CRYPTOGRAPHY CONCEPTS AND TECHNIQUES

• We define some terms. An original message is known as the

plaintext, while the coded message is called the ciphertext.
• The process of converting from plaintext to ciphertext is known as
enciphering or encryption;
• Restoring the plaintext from the ciphertext is deciphering or
decryption.
• The many schemes used for encryption constitute the area of study
known as cryptography.
• Such a scheme is known as a cryptographic system or a cipher.
Techniques used for deciphering a message without any knowledge
of the enciphering details fall into the area of cryptanalysis.
• Cryptanalysis is what the layperson calls “breaking the code.” The
areas of cryptography and cryptanalysis together are called
cryptology.
 SYMMETRIC CIPHER MODEL
• Plaintext: This is the original intelligible message or data that is fed
into the algorithm as input.
• Encryption algorithm: The encryption algorithm performs various
substitutions and transformations on the Plaintext.
• Secret key: The secret key is also input to the encryption algorithm.
The key is a value independent of the plaintext and of the algorithm.
The algorithm will produce a different output depending on the specific
key being used at the time. The exact substitutions and transformations
performed by the algorithm depend on the key.
• Ciphertext: This is the scrambled message produced as output. It
depends on the plaintext and the secret key. For a given message, two
different keys will produce two different ciphertexts. The ciphertext is
an apparently random stream of data and, as it stands, is unintelligible.
• Decryption algorithm: This is essentially the encryption algorithm
run in reverse. It takes the ciphertext and the secret key and produces
the original plaintext.
SYMMETRIC CIPHER
 CRYPTANLYTIC ATTACKS

•A cryptanlytic attacks are done generally by

cryptanalysis or by brute force attcks.
•There are several ways for attack for cryptanalyst.
1. Ciphertext Only
2. Known Plaintext
3. Chosen Plaintext
4. Chosen Ciphertext
5. Chosen Text
• The details about the above attacks are given
below.
 SUBSTITUTION TECHNIQUES

• The two basic building blocks of all encryption

techniques are substitution and transposition.
substitution
• 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.
• We Discuss about few substitution techniques
below.
 SUBSTITUTION TECHNIQUES

•Caeser cipher
•Monoalphabetic cipher
•Playfair cipher
•Hill cipher
•Polyalphabetic cipher
 Caesar Cipher

• The earliest known, and the simplest, use of a substitution

cipher was by Julius Caesar.
• The Caesar cipher involves replacing each letter of the
alphabet with the letter standing three places further down
the alphabet. For example,
• plain: meet me after the toga party
• cipher: PHHW PH DIWHU WKH WRJD SDUWB
• Note that the alphabet is wrapped around, so that the letter
following Z is A. We can define the transformation by listing
all possibilities, as follows:
• plain: 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
• cipher: d e f g h i j k l m n o p q r s T u v w x y z a b c
• Let us assign a numerical equivalent to each letter:
 Caesar cipher
• Representation of alphabets
 Caesar cipher
• Then the algorithm can be expressed as follows.
For each plaintext letter p, substitute the
ciphertext letter C:2
• C = E(3, p) = (p + 3) mod 26
• A shift may be of any amount, so that the general
Caesar algorithm is
• C = E(k, p) = (p + k) mod 26
• where k takes on a value in the range 1 to 25. The
decryption algorithm is simply
• p = D(k, C) = (C - k) mod 26
 Caesar cipher
• If it is known that a given ciphertext is a Caesar
cipher, then a brute-force cryptanalysis is easily
performed: simply try all the 25 possible keys.
• Three important characteristics of this problem
enabled us to use a brute-force cryptanalysis:
• 1. The encryption and decryption algorithms are
known.
• 2. There are only 25 keys to try.
• 3. The language of the plaintext is known and
easily recognizable.
 EXAMPLE

• KEY K=5
• WORD = NEW
• N value is 13 C=(13+5) mod 26 = 18 is ‘S’
• E value is 4 C=(4+5) mod 26 = 9 is ‘J’
• W value is 22 C =(22+5)mod 26 =1 is ‘B’
• For NEW the cipher text is SJB.
• Caeser cipher also callesd as additive ciphers or
shift ciphers.
 MONOALPHABETIC SUBSTITUTION CIPHER
•In monoalphabetic substitution the relation ship between a
symbol in the plain text to a symbol in the cipher text is always
one-to-one.
•After sender and receiver agreed to a single key that key is
used to encrypt each letter in the plain text or decrypt each
letter in the cipher text.
•A better solution is to create a mapping between each plain
text character and corresponding cipher text character.
•An example key for monoalphabetic cipher.
Plain Text
abcdefghIjklmnopqrstuvwxyz
Cipher text
noatrbecfuxdqgylkhvIjmpzsw
 MONOALPHABETIC SUBSTITUTION CIPHER
• With only 25 possible keys, the Caesar cipher is far from
secure.
• A dramatic increase in the key space can be achieved by
allowing an arbitrary substitution.
• Before proceeding, we define the term permutation.
• A permutation of a finite set of elements S is an ordered
sequence of all the elements of S, with each element
appearing exactly once.
• For example, if S = {a, b, c}, there are six permutations of S:
abc, acb, bac, bca, cab, cba.
• In general, there are n! permutations of a set of n elements,
because the first element can be chosen in one of n ways,
the second in n - 1 ways, the third in n – 2 ways, and so on.
 MONOALPHABETIC SUBSTITUTION CIPHER
EX: Message : encrypt
Plain text : encrypy
Ciphertext : rgahsli
 PLAYFAIR CIPHER
•After completion of this session. Students will
learn about
•Understand the basic working of playfair cipher
•Know about diagrams and generation of diagrams
with/without filter characters.
•This cipher is also called as wheatstone playfair
cipher.
•This cipher is manual symmetric encryption
technique.
 PLAYFAIR CIPHER contd..
•This cipher is multiple letter encryption cipher.
•In this cipher plain text alphabets are split as
diagrams.
•This technique uses 5*5 matrix constructed using a
keyword (EX: Monarchy).
M O N A R

