Symmetric Key Generation Algorithm in Linear Block Cipher Over LU Decomposition Method

International Journal of Trend in Scientific Research and Development, Volume 1(4), ISSN: 2456-6470
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IJTSRD | May-Jun 2017
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Symmetric Key Generation Algorithm in Linear Block Cipher Over
LU Decomposition Method
P.Sundarayya
Department of Mathematics,
GITAM University, Visakhapatnam,
Andhra Pradesh, India
M.G.Vara Prasad
Department of Mathematics,
NSRIT, Visakhapatnam,
Andhra Pradesh, India
ABSTRACT
In symmetric key algorithm in linear block cipher to
encrypt and decrypt the messages using matrix and
inverse matrix. In this proposed technique generate
lower and upper triangular matrices from square
matrix using decomposition. In encryption process,
the key is lower triangular matrix and decryption
process, the key is upper triangular matrix under
modulation of prime number. We illustrate the
proposed technique with help of examples.
Keywords: liner block cipher, symmetric matrix key,
encryption, and decryption, lower and upper
triangular decomposition
1. INTRODUCTION
Cryptography is the science of making messages in
secret code and having lived art. Cipher is
mathematical function which is used in encryption
and decryption process. Cryptography systems can be
divided into two cryptosystems.
 Symmetric
 Asymmetric.
Symmetric key cryptography is classical cryptography
is divided into four parts.
 The encryption algorithm
 The encryption key
 The decryption algorithm
 The decryption key
Symmetric cryptosystems use the secret keyto encrypt
and decrypt message, and asymmetric cryptosystems
use the public key to encrypt a message and the
private key to decrypt it .Symmetric encryption is
described to as conventional encryption or single key
encryption. Conventional encryption can be divided
into two categories.
 Classical techniques
 Modern techniques
The hallmark of Symmetric key encryption is that the
cipher or key to the algorithm is shared. Linear block
Cipher is one of the basic components of classical
ciphers. A Liner block cipher is a method of
encryption by which units of plaintext are substituted
with cipher text according to a regular system; the
units maybe pairs
of letters, triplets of letters, poly of letters and
mixtures of the above. The receiver deciphers the text
by performing an inverse function [2]. Hill cipher is a
block cipher that has several advantages such as
disguising
letter frequencies of the plaintext, its simplicity
because of using matrix multiplication for enciphering
and deciphering, its high speed, and high throughput
[4].In this proposed work, Instead of matrix and
inverse matrix, idea of generate the symmetric key
generation can be decomposed matrix into lower and
upper triangular matrices. In encryption process using
lower triangular matrix and decryption process the
cipher text convert into plain text using upper
triangular matrix under modulation of prime number.
In this proposed technique overcome of known plain
text attack when the order of the key matrix known.

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2. The Hill cipher
The Hill cipher algorithm takes m successive plaintext
letters and substitute’s m cipher text letters for them.
The substitution is determined by m where m is a
positive integer, the idea is to take m linear
combinationsof the m alphabetic characters in one
plaintext elementand produce m alphabetic characters
in onecipher text element. Then, an m×mmatrix K is
used asa key of the system such that K is invertible
modulo n (Peterson, 2000; Lerma, 2005) andg.c.d((det
K)mod n, n)=1. Let kijbe the entryof K. For the
plaintext block R=(x1, x2,..,xm) (thenumerical
equivalents of m letters) and a key matrix K,the
corresponding cipher text block S=(y1, y2, …, ym)
Encryption: The cipher text is obtained from the
plaintext by means of a linear transformation.
S=
⋯
⋯
… … ⋯ ⋯
⋯
(mod n)
Decryption: The reverse process, deciphering, is
computed by
R=
⋯
⋯
… … ⋯ ⋯
⋯
( mod n)
There might be some complications with the
procedure outlined above due to the fact that not all
the matrices K have an inverse K−1
over Zn. In fact,
those matrices K with determinant 0, or with a
determinant that has common factors with the
modulus n, will be singular over Zn, and therefore
they will not be eligible as key matrices in the Hill
cipher scheme (Overbeyet al., 2005)[1]. Furthermore,
due to its linear nature, the basic Hill cipher succumbs
to known-plaintext attacks. Indeed, it is easy to show
that an opponent able to obtain mplaintext/cipher text
character pairs has a high probability of completely
breaking the system, i.e., he/she can obtain the matrix
K by solving the linear system of equations.
3. Cryptanalysis of known plain-text attack when
m is known of Hillcipher
Despite Hill cipher being difficult to break with a
cipher text-only attack, it succumbs to a known
plaintext attack assuming that the opponent has
determined the value of the m being used. Let mbe
distinct plaintext-cipher text pairs, say, xj=(x1j, x2j,
…,xmj) and yj=(y1j, y2j, …, ymj), 1≤j≤m, such that
yj=ek(xj),Define two m× m matrices R=(xij) and
S=(yij).Whenever K is invertible in the encryption
equation S=RK, the opponent can compute the
unknown key of ciphering as K=R−1
S and thereby
break the cipher (Barr, 2002). If R is not invertible,
then it will be necessary to try other sets of m
plaintext-cipher text pairs. When m is unknown,
assuming that m is not too large, the opponent could
simply try m=2, 3, …, until the key is found. If the
guessed value of m was incorrect, the obtained key
matrix would be not agree with further plaintext-
cipher text pairs (Stinson,2002).[1]
4. The Proposed Technique
The proposed technique takes m successive plaintext
letters and substitute’s m cipher text letters for them.
The substitution is determined by m where m is a
positive integer, the idea is to take m linear
combinationsof the m alphabetic characters in one
plaintext elementand produce m× constant matrix
B. In this proposed technique has generation of key
matrices using LU Decomposition method, i.e A=LU
and g.c.d((det A)mod q, q)=1 . Now constant matrix
B=AP ⟹ = . In encryption process LC=B⟹
C=L-1
B and decryption process UP=C⟹ P=U-1
B
where P is plain text and C is cipher text.

