The document discusses two cipher techniques: 1) Homophonic substitution cipher where each plaintext letter can be replaced by multiple ciphertext letters. This makes it more difficult to break than a standard substitution cipher. 2) The Polybius square cipher which maps letters to numbers based on their coordinates in a 5x5 grid. An example is provided to encrypt the word "university".

1. HOMOPHONIC SUBSTITUTION
CIPHER
Prof. Neeraj Bhargava
Mrs. Shubha Chaturvedi
Department of Computer Science, School of Engineering & System Sciences
MDS University Ajmer, Rajasthan

2. • The Homophonic Substitution cipher is a substitution cipher in
which single plaintext letters can be replaced by any of several
different ciphertext letters. They are generally much more
difficult to break than standard substitution ciphers.
The number of characters each letter is replaced by is part of
the key, e.g. the letter 'E' might be replaced by any of 5
different symbols, while the letter 'Q' may only be substituted
by 1 symbol.
HOMOPHONIC SUBSTITUTION
CIPHER

3. • The Polybius square, also known as the Polybius checkerboard.
• It is 5*5 grid that allows to encrypt letters into numbers .
• It is a device invented by the ancient Greeks Cleoxenus and
Democleitus, and made famous by the historian and scholar Polybius.
• Each letter is then represented by its coordinates in the grid.
• The 26 letters of the Latin/English alphabet do not fit in a 5 × 5
square, two letters must be combined (usually I and J as above,
though C and K is an alternative). Alternatively, a 6 × 6 grid may be
used to allow numerals or special characters to be included as well as
letters.
POLYBIUS/POLYBIUS CHECKBOARD

4. EXAMPLE :
Find out the polybius square cipher for the plain text “university”?
STEP-1
First we will create a matrix 5*5 ,in order to fit 26 letters of the English
Alphabet into the 25 spots letter I and J usually combined.
1 2 3 4 5
1
2
3
4
5
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

5. STEP-2
Assign the number to letter in the plain text from the
about to get cipher text.
PLAINTEXT :
U N I V E R S I T Y
CIPHER TEXT :
45 33 24 51 15 42 43 24 44 54

6. Q. Find out the polybius square cipher for the
plain text “RAJASTHAN”?
Assignment