key-Words: - Nonbinary Pseudo Random Sequence, Nonlinear Pseudo Random Number Generation, pLFSR,
Cryptography, Multilevel Signals, PAM-5.
Received: April 23, 2022. Revised: August 21, 2023. Accepted: November 15, 2023. Published: December 31, 2023.
1 Introduction
Encryption is an effective way to protect sensitive
information while it is stored on media or
transmitted over an untrusted communication
channel over a network, allowing the information to
be read and processed only by the authorized
entities for which it is intended. Two basic types of
cryptographic algorithms, also known as ciphers, are
used in cryptography: symmetric ciphers and
asymmetric ciphers. Symmetric ciphers, in turn, are
classified as block ciphers and stream ciphers.
Stream ciphers are very fast and simple because
they are generated by performing a bitwise XOR
operation on a pseudo-random sequence of bits,
called a keystream, and the information which has
to be protected. Stream ciphers effectively
implement the principles of the One Time Pad
cipher, which is considered secure from an
information theory perspective.
Various types of stream ciphers generated using
block ciphers or Linear Feedback Shift Registers
(LFSR) have been developed. Pseudo-random
generators based on block ciphers have high
computational complexity and are not suitable for
application in resource constrained devices. On the
other hand, LFSR registers offer a fast and efficient
method for generating a linear pseudo-random
sequence. To be applicable in cryptography,
additional means of ensuring nonlinearity in the
output sequence are applied to them. The current
research is in the use of structures based on LFSR
registers: filter generators, combinational
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Zhaneta N. Savova, Antoniya T. Tasheva,
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generators, or clock-controlled generators, and in
the use of generators in finite fields, [1], [2], [3]. For
instance, modern 3GPP over-the-air standards
employ such structures to provide security
technologies: confidentiality and integrity. For
example, the word-oriented stream cipher SNOW
3G, [4], is utilized for both encryption and integrity
purposes in UMTS, Extended Coverage GSM for
IoT (EC-GSM-IoT), and 5G-NR. Another example
is the stream ciphers ZUC, [5], [6], which is applied
in LTE and 5G-NR.
Multilevel signaling has played a crucial role in
enhancing the data transfer rates that current wire
infrastructures can support. It provides high-speed
symbol transfer rates while keeping the observed
line rates relatively low. For example, a version
known as PAM-5, which uses 5 levels of Pulse
Amplitude Modulation, enables Gigabit Ethernet
(GbE) to achieve 1Gbps data rates and 10-10 or
lower bit error rates, [7]. A new PWAM signaling
scheme is proposed in, [8], which combines dual-
mode PAM-5 and PWM-2 to enhance high-speed
data transmission capabilities by increasing the
minimum pulse width, in comparison to the
traditional PWAM scheme.
In, [9], is introduced a new method 4D PAM-7
for data center applications, involving a
combination of trellis coded modulation and seven-
level pulse amplitude modulation (PAM-7).
Nowadays, the term „Automotive Ethernet“, [10]
primarily refers to in-vehicle networking, which
encompasses communication between the different
Electronic Control Units (ECUs) within a car. The
authors state that two automotive Ethernet Physical