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4.1. Generating lower and upper triangle matrices
using LU decomposition method
Theorem: Every m-square matrix A can be expressed
as product of two triangular matrices, one lower
triangular and another upper triangular thus A=LU,
where q is prime number and A∈ ×
⋯
⋯
… … ⋯ ⋯
⋯
=
0 ⋯ 0
⋯ 0
… … ⋯ ⋯
⋯
0
⋯
⋯
… …
0 0
⋯ ⋯
⋯
Where L=
0 ⋯ 0
⋯ 0
… … ⋯ ⋯
⋯
and
U=
0
⋯
⋯
… …
0 0
⋯ ⋯
⋯
To simplify calculations we may choose
( , , … … . . ) =
(1,1, … . . ,1) ( ′
ℎ )
⋯
⋯
… … ⋯ ⋯
⋯
=
1 0
1
⋯ 0
⋯ 0
… … ⋯ ⋯
⋯ 1
0
⋯
⋯
… …
0 0
⋯ ⋯
⋯
WhereL=
1 0
1
⋯ 0
⋯ 0
… … ⋯ ⋯
⋯ 1
and
U=
0
⋯
⋯
… …
0 0
⋯ ⋯
⋯
If m=2 order of matrix then A=LU
becomes =
1 0
1 0
Multiplying the matrices L and U and equating
corresponding elements from both sides
 = , =
 = ⇒ =
 = + ⇒ = −
Example 4.1.1
ConsiderA=
4 1
2 3
∈ ×
A=
4 1
2 3
can be decomposed into L and
U.A=LU⇒
4 2
6 3
=
1 0
1 0
⇒
4 1
2 3
= + , = 4, =
1, =2⇒ =
+ = 3 ⇒ =
L=
1 0
15 1
=
1 0
1 mod 29, U=
4 1
0 17
=
4 1
0
mod 29, Therefore L, U ∈ ×
If m=3 order of matrix then A=LU becomes

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=
1 0 0
1 0
1
0
0 0
Multiplying the matrices L and U and equating
corresponding elements from both sides
 u11=all, u12=a12, u13=a13.
 l21u11=a21 ⇒ l21=
 l31u11=a31⇒l31=
 l21u21+u22=a22⇒u22=a22-( )
 l21u13+u23=a23⇒u23= − ( )
 l31u13+l32u22=a32⇒ l32=
( )
( )
 l31u13+l33u23=a33⇒u33= − -
( )
( )
− − ( )
4.2. Calculation of constant matrix
Let P is block of given plain text. In that block P
assigns labels fromtable -1according to given plain
text. Let be constant matrix and A is generator key
matrix then B=AP mod q, where q is prime number.
A=
⋯
⋯
… … ⋯ ⋯
⋯
is matrix of order m,
P=
⋯
⋯
… … ⋯ ⋯
⋯
and
B=
⋯
⋯
… … ⋯ ⋯
⋯
⋯
⋯
… … ⋯ ⋯
⋯
=
⋯
⋯
… … ⋯ ⋯
⋯
⋯
⋯
… … ⋯ ⋯
⋯
(
mod q)
4.3. Encryption
Let C be a block of cipher text and B is constant
matrix then encryption can be defined as AC=B and
LC=B⟹ C=L-1
B
C=
⋯
⋯
… … ⋯ ⋯
⋯
be a block of cipher
text
1 0
1
⋯ 0
⋯ 0
… … ⋯ ⋯
⋯ 1
⋯
⋯
… … ⋯ ⋯
⋯
=
⋯
⋯
… … ⋯ ⋯
⋯
⋯
⋯
… … ⋯ ⋯
⋯
=
1 0
1
⋯ 0
⋯ 0
… … ⋯ ⋯
⋯ 1
⋯
⋯
… … ⋯ ⋯
⋯
4.3.1. Encryption algorithm
Step1: Calculate B=AC (mod q)

