Lec 4 : Data Security
Introduction to Classical Cipher systems
1
2-1 Introduction.
Classical ciphers have been used since ancient Egypt to exchange classified
messages between authorized persons. Since then, different methods and techniques
are used in order to increase security level of such information. Most of these
methods and techniques are based on the idea that each natural language has its own
distribution characteristics.
The encryption process aims to uniformly flatten the statistical characteristics of
the language and obscure any correlation and dependencies between plaintext and
ciphertext by diffusion and confusion. In the other hand, cryptanalysis tries to make
use of remaining information (such as structured properties, redundancy, correlation,
and dependencies) to recover the plaintext or the used key during encryption
process.
Cryptographic systems have been classified into different types in many
cryptography literatures. However, most of these literatures classify them into two
main categories; symmetric (one-key) cryptosystems, and asymmetric (two-key)
systems . In symmetric or one-key systems, the enciphering and deciphering key are
the same. In asymmetric cryptosystems enciphering and deciphering keys are
different in such a way the at least one key is computationally infeasible to
determine from the other.
From the ancient ages till the last few decades, all cryptosystems were one-key
systems, thus one-key systems referred to as conventional or (classical) systems.
Lec 4 : Data Security
Introduction to Classical Cipher systems
2
There are four basic types of classical cryptosystems based on substitution. They are
; ( simple substitution , homophonic substitution , polyalphabetic substitution ,
and polygram substitution systems ), and one cipher system based on
transposition (permutation).
2.2 TRANSPOSITION CIPHERS.
Transposition ciphers rearrange characters according to some scheme. This is
done classically by some geometric figure, the figure is 2-dimentional array, and
often
called"Permutation".
Example : Encrypt the word " RENAISSANCE " using 3*4 figure, using [2 4 1 3]
scheme.
1
2
3
4
R
E
N
A
I
S
S
A
N
C
E
Ciphertext : ESCAARINNSE.
Many transposition ciphers permute characters of the plaintext with fixed
period Let Zd be the integers 1... d, and f:Zd
Zd be a permutation over Zd,
then successive blocks of d characters are encrypted by permuting the characters
according to f.
M  m1.....md 1 ,.....m2 d ..... is encrypted as
Ek (M )  m f (1) ...m f ( d ) , m fd  f (1) ....md  f ( d )
Lec 4 : Data Security
Introduction to Classical Cipher systems
3
Example :suppose d= 4 and ƒ E = [2 4 1 3] and ƒ D= [3 1 4 2], thus:
M
=
RENA
ISSA
NCE
Ek (M) =
EARN
SAIS
CNE
Dk (C) =
RENA
ISSA
NCE
1
2
3
4
ƒE =[2413]
1
2
3
4
1
2
3
4
ƒD= [3 14 2]
Figure ( 2-1 ) Encryption and decryption permutation
To determine the expected number of characters required to break a permutation
cipher with period d :
possible arrangement of d characters (possible keys) = d!