Symmetric Cipher Model, Substitution techniques, Transposition techniques, Steganography, Unconditional Security

Shri S’ad Vidya Mandal Institute Of Technology
NAME ENROLLMENT NO.
Awadh Kishor Singh 150450116005
Raj Bhavsar 150450116009
Jainam Kapadiya 150450116015
Guided by :-
Prof. Ghanshyam Prajapati
Topic:- Symmetric Cipher Model, Substitution techniques,
Transposition techniques, Steganography
Subject:- Information and Network Security (2170709)

Symmetric Cipher Model
❑ A symmetric encryption scheme has five ingredients:
➢ Plaintext: original message to be encrypted.
➢ Ciphertext: the encrypted message.
➢ Encryption algorithm: The encryption algorithm performs various
substitutions and transformations on the plaintext.
➢ Decryption algorithm: This is essentially the encryption algorithm
run in reverse. It takes the ciphertext and the secret key and
produces the original plaintext.
➢ Secret key: A secret key is the piece of information or parameter
that is used to encrypt and decrypt messages in a symmetric, or
secret-key, encryption.

Simplified Model of Symmetric encryption

There are two requirements for secure use of symmetric
encryption:
➢ a strong encryption algorithm.
➢ a secret key known only to sender / receiver.

Model of Conventional Cryptosystem

Mathematically:
Y = EK(X) or Y = E(K, X)
X = DK(Y) or X = D(K, Y)
• X = plaintext
• Y = ciphertext
• K = secret key
• E = encryption algorithm
• D = decryption algorithm
• Both E and D are known to public
Symmetric Encryption

Cryptography
Cryptographic systems are characterized along three independent dimensions:
➢The type of operations used for transforming plaintext to ciphertext.
➢The number of keys used.
➢The way in which the plaintext is processed.

Cryptanalysis
Cryptanalytic attacks rely on the nature of the algorithm plus perhaps some
knowledge of the general characteristics of the plaintext or even some sample
plaintext–ciphertext pairs. This type of attack exploits the characteristics of the
algorithm to attempt to deduce a specific plaintext or to deduce the key being
used.
Kirchhoff's principle: the adversary knows all details about a cryptosystem
except the secret key.
Two general approaches:
• brute-force attack
• non-brute-force attack (cryptanalytic attack)

Brute-force attack:
The attacker tries every possible key on a piece of ciphertext until an
intelligible translation into plaintext is obtained. On average, half of all possible
keys must be tried to achieve success.

Unconditional Security
• An encryption scheme is unconditionally secure if the ciphertext generated
by the scheme does not contain enough information to determine uniquely
the corres-ponding plaintext, no matter how much ciphertext is available.
• Therefore, all that the users of an encryption algorithm can strive for is an
algorithm that meets one or both of the following criteria:
(1) The cost of breaking the cipher exceeds the value of the encrypted
information.
(2) The time required to break the cipher exceeds the useful lifetime of
the information.
• An encryption scheme is said to be computationally secure if either of the
foregoing two criteria are met.

The two basic building blocks of all encryption techniques are :
1. Substitution techniques: A substitution technique is one in which
the letters of plaintext are replaced by other letters or by
numbers or symbols. If the plaintext is viewed as a sequence of
bits, then substitution involves replacing plaintext bit patterns
with ciphertext bit patterns.
Different types of Substitution techniques are:
1. Caesar Cipher
2. Monoalphabetic Ciphers
3. Playfair Cipher
4. Hill Cipher
5. Polyalphabetic Ciphers
6. One-Time Pad

Caesar Cipher
The Caesar Cipher technique is one of the earliest and simplest method of
encryption technique. It’s simply a type of substitution cipher, i.e., each letter
of a given text is replaced by a letter some fixed number of positions down
the alphabet. The method is apparently named after Julius Caesar, who
apparently used it to communicate with his officials.
Formula:
Ciphertext(C):E(k,p)=(p+k)mod26
Plaintext(p):D(k,C)=(C-k)mod26
➢ For Example, key=3
plaintext: hello how are you
ciphertext: KHOOR KRZ DUH BRX