layer technologies were developed, namely
100BASE-T1 and 1000BASE-T1. The current
standard 802.3ab-1999 (CL40) for 1000BASE-T
uses Four-Dimensional Five Level Pulse Amplitude
Modulation (4D-PAM-5) and 802.3bp-2016 for
1000BASE-T1 uses PAM-3 idle symbols.
The paper compares PAM-N (N = 4, 5, 6, 7, and
8) optical signals operating at 103.12 Gbps, [11].
From the results, the PAM-5 signal is more suitable
than other PAM-N signals considering the effect of
chromatic dispersion for 10km single-mode fiber
transmission using LAN-WDM (Wavelength-
Division Multiplexing) wavelength.
As can be seen from the above, multi-level
signals and sequences are emerging as a prominent
feature in today's high-speed communication
systems. Therefore, to ensure the security of these
multi-level sequences, advanced methods and
devices are required to generate not only binary but
also nonbinary keystreams with strong
cryptographic properties for use in embedded
applications utilizing microcontrollers with low
computational power and small memory.
In the context of improving higher education in
security and defense, [12], innovative practical
solutions would increase the knowledge of academic
staff and students in the field of pseudo-random
sequences and their mathematical principles.
2 Problem Formulation
In this section, we give a brief description of the
problem formulation and some preliminaries that we
use for the solution.
To simultaneously generate both binary and
nonbinary keystreams, we make use of the
mathematical background of the extended Galois
Field GF(pn) with an arbitrary prime number p, in
conjunction with the properties of devices that
generate p-ary linear recurrence sequences. The
implementation of finite Galois fields in
cryptographic algorithms is motivated by their
ability to perform precise calculations without any
rounding.
Linear Feedback Shift Registers (LFSR) based
on a Galois Field GF(p) with a base p other than 2
can be used to generate non-binary linear recurrent
sequences. For instance, the ternary and quinary
recurrent sequences can form the foundation of
devices that generate ternary and quinary keystream
sequences. These sequences are suitable for
synchronous stream ciphers within multi-level PAM
signals that have 3 and 5 levels, correspondingly.
In the following third section, we present
techniques that transform a linear sequence into a
nonlinear one to obtain cryptographically secure
nonlinear key sequences.
2.1 Mathematical Background of Extended
Galois Field GF(pn)
If the order of the Galois Field GF(q) can be
expressed as a power of the prime p (q = pn), where
n is a positive integer greater than or equal to 2, then
the field GF(q) is considered as an extension of the
Galois Field GF(p) with a degree of n. In such cases,
the notation Extended Galois Field GF(pn) is
applied.
To create an Extended Galois Field GF(pn), an
irreducible polynomial p(x) over GF(p) is selected,
[13]. Let α be an element that satisfies the equation
p(α) = 0, then α is a root of p(x). In this way, the
elements of the field GF(pn) are all polynomials of
degree n 1, which are elements of the ring
GF(p)[x]. The coefficients of the polynomials
belong to the GF(p).
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󰇛󰇜󰇝
󰇛󰇜󰇞
(1)
The arithmetic is mostly computed using
polynomial arithmetic modulo the irreducible
polynomial p(x). Two main algebraic operations,
addition (4) and multiplication (5), between two
elements 󰇛󰇜 and 󰇛󰇜 of GF(pn), are defined:
󰇛󰇜