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Step2: Select Key L as square matrix order m from
A=LU.
Step3: Calculate C=L-1
B
Step4:Calculate C= C (mod q)
4.4. Decryption
Let P be a block of plain text and C be a block of
cipher text thenUP=C⟹ P=U-1
C
0
⋯
⋯
… …
0 0
⋯ ⋯
⋯
⋯
⋯
… … ⋯ ⋯
⋯
=
⋯
⋯
… … ⋯ ⋯
⋯
⋯
⋯
… … ⋯ ⋯
⋯
=
0
⋯
⋯
… …
0 0
⋯ ⋯
⋯
⋯
⋯
… … ⋯ ⋯
⋯
4.4.1. Decryption algorithm
Step1: Select Key U as square matrix order m from
A=LU.
Step2: Calculate P= U-1
C
Step3: Calculate P= P (mod q)
5. Example of proposed technique
Consider the plain text P=’GOOGLE’, is
block of plain text.
In the 29-letter alphabet in which A-Z have numerical
equivalent 1-26,?=27,space=28
P=
6 14 11
14 6 4
is block of plain text
TakingFrom Example4.1.1
A=
4 1
2 3
can be decomposed into L and
U.A=LU⇒
4 1
2 3
=
1 0
15 1
4 1
0 17
Encryption:
B= AP mod 29
B=
4 1
2 3
6 14 11
14 6 4
(mod 29)
B=
9 4 19
25 17 5
(mod 29)
Let C= be a block of cipher text.
Encryption key L=
1 0
15 1
LC=B⟹ C=L-1
B mod 29
C=
1 0
15 1
9 4 19
25 17 5
( mod 29)
C=
9 4 19
14 15 10
=
This gives Cipher text C=’JOEPTK’
Decryption:
Decryption key U=
4 1
0 17
UP=C⟹ P=U-1
C mod 29
P=
4 1
0 17
9 4 19
14 15 10
(mod 29)
P=
6 14 11
14 6 4
Which gives Plain text =’GOOGLE’

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6. Cryptanalysis of proposed technique
The block cipher can be difficult to break with a
cipher text only attack. In this section, we discuss
Cryptanalysis of known plain-text attack. We
assumed that K is key matrix is an element of ×
6.1.Algorithm for known plain-text attack when m
is known
Step1: Let P= (P1,P2,……,Pn) be a block of plain text.
Where Pi= ⋮ Let C= (C1,C2,……,Cn) be a
block of cipher text. Where
Ci= ⋮ and select randomly pairs of plaintext and
corresponding cipher text are
(Pi,Ci)for i=1,2,3…….n.
Step2: The known plaintext – cipher text pairs
gives us a system of equations of the form
[Ci- Cj]=K[Pi- Pj] for 1≤ i≠j≤m , we
get general equation C=KP
Step 3: Determine K=P-1
C mod q
6.2. Example for cryptanalysis of proposed
technique when m=2 is known
Taking from 6.Example we get plaintext blocks
P1=
6
14
, P2=
14
6
,P3=
11
4
, cipher text
blocksC1=
9
6
, C2=
4
15
,C3=
19
10
then( − − ) = K( − − )
5 −15
−9 5
=K
−8 3
−8 2
⇒K=
5 −15
−9 5
−8 3
−8 2
=
4 1
0 17
(mod 29)
Conclusion
In the proposed work the symmetric key matrix of
classical Hill cipher is to make changes in order to
make greatest security of communication text. In this
paper symmetric key generation is more secure due to
the decomposition of the matrix into lower and upper
triangle matrices to encrypt and decrypt the messages
.The Proposed Cryptosystem to get better the plaintext
attack and also to get better cipher text attack, since
the lower triangle matrix is using for encryption
process and upper triangle matrix is using for
decryption process over residue modulo prime
number
References
[1] Douglas R. Stinson,Cryptography Theory and
practice ,third edition (2006)by chapman & Hall/CRC
Taylor &Francis Group.
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[3]A. Menezes, P. van Oorschot, and S. Vanstone,
Handbook of Applied Cryptography, CRC Press,
1996
[4]G.R. Blakley, Twenty years of cryptography in the
open literature, Security and Privacy 1999
Proceedings of the IEEE Symposium, 9-12 May1999
[5]J. Overbey, W. Traves, J. Wojdylo, On the key
space of the Hill cipher. Cryptologia, 29(1), 2005, 59-
72
[6]Introduction to Analytic Number Theory, fifth
edition. T. Apostol .Undergraduate Texts in
Mathematics, Springer-Verlag, New York, 1995
[7]Higher engineering mathematics khanna publishers
by Dr. B.S. Grewal 40th edition

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[8]P.B. Bhattachary SK Jain S.R. Nagpaul First
course in Linear Algebra 1983
[9] W. H.; Flannery, B. P.; Teukolsky, S. A.; and
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[10] Bunch, James R.; Hopcroft, John (1974),
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(125): 231–236
[11]Cormen, Thomas H.; Leiserson, Charles E.;
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[12]Okunev, Pavel; Johnson, Charles R. (1997),
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[14] SurajoAbubakarWadaInt. Journal of Engineering
Research and Applications Vol. 6, Issue 2, February
2016

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