Encryption using Caesar cipher

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Topic
Caesar Cipher Encryption Technique
MSC (Cs)-I (1ST SEM)
Year 2017-18
Submitted to Submitted by
Dr. H.S. HOTA (HOD) Dushhyant Kumar
(Department of CSA) Roll No. 06

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Content

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Introduction
The word ‘cryptography’ was coined by combining two Greek words, ‘Krypto’
meaning hidden and ‘graphene’ meaning writing.
Hieroglyph − The Oldest Cryptographic Technique
The first known evidence of cryptography can be traced to the use of ‘hieroglyph’.
Some 4000 years ago, the Egyptians used to communicate by messages written in
hieroglyph. This code was the secret known only to the scribes who used to transmit
messages on behalf of the kings.
Why need Cryptography?
Goal :- Secure communication(Protect data from Hackers & Crackers)
1. Confidentiality:- protection of data from unauthorized disclosure
2. Authentication:- The authentication service is concerned with assuring that a
communication is authentic persion.
3. Integrity:- assurance that data received is as sent by an authorized entity

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Introduction

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Introduction
Passive Attack: Traffic Analysis

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Introduction
Active Attack: Interruption
Block delivery of message

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Introduction
Fabricate message
Active Attack: Fabrication

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Introduction
Modify messageModify message
Active Attack: Modification

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Introduction
Human being from ages had two inherent needs −
(a) to communicate and share information and
(b) to communicate selectively.
These two needs gave rise to the art of coding the messages in such a way
that only the intended people could have access to the information.
Unauthorized people could not extract any information, even if the
scrambled messages fell in their hand.

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What is cryptography?
Cryptography:-
The art or science encompassing the principles and methods
of transforming an intelligible message into one that is unintelligible, and
then retransforming that message back to its original form.
Cryptography can characterize by:
– type of encryption operations used
• substitution / transposition
– number of keys used
• single-key or private / two-key or public
– way in which plaintext is processed
• block / stream

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Basic Terminology

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Symmetric Encryption

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Symmetric Cipher Model

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Requirements
Two requirements for secure use of symmetric encryption:
– a strong encryption algorithm.
– a secret key known only to sender / receiver.
Y = EK(X)
X = DK(Y)
¨assume encryption algorithm is known
¨implies a secure channel to distribute key
Cipher text (y)
Plain text (x)
Key (K)
Encryption(E)
Decryption(D)

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Classical Encryption Technique
1. Substitution Ciphers:-
where letters of plaintext are replaced by
other letters or by numbers or symbols.
OR
if plaintext is viewed as a sequence of bits, then substitution involves replacing
plaintext bit patterns with cipher text t bit patterns.
Types of Substitution technique are :-
A. Caesar Cipher
B. Hill Cipher
C. Polyalphabetic Ciphers
D. One-Time Pad
E. Playfair Cipher
F. Monoalphabetic Ciphers
2. Transposition technique

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Caesar Cipher
Caesar Cipher earliest known substitution cipher “by Julius Caesar” first
attested use in military affairs replaces each letter by 3rd letter.
example:
meet me after the toga party
PHHW PH DIWHU WKH WRJD SDUWB
Caesar Cipher
¨can define transformation as:
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
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
replaces each letter by 3rd letter

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Caesar Cipher
Caesar Cipher can define Mathematical transformation as:
1. Now we apply the same method to calculation for encryption using the
Caesar cipher.
2. First we convert the letters of the alphabet to number so that we can
operate on them mathematically.
3. We convert ‘a’ to 0, ‘b’ to 1, ‘c’ to 3.right through to ‘z’ 25 as shown below.
then have Caesar cipher as:
C = E(p) = (p + k) mod (26)
p = D(C) = (C – k) mod (26)

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Caesar Cipher
Caesar Cipher transformation Example:
1. Steps- Encryption and decryption process using Caesar cipher
2. Choose Plain text (original message).
3. Applying Caesar cipher replaces each letter by 3rd letter algorithm
4. Use key for Encryption
Key= Number of shift alphabet called key
Hello friend Plain text
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
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

19. 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
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
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Caesar Cipher
Caesar Cipher transformation Example:
1. Encryption Process
= Plain text
K H O O R I U L H Q G
h e l l o f r i e n d
= E
= C

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Caesar Cipher
K H O O R I U L H Q G
Plain text
Cipher text =
Encryption process
h e l l o f r i e n d
Decryption process
h e l l o f r i e n dPlain text =
Key=Shift 3rd place alphabet
C = E(p) = (p + k) mod (26)
p = D(C) = (C – k) mod (26)