(2)
󰇛󰇜



(3)
Addition:
󰇛󰇜󰇛󰇜
󰇟󰇛󰇜󰇠󰇛󰇜


(4)
Multiplication:
󰇛󰇜󰇛󰇜 󰇛󰇜
where 󰇛󰇜 is the remainder, when dividing the
result c() (5) by the irreducible polynomial p() of
degree less than n
󰇛󰇜󰇛󰇜  󰇛󰇜
󰇛󰇜

(5)
with coefficients

 
  
(6)
The basic operations in the extended Galois
fields are elucidated by an example of the field
GF(32). The irreducible polynomial p(x) = x2 + x + 2
is used. Table 1 presents all elements of GF(32) in
the form of polynomials, as ordered pairs of
coefficients and as the power of the primitive
element .
The polynomial and n-tuple representations are
better suited for addition and subtraction arithmetic
operations, whereas the power of the primitive
element representation leads to faster computation
for multiplication and division arithmetic
operations. The following formulas are used:
If a = x, b = y GF(pn), then their product c
is:
 󰇛󰇜󰇛󰇜
(7)
and their quotient d is
󰇛󰇜󰇛󰇜
(8)
As corollary of the division (8) the multiplicative
inverse element a-1 is given by


󰇛󰇜󰇛󰇜
(9)
Table 1. GF(32) elements with irreducible
polynomial p(x) = x2 + x + 2
Polynomial
Representation
Representation
as 2-tuple
Power of
0
0. + 0
0 0
0
1
0. + 1
0 1
0
2
0. + 2
0 2
4
3
1. + 0
1 0
1
4
1. + 1
1 1
7
5
1. + 2
1 2
6
6
2. + 0
2 0
5
7
2. + 1
2 1
2
8
2. + 2
2 2
3
The following are examples of the arithmetic
operations addition, subtraction, multiplication and
division of two elements 2 and 7 from GF(32).
Addition: 2 + 7 = (2 + 2. + 1) = 2. = 6.
Subtraction: 2 – 7 = (2 – 2. – 1) = 1 + 1. = 4.
Multiplication:
2.7 = 4.2 = 6 mod 8 = 6 = 1 2 = 5.
Division: 2/7 = 4 / 2 = 2 mod 8 = 2 = 2 1 = 7.
Multiplicative inverse:
2-1 = 1/4 = 8/4 = 4 = 0 2 = 2.
7-1 = 1/2 = 8/2 = 6 = 1 2 = 5.
2.2 p-ary Linear Feedback Shift Register
with Galois Architecture
In this subsection, we briefly recall the basics of the
p-ary Linear Feedback Shift Register (pLFSR) with
Galois Architecture and its properties, [14].
The block diagram of the p-ary linear feedback
shift register with Galois Architecture is shown in
Figure 1. It consists of L-count delay elements ai, i
= 0, 1, …, L 1, each of which can store one p-ary
digit in the interval [0, p], where p is a prime. The
output of the last significant element is put in
each multiplier in a Galois Field GF(p) as it is
multiplied by the coefficients qi+1, i =0, 2, …, L - 2,
and it is added to the content of the previous
element ai through the adder in the Galois Field
GF(p). The content of the element a0 is taken out to
the output and it forms a part of the output p-ary
linear sequence . During the work of the pLFSR
register, whenever the content of the element i is
being shifted into element i-1 for each i,
1
i
L − 1, the following recurrence relations (1)
are executed:
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󰆒󰇛 󰇜  


󰆒 
(10)
The multipliers of the feedbacks 
in the pLFSR register are determined by the
coefficients of the primitive polynomial 󰇛󰇜 (11)
in GF(pL) to create p-ary m-sequence with a
maximum period .
Fig. 1: p-ary Linear Feedback Shift Register with
Galois architecture
If qL 0, the feedback polynomial 󰇛󰇜
󰇛󰇜  
(11)
of degree L and the polynomial 󰇛󰇜 of the initial
state 󰇛 󰇜
󰇛󰇜  
(12)
define the generated output p-ary m-sequence
󰇛 󰇜. In this case, the generating function
󰇛󰇜 of the pLFSR output sequence is
󰇛󰇜 󰇛󰇜
󰇛󰇜
(13)
3 Problem Solution
In this section, we provide a method for generating a
5-ary nonlinear pseudo-random sequence and a
binary key sequence, which is suitable for use as a
synchronous stream cipher in a communication and
network environment involving resource-
constrained devices with increased security and
efficiency of the encryption and decryption
processes.
The block diagram of a keystream generator for
stream ciphers is shown in Figure 2. It consists of a
5LFSR register, a selection rule block, and a binary
sequence converter. The selection rule block
introduces nonlinearity into the output of the 5LFSR
sequence A using shrinking and multiplexing. As a
result, the selection rule block produces balanced
nonlinear 5-ary sequence B. The binary sequence
converter ensures the uniform distribution of the
zero and one bits in the output keystream C by
involving a running zero mechanism.
Fig. 2: Generator of a 5-ary nonlinear pseudo-
random sequence and a binary keystream sequence
3.1 5-ary Linear Feedback Shift Register
with Galois Architecture
For higher speed performance, the Galois field
GF(5L) was chosen based on the byte organization
of processors in resource-constrained devices. Thus,
the period of the output sequence A from the 5LFSR
register is . For ease of comprehension
the proposed method for generating a 5-ary
nonlinear pseudo-random sequence and a binary key
sequence will be explained with the specific
example shown in Figure 3.
The example uses a 5LFSR register of length 8.
The primitive polynomial q(x) (5)
󰇛󰇜 

(14)
is chosen from all 48 750 primitive polynomials in
GF(58). To construct the concrete 5LFSR scheme,
the primitive polynomial (14) must be transformed
into the form (11). For the transformation, the
algorithm proposed in, [15], is used to determine the
feedback multipliers by multiplying with the
constant