C H Y B D

E F G I/J K

L P Q S T

U V W X Z
 PLAYFAIR CIPHER contd..
• Here the key is MONARCHY. We can take any
other key also.
•In 5*5 matrix we can enter non repeated words.
•I and J occupies same cell.
Rules for encryption using playfair cipher.
1. Diagrams
2. Repeating letter-filter it
3. Same column select immediate down element
↓ wrap around.
4. Same row select immediate right element →
wrap around
5. Rectangle ⇆ swap
 EXAMPLE
•Plain text: ATTACK
•Diagram : AT TA CK
 EXAMPLE-2
• MOSQUE
HILL CIPHER
 POLYALPHABETIC CIPHER
• 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.
• Polyalphabetic substitution ciphers are
1. Vigenere cipher
2. One-time pad cipher
VIGENERE CIPHER
 One Time Pad
• In cryptography, a one-time pad is a system in which
a randomly generated private key is used only once
to encrypt a message that is then decrypted by the
receiver using a matching one-time pad and key.
• One-time pad cipher is a type of Vignere cipher which
includes the following features −
• It is an unbreakable cipher.
• The key is exactly same as the length of message which
is encrypted.
• The key is made up of random symbols.
• As the name suggests, key is used one time only and
never used again for any other message to be
encrypted.
 One Time Pad contd..
• Due to this, encrypted message will be vulnerable to
attack for a cryptanalyst. The key used for a one-time
pad cipher is called pad, as it is printed on pads of
paper.
Encryption
• To encrypt a letter, a user needs to write a key
underneath the plaintext.
• The plaintext letter is placed on the top and the key
letter on the left.
• The cross section achieved between two letters is the
plain text.
• It is described in the example in the next slide.
One Time Pad contd..
 One Time Pad contd..
Decryption
• To decrypt a letter, user takes the key letter on
the left and finds cipher text letter in that row.
• The plain text letter is placed at the top of the
column where the user can find the cipher
text letter.
 TRANSPOSTION TECHNIQUES

• Transposition Cipher is a cryptographic

algorithm where the order of alphabets in
the plaintext is rearranged to form a cipher
text.
• In this process, the actual plain text alphabets
are not included.
• We are considering two techniques
1. Rail-Fence Technique
2. Simple columnar transposition techniques
 RAIL FENCE
• Rail-Fence is the simple Transposition technique that involves writing
plain text as a sequence of diagonals and then reading it row by row
to produce the ciphertext.

• Now read the plain text by row-wise, i.e. croaerdeoprtbig.

• So, here the plain text is a corporate bridge, and ciphertext is
croaerdeoprtbig
 Simple columnar transposition techniques
• The simple columnar transposition technique
can be categorized into two parts – Basic
technique and multiple rounds.
• Simples columnar transposition technique –
basic technique.
• The simple columnar transposition technique
simply arranges the plain text in a sequence of
rows of a rectangle and reads it in a columnar
manner.
 ALGORITHM
• Step 1: Write all the characters of plain text
message row by row in a rectangle of
predefined size.
• Step 2: Read the message in a columnar
manner, i.e. column by column.
• Note: For reading the message, it needs not to
be in the order of columns. It can happen in
any random sequence.
• Step 3: The resultant message is ciphertext.
 Example
• Let’s assume that Plain text is a corporate
bridge, and we need to calculate the cipher
text using a simple columnar transposition
technique.
• Let’s take 6 columns and arrange the plain text
in a row-wise manner.
• The Table is taken as in the next slide.
 Example contd..
COLUMN-1 COLUMN-2 COLUMN-3 COLUMN-4 COLUMN-5 COLUMN-6

C O R P O R

A T E B R I

D G E

Decide the column order for reading the message – let’s

assume 1,3,5,2,4,6 is an order.
Now read the message in a columnar manner using the
decided order. – cadreeorotgpbri
cadreeorotgpbri is a ciphertext.
 Steganography

• The purpose of steganography is to conceal

and deceive.
• It is a form of coverting communication and
can involve the use of any medium to hide
messages.
• It's not a form of cryptography, because it
doesn't involve scrambling data or using a
key.
• Instead, it is a form of data hiding and can be
executed in clever ways.
Types of Steganography techniques
• Depending on the nature of the cover
object(actual object in which secret data is
embedded), steganography can be divided
into five types:
• Text Steganography.
• Image Steganography.
• Video Steganography.
• Audio Steganography.
• Network Steganography.