1. SUBSTITUTION CIPHERS
A substitution technique is one in which the letters/number/symbols of plain text are replaced by other letters/numbers/symbols.
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2. Substitution cipher techniques
Caesar's Cipher –right shift by 3
Mono-Alphabetic Cipher –random substitution
A Homophonic Substitution Cipher-one to many substitution
A Polygram Substitution Cipher-many to many substitution
A Polyalphabetic Substitution Cipher-key to each plain text
e g. Vigenere Cipher
One time Pad( Vernam Cipher)
[Note: write plain text in lower letter and cipher text in capital letters]

3. 1.Caesar Cipher It is earliest known substitution cipher
by Julius Caesar
first attested use in military affairs
replaces each letter by 3rd letter after it
Example:
meet me after the party
PHHW PH DIWHU WKH SDUWB

4. 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
mathematically give each letter a number
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
then have Caesar cipher as:
C = E(p) = (p + k) mod (26)
p = D(C) = (C – k) mod (26)

5. 2. Mono alphabetic Cipher
The week ness of Caesar's cipher is it’s predictability.
Instead of using a unique form for all alphabets in a plain text message use a random substitution.
Rather than just shifting the alphabet we could shuffle (jumble) the letters arbitrarily
Each plaintext letter maps to a different random cipher text letter
hence key is 26 letters long

6. Plain text: Hello word Cipher text: ACDER PQSAK

7. 3.A Homophonic Substitution Cipher
In this technique one plain text is mapped with more than one cipher text alphabet.
h-[ABXY]
I –[MNQR]
Plain text: hi
Cipher text: ABXYMNQR

8. 4. A Polygram Substitution Cipher
Rather than replacing one plain text with another cipher text at time block of alphabet is replaced with another.
Plain text: hi hello
Cipher text: JH KNHPL

9. 5. Polyalphabetic Ciphers
It uses multiple one character key.
Each of the key encrypts one plain text character.
First key encrypts 1st key , second key encrypts 2nd key, so on..
E.g
Vigenere cipher is example of poly alphabetic cipher

10. Vigenere Cipher It is based on vigenere table.
It is made up of 27 rows.
1st row of lower case letters represents plain text character.
Each subsequent row represent cipher alphabet.

12. Vigenere Cipher Example
Key (K) : FRINGE
Plain text(P): get all soldier a meal.
Cipher text(C):
See key in vertical column and plain text in 1st row and write down intersection letter as cipher text.

13. 6.One time pad ( Vernam Cipher)
It is implemented using random set of non repeating characters as the input cipher text.
Once the key is used to encrypt and decrypt a single message it will never used again for any other message so it is called one time pad
The length of key must equal to length of original plain text.
Plain text: how are you
One time pad: NCBTZQARX

14. 6.One time pad ( Vernam Cipher)
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