Monoalphabetic Ciphers
• Better than Caeser Cipher
• For each character of alphabet, assign different-abrupt concerned character
• Example:
• Monoalphabetic ciphers are easy to break because they reflect the frequency data
of the original alphabet.
• A countermeasure is to provide multiple substitutes, known as homophones, for a
single letter.
• For Example:
Plaintext: goodmorning
Ciphertext: TLLWNLIMRMT
a b c d e f g h i j k l m n o p q r s t u v w x y z
Z Y X W V U T S R Q P O N M L K J I H G F E D C B A

Playfair Cipher
• The best-known multiple-letter encryption cipher is the Playfair, which
treats diagrams in the plaintext as single units and translates these units
into ciphertext diagrams.
• The Playfair algorithm is based on the use of a 5 * 5 matrix of letters
constructed using a keyword.
➢ For Example,
Keyword: security
Plaintext: pattern
• In this case, the keyword is security. The matrix is constructed by filling in
the letters of the keyword (minus duplicates) from left to right and from top
to bottom, and then filling in the remainder of the matrix with the remaining
letters in alphabetic order.
▪ Note: The letters I and J count as one letter.

S E C U R
I/J T Y A B
D F G H K
L M N O P
Q V W X Z
Plaintext is encrypted two letters at a time, according
to the following rules:
1) Repeating plaintext letters that are in the same pair are separated with
a filler letter, such as x, so that pattern would be treated as pa tx te
rn.
2) Two plaintext letters that fall in the same row of the matrix are each
replaced by the letter to the right, with the first element of the row
circularly following the last. For example, op is encrypted as PL.
3) Two plaintext letters that fall in the same column are each replaced by
the letter beneath, with the top element of the column circularly
following the last. For example, mv is encrypted as VE.

4) Otherwise, each plaintext letter in a pair is replaced by the letter that
lies in its own row and the column occupied by the other plaintext
letter. Thus, pa becomes OB.
Plaintext: pa tx te rn
Ciphertext: OB VA FT CP
➢ The Playfair cipher is a great advance over simple monoalphabetic ciphers.
➢ For one thing, whereas there are only 26 letters, there are 26 * 26 = 676
diagrams, so that identification of individual diagrams is more difficult.

Hill Cipher
• This encryption algorithm takes m successive plaintext letters and substitutes
for them m ciphertext letters.
• The substitution is determined by m linear equations in which each character
is assigned a numerical value.
In general terms , the hill system can be expressed as,
C=E(P,K)=PKmod26
P=D(C,K)=C𝐾−1mod26
a b c d e f g h i j k l m n o p q r s t u v w x y z
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Example
2 −1
5 3
=
Plaintext: abcd
Keyword: 3 1
5 2
C=E(P,K)=PKmod26
C=
0 1
2 3
3 1
5 2
mod26
C=
5 2
21 8
mod26
C=
5 2
21 8
C=fcvi
𝑘−1
=
1
𝑎c − 𝑏𝑑
2 −1
5 3
P=D(C,K)=C𝐾−1
mod26
P=
5 2
21 8
2 −1
5 3
mod26
P=
0 1
2 3
mod26
P=
0 1
2 3
P=abcd

Polyalphabetic Ciphers
• Another way to improve on the simple monoalphabetic technique is to use different
monoalphabetic substitutions as one proceeds through the plaintext message.
• The general name for this approach is polyalphabetic substitution cipher.
• All these techniques have the following features in common:
1. A set of related monoalphabetic substitution rules is used.
2. A key determines which particular rule is chosen for a given transformation.
For Example,
key: deceptivewearediscoveredsav
plaintext: wearediscoveredsaveyourself
ciphertext: ZICVTWQNGKZEIIGASXSTSLVVWLA
𝐶 ሶ𝑖 = 𝑃𝑖 + 𝑘𝑖 mod 𝑚 mod26
𝑃ሶ𝑖 = 𝐶𝑖 − 𝑘𝑖 mod 𝑚 mod26

One-Time Pad (Vernam cipher)
ANKYODKYUREPFJBYOJDSPLREYIUNOFDOIUERFPLUYTS
We now show two different decryptions using two different keys:
ciphertext: ANKYODKYUREPFJBYOJDSPLREYIUNOFDOIUERFPLUYTS
key: pxlmvmsydofuyrvzwc tnlebnecvgdupahfzzlmnyih
plaintext: mr mustard with the candlestick in the hall
ciphertext: ANKYODKYUREPFJBYOJDSPLREYIUNOFDOIUERFPLUYTS
key: mfugpmiydgaxgoufhklllmhsqdqogtewbqfgyovuhwt
plaintext: miss scarlet with the knife in the library