  
(15)
The transformation gives the polynomial
󰇛󰇜 

(16)
that defines the 5LFSR scheme in Figure 3.
3.2 Selection Rule Block
For better resistance against correlation attacks, the
choice of the nonlinear function is made in
correspondence with the value of the first 5-ary digit
 of each group of five digits
󰇛󰇜  stored in a buffer.
The first 5-ary digit determines which of the
nonlinearity techniques is to be executed. If it is
equal to zero, shrinking technique is executed and
the 5-ary digit is not outputted. If it is greater than
zero, first it is decremented and then the new value
controls the output of the 4:1 multiplexer MX which
forms a nonlinear 5-ary sequence. The new value
 determines which of the other four 5-ary
digits 󰇛󰇜  is to be
5LFSR
Selection
Rule
Block
Binary
Sequence
Converter
A
B
C
aL-1
qL-1
aL-2
a1
a0
qL
q2
q1
mod p
mod p
mod p
mod p
mod p
mod p
mod p
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displayed at the output of the nonlinear 5-ary
sequence. Thus, the 5-ary nonlinear output sequence
B is a shrunken and multiplexed version of the
linear output 5-ary sequence A.
Fig. 3: Example of a generator of a 5-ary nonlinear pseudo-random sequence and a binary keystream
sequence
An example of the work of the generator of a
5-ary nonlinear pseudo-random sequence and a
binary keystream sequence from Figure 3 are shown
in Table 1, but it only presents the first 16 outputs of
the choice rule. The content of the buffer is
presented in the second column of the table, the
output of the choice rule in the third column, and
the fourth the output of the balanced binary
sequence converter.
In the example shown in Table 2, the initial seed
of the 5LFSR is the 5-ary sequence 󰇛󰇜
󰆄
󰆈
󰆈
󰆅
󰆈
󰆈
󰆆
. The
buffer contains five in number 5-ary digits.
If the first one of them is a 0, the nonlinear
operation of the shrinking technique is executed and
there is no value produced at the output (in Table 2,
it is marked with “–“).
If the first digit is different from 0, its
decremented value determines which input of the
4:1 multiplexer will be produced at the output of the
choice rule (in Table 2, it is marked with a bold 5-
ary digit).
The period of the nonlinear 5-ary sequence,
[16], is , and the appearance count of
each 5-ary digit  is equal to:
 
(17)
Hence, the 5-ary nonlinear pseudo-random
sequence is balanced.
Table 2. Example of the work of the generator
from Figure 3
Buffer
Choice
rule
Binary
representation
a5i
a5i+1
a5i+2
a5i+3
a5i+4
0
1
2
3
0
0
2
01
1
2
3
4
1
4
4
11
2
2
0
2
1
1
2
01
3
2
3
2
1
4
2
01
4
4
0
4
3
0
0
01
5
3
4
3
1
2
1
00
6
3
1
1
3
2
3
10
7
2
2
2
3
1
2
01
8
4
4
2
1
2
2
01
9
0
1
0
4
2
10
1
4
1
4
2
4
11
11
1
2
0
4
0
2
01
12
2
1
4
0
3
4
11
13
0
4
1
0
2
14
2
1
1
2
2
1
00
15
0
2
3
3
1
3.3 Binary Sequence Converter
The binary sequence converter converts every 5-ary
digit in a 2-bit sequence as the output binary
sequence has an equal number of 0’s and 1’s, i.e. it
is balanced. Every 5-ary digit bi, i = 0, 1, of the
nonlinear 5-ary sequence is checked whether it has 0
a6
mod 5
mod 5
a7
mod 5
mod 5
a5
mod 5
mod 5
a4
mod 5
a3
mod 5
a2
mod 5
mod 5
a1
mod 5
mod 5
a0
3
2
2
4
2
2
2
4
a5i
a5i+1
a5i+2
MX
a5i > 0
Yes (no Output)
a5i+3
a5i+4
1
Buffer
1
bi > 0
+ 1
mod 4
z0
2
bi = 0
MX
5-ary Nonlinear Output B
Binary
Output
C
5LFSR
Binary Sequence Converter
Selection Rule Block
a5i == 0
bi == 0
5-ary
Linear
Output A
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value. If bi = 0, the value of the MOD 4 counter is
increased by 1 and the acquired 2 output bits of the
counter are produced at the output of the generator
through the 2:1 multiplexer. The initial value of the
counter is the 2-bit representation of a digit q0 that
determines the initial state of the running zero. If bi
> 0, the value of bi is decremented and through the
2:1 multiplexer, the binary code of bi is outputted.
The balanced binary sequence converter presents
every one of the 5-ary digits in the interval
󰇟󰇠through the binary representation of the
number, and the 5-ary zero as a running zero with
an initial value of z0. Every i-th appearance of the
zero is presented by the value of a running zero
, realized by an equality:
󰇛 󰇜
(18)
where i is the i-th occurrence of zero in the 5-ary
nonlinear sequence.
The running zero increases additionally the
security of the method for generating binary
keystream, because it masks the 5-ary zero
consecutively with different 5-ary digits, which are
different from zero.
The fourth column of Table 1 shows the output
of the balanced binary sequence converter. In the
given example the initial value of the running zero,
z0 = 0, is chosen. Because of that, the first
appearance of 5-ary zero in row 4 is represented by
the 2-bit binary representation of the digit 1.
The period of the balanced binary sequence is
, and the appearance count of each bit
is, [16]:

