08-05-2012, 12:07 PM
A Competitive Study of Cryptography Techniques over Block Cipher
A Competitive Study of Cryptography Techniques over Block Cipher.pdf (Size: 298.16 KB / Downloads: 36)
Abstract
The complexity of cryptography does not allow
many people to actually understand the motivations and
therefore available for practicing security cryptography.
Cryptography process seeks to distribute an estimation of
basic cryptographic primitives across a number of
confluences in order to reduce security assumptions on
individual nodes, which establish a level of fault-tolerance
opposing to the node alteration. In a progressively networked
and distributed communications environment, there are more
and more useful situations where the ability to distribute a
computation between a number of unlike network
intersections is needed. The reason back to the efficiency
(separate nodes perform distinct tasks), fault-tolerance (if
some nodes are unavailable then others can perform the task)
and security (the trust required to perform the task is shared
between nodes) that order differently. Hence, this paper aims
to describe and review the different research that has done
toward text encryption and description in the block cipher.
Moreover, this paper suggests a cryptography model in the
block cipher.
Keywords: Cryptography, text encryption, block cipher, AES
I. INTRODUCTION
Unclassified nature of the algorithm cannot be stressed
enough. However, by publishing the algorithm, it gives the
cryptographer choices to be seen by a wide range of
academic cryptography, keen to break into the system to
publish articles demonstrating how smart they are. The real
secret is that the key and its length are very important,
considering a simple combination is safer. The general
principle is that figures are inserted in sequence and the key
is secret. A key length of two digit means that there are 100
possibilities. A three-digit key length is 1000 possibilities
and a key length of six figures means a million. As longer
the key is, with greater workload (work factor) that the
cryptanalyst has to do. Work factor to break the system by
the exhaustive search in the digit space is exponential in
relation to the key length [1]. The secret comes from
having a strong algorithm (but public) and a long key. To
prevent the younger brother to read other mail, there are
enough 64-bit keys. To keep at distance powerful enemies
the needed are at least 256 bits keys [2].
Encryption methods have historically been divided into
two categories: substitution ciphers and transposition
ciphers. Stallings had explained each of these ciphers as
essential information for understanding modern
cryptography [3].
An example of encryption algorithms is AES (Rijndael)
which identifies as a symmetric algorithm. This means that
the encryption key can be calculated from the
corresponding decryption and vice versa [4]. Security an
algorithm based on symmetric key, which must be remains
secret [5]. The AES block cipher as acting in plaintext in
groups of each bit time which are called blocks [6]. Typical
size of a block is 64 bits. Each round transformation
consists of three separate transformations called layers:
Linear mixing layer;
Non-linear layer;
Key addition layer.
Before the first round of AES processing algorithms, a
key addition layer takes place. The linear mixing layer of
the final round is different than the other rounds. Each
round of treatment consists of four different
transformations that compose 3 layers [7].
However, AES is an iterative algorithm with variable
size block processing and key which can be 128, 192 or
256 bits. The interim results of the algorithm after each
transformation called State [8]. Each State is expressed as a
rectangular table of data bytes [9]. The table blow has 4
rows, while the number of batteries (NB) is the size of the
block processing divided by 32. Similarly, the encryption
key (cipher key) expressed as rectangular table with data
bytes. The table has 4 rows and number of columns [10] is
the key length divided by 32. Each table element is one
byte.
Table1: State (with Nb = 6) and encryption key (with Nk =
4)
Round (State, RoundKey) {ByteSub (State);
ShiftRow (State); MixColumn (State);
AddRoundKey (State, RoundKey);}The final round
is defined as follows:
FinalRound (State, RoundKey) {{ByteSub (State);
ShiftRow (State); AddRoundKey (State,
RoundKey);}
2011 UKSim 13th International Conference on Modelling and Simulation
978-0-7695-4376-5/11 $26.00 © 2011 IEEE
DOI 10.1109/UKSIM.2011.85
415
Moreover, each column is referred as "word" or "4-byte
vector". Each table can be considered as one-dimensional
table of elements "4-byte vectors". The entrance and exit of
AES can be regarded as dimensional data tables with 8-bit
(byte) numbered from 4 * Nb-1. Similarly, the key
numbered 0 to 4 * Nk-1. H entrance cipher (plaintext) is
shown in bytes of the table with the State series:
The number of laps made by the algorithm denoted by
Nr and depends on the values Nb and Nk as shown in table
1.
Table 2: The values of Nr, Nb, Nk
A block cipher cryptosystem consists of two algorithms,
the encryption algorithm and decryption algorithm that are
illustrated in Figure 1. The encryption algorithm takes as
input an n-bit plaintext M and a k-bit key K and outputs an
n-bit ciphertext C; the decryption algorithm takes as input
an n-bit ciphertext C and a k-bit key K and outputs an n-bit
plaintext M [11]. For any fixed key, the decryption
algorithm acts as the inverse process of the encryption
algorithm as in following equation (1.1), (1.2).
C=Ek(M) (1.3)
M=Dk © =E-1(Ek(M)) (1.4)
The block cipher breaks M into successive blocks M1,
M2, and enciphers each M1 with the same key K; that is as
in equation (1.5).
Ek(M)=Ek(M1) Ek(M2) (1.5)
Typically, each block is several characters long. Two
important block ciphers classes are substitution and
transposition ciphers. Simple substitution and homophonic
substitution ciphers are blocks ciphers even thought the
unit of encryption is a single character. This is due to the
same key being used for each character [12].