• In theory, we need look no further for a cipher. The one-time pad offers
complete security but, in practice, has two fundamental difficulties:
1.There is the practical problem of making large quantities of
random keys. Any heavily used system might require millions
of random characters on a regular basis. Supplying truly random
characters in this volume is a significant task.
2.Even more daunting is the problem of key distribution and
protection. For every message to be sent, a key of equal length
is needed by both sender and receiver. Thus, a mammoth key
distribution problem exists.
• Because of these difficulties, the one-time pad is of limited utility and is
useful primarily for low-bandwidth channels requiring very high security.
• The one-time pad is the only cryptosystem that exhibits what is referred to as
perfect secrecy.

Transposition Techniques
➢ A very different kind of mapping is achieved by performing some sort of
permutation on the plaintext letters. This technique is referred to as a
transposition cipher.
➢ The simplest such cipher is the rail fence technique, in which the plaintext is
written down as a sequence of diagonals and then read off as a sequence of
rows.
For Example,
Plaintext: meet me after the toga party
rail fence of depth: 2
m e m a t r h t g p r y
e t e f e t e o a a t
The encrypted message is: MEMATRHTGPRYETEFETEOAAT

A more complex scheme is to write the message in a rectangle, row by row, and
read the message off, column by column, but permute the order of the columns.
The order of the columns then becomes the key to the algorithm.
Key: 4 3 1 2 5 6 7
Plaintext: a t t a c k p
o s t p o n E
d u n t I l T
wo amx y z
Ciphertext: TTNAAPTMTSUOAODWCOIXKNLYPETZ
The transposition cipher can be made significantly more secure by performing
more than one stage of transposition. The result is a more complex permutation
that is not easily reconstructed. Thus, if the foregoing message is reencrypted
using the same algorithm,

Key: 4 3 1 2 5 6 7
Plaintext: t t n a a p t
m t s u o a o
dw c o i x k
n l y p e t z
Output: NSCYAUOPTTWLTMDNAOIEPAXTTOKZ
• To visualize the result of this double transposition, designate the letters in the
original plaintext message by the numbers designating their position. Thus,
with 28 letters in the message, the original sequence of letters is,
01 02 03 04 05 06 07 08 09 10 11 12 13 14
15 16 17 18 19 20 21 22 23 24 25 26 27 28

• After the first transposition we have,
03 10 17 24 04 11 18 25 02 09 16 23 01 08
15 22 05 12 19 26 06 13 20 27 07 14 21 28
• which has a somewhat regular structure. But after the second transposition,
we have,
17 09 05 27 24 16 12 07 10 02 22 20 03 25
15 13 04 23 19 14 11 01 26 21 18 08 06 28
• This is a much less structured permutation and is much more difficult to
cryptanalyze.

Steganography
▪ A plaintext message may be hidden in one of two ways. The methods of
steganography conceal the existence of the message, whereas the methods
of cryptography render the message unintelligible to outsiders by various
transformations of the text.
Various other techniques have been used historically; some examples are the
following :
➢Character marking: Selected letters of printed or typewritten text are
overwritten in pencil. The marks are ordinarily not visible unless the paper is
held at an angle to bright light.
➢Invisible ink: A number of substances can be used for writing but leave no
visible trace until heat or some chemical is applied to the paper.
➢Pin punctures: Small pin punctures on selected letters are ordinarily not
visible unless the paper is held up in front of a light.

➢Typewriter correction ribbon: Used between lines typed with a black ribbon,
the results of typing with the correction tape are visible only under a strong
light.
• Steganography has a number of drawbacks when compared to encryption. It
requires a lot of overhead to hide a relatively few bits of information,
although using a scheme like that proposed in the preceding paragraph may
make it more effective.
• Also, once the system is discovered, it becomes virtually worthless. This
problem, too, can be overcome if the insertion method depends on some
sort of key.
• Alternatively, a message can be first encrypted and then hidden using
steganography.

• The advantage of steganography is that it can be employed by parties
who have something to lose should the fact of their secret
communication (not necessarily the content) be discovered.
• Encryption flags traffic as important or secret or may identify the sender
or receiver as someone with something to hide.
THANK YOU

Symmetric Cipher Model, Substitution techniques, Transposition techniques, Steganography, Unconditional Security

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