(19)
4 Conclusion and Feature Work
The article proposes an approach and an apparatus
for a synchronous stream cipher that generates both
a balanced nonlinear 5-ary pseudo-random sequence
and a binary keystream sequence.
This method is suitable for the cryptographic
protection of confidential data using a stream cipher
with a proposed balanced nonlinear 5-ary pseudo-
random sequence as a key stream when 5-level
signals are transmitted in a communication
environment. As a supplement, the bаlanced binary
pseudo-random sequence is applicable to binary
data encryption in a network environment that
includes resource-constrained devices.
Two key features determine the enhanced
reliability and cryptographic resistance of the
proposed method. The main one is the use of
shrinking and multiplexing techniques to introduce
nonlinearity into the generated linear p-ary pseudo-
random sequence, where p is an arbitrary prime. The
second one, a running zero is introduced when
converting the nonlinear 5-ary sequence into a
balanced binary sequence. This not only enhances
the security of the method but also masks the 5-ary
zero with different 5-ary digits that are not zero in a
consecutive manner.
However, there are some practical issues with the
hardware design of the proposed method that need
to be addressed. With the current focus on the fourth
industrial revolution Industry 4.0, [17], companies
are integrating new technologies, including the
Internet of Things (IoT), cloud computing, and big
data, as well as artificial intelligence and machine
learning, into their production facilities and
throughout their operations to ensure cybersecurity
in the operations of companies that manufacture,
improve and distribute their products.
The majority of network traffic on IoT devices
lacks encryption, exposing sensitive and personal
data to cyberattacks such as malware attacks,
including ransomware, phishing attacks, denial of
service attacks as well as other forms of data
breaches or thefts. The suggested method and
apparatus are optimal for employing stream cipher
to encrypt network traffic involving IoT devices
with limited resources. From a practical
consideration, we need to design the Field
Programmable Gate Arrays (FPGAs), [18], [19], or
Application Specific Integrated Circuits (ISICs),
[20], [21], hardware implementation of the proposed
apparatus, which will allow faster execution of the
algebraic addition and multiplication operations in
the Galois field GF(5).
Acknowledgement:
The authors would like to thank the reviewers for
their helpful comments.
References:
[1] S. W. Golomb and G. Gong, Signal Design
for Good Correlation: For Wireless
Communication, Cryptography, and Radar,
Cambridge University Press, New York, NY,
USA, 2005.
[2] Chenchev, Ivaylo, Adelina Aleksieva-Petrova,
and Milen Petrov. Authentication
Mechanisms and Classification: A Literature
Survey. Intelligent Computing: Proceedings
of the 2021 Computing Conference, Vol. 3.
Springer International Publishing, 2021.
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WSEAS TRANSACTIONS on ELECTRONICS
DOI: 10.37394/232017.2023.14.9
Zhaneta N. Savova, Antoniya T. Tasheva,
Rosen A. Bogdanov
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finite field multipliers using fast XNOR cells.
In Proceedings of the Great Lakes Symposium
on VLSI 2023, 2023, pp. 163-166.
Contribution of Individual Authors to the
Creation of a Scientific Article (Ghostwriting
Policy)
The authors equally contributed to the present
research, at all stages from the formulation of the
problem to the final findings and solution.
Sources of Funding for Research Presented in a
Scientific Article or Scientific Article Itself
The article was prepared with the financial support
of the National Scientific Program “Security and
Defence”, funded by the Ministry of Education and
Science of the Republic of Bulgaria, in
implementation of Decision № 731 of 21.10.2021 of
the Council of Ministers of the Republic of
Bulgaria.
Conflict of Interest
The authors have no conflicts of interest to declare
that are relevant to the content of this article.
Creative Commons Attribution License 4.0
(Attribution 4.0 International, CC BY 4.0)
This article is published under the terms of the
Creative Commons Attribution License 4.0
https://creativecommons.org/licenses/by/4.0/deed.en
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WSEAS TRANSACTIONS on ELECTRONICS
DOI: 10.37394/232017.2023.14.9
Zhaneta N. Savova, Antoniya T. Tasheva,
Rosen A. Bogdanov
E-ISSN: 2415-1513
78
Volume 14, 2023