Figure 1: The encryption and decryption operations in
block cipher algorithm (ref…)
Several groups of researchers that have analyzed
some of the block algorithms already found a way to
determine their strength or weakness. Apparently, there are
some properties that determine the block algorithms
strength or weakness.
Complementation
The complexity of a brute-force attack is reduced factor
of two by using this complementation property. The simple
relation can be defined by the following rule IF Ek (P) = C
THEN EK’ (P’) = C' P’, C’, and K' are the bit-wise
complements of P, C and K [13].
There are no simple relations in a high-quality block
cipher. Weaknesses in the block cipher are created by this
property. An example that has this property is the DES
algorithm.
The Strict Avalanche Criteria (SAC)
The avalanche effect is a property that seems to be very
important: it deals with the number of S-Box output bits
change when the subsets of the inputs bits are changed.
Conditions can be easily imposed on the Boolean
function to satisfy particular avalanche criteria but the
difficult task is constructing them [14].
SAC guarantees that exactly half of the output bits
change when one input bit is changed [15].
II. EXISTING ISSUES
Generally, the utilization of the encryption techniques
has raises different security issues, which consisted mostly
on how to effectively manage the encryption keys to ensure
that they are safeguarded throughout their life cycle and are
protected from unauthorized disclosure and modification.
a0,0, a1,0, a2,0, a3,0, a0,1,a1,1, a2,1, a3,1, a4,1, … (1)
*While byte key shown in the table key in the order:
k 0,0, k 1,0, k 2,0, k 3,0, k 0,1, k 1,1, k 2,1, k 3,1, k 4,1, ..(2)
416
Encryption keys are a sequence of symbols used with a
cryptographic algorithm, which enables encryption and
decryption. It is imperative that an efficient key
management program be established and facilitated
throughout public safety agencies. Key management
ensures that critical and sensitive radio transmissions are
protected with proper encryption methods and that
encryption keys are controlled and securely stored during
their life cycle. For purposes of this report, encryption is
defined as the process of transforming plain text into
unintelligible form by using a cryptographic system. The
cryptosystem is hardware and software providing the
means to encrypt and decrypt transmissions. Figure 2
presents a basic encryption concept.
The basic meteorological of encryption comprise the
algorithm (i.e., a mode of changing information), the key
(i.e., a secret introducing point for the algorithm), and the
key authority (i.e., key management). The key is
characteristically recognized as a binary number used with
a cryptographic algorithm to authorize the encryption and
decryption of information over the block cipher. The key
jurisdictions the algorithmic alteration executed to
information transmission during encryption and description
process that must be anticipated so that a corresponding
decryption algorithm can backtrack the operation by
employing a suitable key. Several reasons in the encryption
of information over block cipher are observed in terms
of key management, which known as an important issue to
the public safety community, most of these issues
addressed the following:
Difficulties in addressing the security issues
regarding encryption key management;
Lacks in providing a suitable details about the
different threats in terms of decision makers on
the importance of key management;
Difficulties in generating the suitable
recommendations for establishing proper key
management.
Figure 2: Basic Encryption Concept
III. RELATED WORKS
Chan & Fekri develooped a new private key
cryptosystem based on the finite-field wavelet. The
encryption and decryption are performed by the synthesis
and analysis banks of the nonlinear finite-field wavelet
transform whose filter coefficients are determined by the
keys of the users. Authors illustrate the polyphone
representation of the wavelets to introduce a shared key
mechanism for the wavelet cryptosystem. As well as adopt
the wavelets that operate over GF (256) and a nonlinear
device that performs a mapping on the field elements to
their inverse in the field. The block cipher system has a key
length of 16 symbols (128 bits) and an input block size of
30 symbols (240 bits). To evaluate the efficiency of the
developed two-round wavelet cryptographic scheme, the
study also has compared with DES and AES. The results
indicated that the wavelet cryptosystem has comparable
computational complexity to AES and approximately half
the complexity of DES. The security is tied to the length of
the wavelet basis function and to the nonlinearity within
the wavelet transform. Finally, Chan & Fekri conclude that
the lowest complexity of any of these attacks is greater than
an exhaustive key search [11].
Another study by Mousa & Hamad invistigates the
analysis process of the effect of different parameters of the
RC4 encryption algorithm that was performed to illustrate
the performance of RC4 algorithm based on changing some
of these parameters. Mousa & Hamad examined the
execution time as a function of the encryption key length
and the file size, which recognized as a complexity and
security. Meanwhile, the study demonstrated a different
data types and the role of the data type. The results have
been analyzed and interpreted as mathematical equations
showing the relationship between the examined data and
hence can be used to predict any future performance of the
algorithm under different conditions. The order of the
polynomial to approximate the execution time was justified
[16].
Additionally, Ray & Das, descirbed the Cellular
Automata [5] as a computing model of complex System
using simple rule. Ray & Das highlights the main issues in
the space, which divided into number of cell and each cell
can be one or several final state. Cells are affected by
neighbors with the application of simple rule. Furthermore,
the study deals with the Cellular Automata in cryptography
for a class of Block Ciphers through a new block
encryption algorithm based on programmable cellular
automata. The proposed algorithm belongs to the class of
symmetric key systems [17].