This document summarizes and compares different types of substitution cipher techniques. It discusses monoalphabetic ciphers, homophonic ciphers, polyalphabetic ciphers, and polygraphic substitution ciphers. Monoalphabetic ciphers have weak security as the entire message can be deciphered with 25 attempts or by analyzing letter frequencies. Homophonic ciphers are similar but map letters to multiple keys. Polyalphabetic ciphers use multiple shifting keys for improved security. Polygraphic ciphers substitute blocks of text rather than single letters, providing the strongest security of the classic techniques. The document concludes modern encryption methods are superior to these classic substitution ciphers for strong confidentiality.

1. COOCHBEHAR GOVERNMENT
ENGINEERING COLLEGE
NAME- SUBHADIP BARMAN
ROLL NO- 34900121037
DEPT. – CSE
SEM- 8TH
SUBJECT- CRYPTOGRAPHY AND NETWORK SECURITY
CONTINUOUS ASSESSMENT-1

2. TOPIC- " Different types of substitution cipher
techniques and their comparative study."
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Substitution ciphers are classical encryption techniques that replace individual units of
plain text (usually letters) with different symbols or letters based on a predetermined rule or
key. This disguises the message and protects its confidentiality.
Here, we'll explore various types of substitution ciphers and compare their strengths and
weaknesses.
Introduction

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Types of Substitution Ciphers
There are four main types of substitution ciphers. These include monoalphabetic ciphers,
homophonic ciphers, polyalphabetic ciphers, and polygraphic substitution ciphers. However,
multiple examples of substitution ciphers exist within these four main categories.

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1) Mono-alphabetic Cipher
Predictability of Caesar Cipher was its
weakness once any key replacement of a
single alphabet is known then, the whole
message can we decipher and almost 25
attempts are required to break it.
In this technique, we simply substitute any
random key for each alphabet letter, that
is 'A' can be being replaced with any letters
from B to Z and 'B' can be changed to rest of
the Alphabets but itself and so on. Let's say
we substitute A with E that doesn't mean
that B will be replaced by F.
Mathematically, we have 26 alphabet
permutation which means (26 x 25 x 24
x...2) which is about 4 x 1026 possibilities.
There is only one problem with it and that is
short text created using this technique, a
crypto analyst can try different attacks solely
based on her knowledge of the English
language. English analyst found that the
probability of occurrence of the
letter P is 13.33% which highest followed
by Z with 11.67% and occurrence of letters
like C, K, L, N or R is negligible. A cryptanalyst
can try various alphabets in place of cipher-
text alphabet or she can look for repeated
patterns of the word for is example word 'to'
or 'is' occur frequently in English so she can
try replacing all the T's and O's from the
cipher-text and deduce further to three letter
words like 'the', 'and' and so on.

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2) Homophonic Substitution Cipher
The Homophonic substitution and mono-alphabetic substitution are very much alike. Like in
plain cipher substation we replace an alphabet with a key but in case of Homophonic
Substitution, we map an alphabet with a set of fixed keys (more than one key). For
instance, A can be replaced with H, J, O, P and B will replace with any of the following inspite
of A's key set D, I, W, Z etc.

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3) Polygram Substitution Cipher
In Polygram substitution cipher, instead of replacing one plain-text alphabet we simply
replace a block of the word with another block of a word. Example, 'INCLUDEHELP' will
change to 'WDSAEQTGTAI' whereas 'HELP' will replace to 'RYCV'. This is true that the
last four letters are the same but still different in both words.

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4) Polyalphabetic Substitution Cipher
Polyalphabetic Substitution cipher was introduced by Leon Battista in the year 1568,
and its prominent examples are Vigenère cipher and Beaufort cipher.
We use multiple one-character keys, each key encrypts one plain-text character. This
first key encrypts the first plain-text character, the second the key encrypt the second plain-
text character and so on, after all, keys are used then they are recycled. If 50 one-letter
keys, every 50th character in the plain text would be placed with the same key and this
number (in our case, 50) is period of the cipher.
The key points of the polyalphabetic substation cipher are the following:
i. It uses a set of related mono-alphabetic substitution rules.
ii. The ruleused for transformations determined by the key it uses.

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Feature Monoalphabetic Polyalphabetic Homophonic Polygram
Security Weak Moderate Moderate Strong
Complexity Low Moderate High High
Key Management Simple Moderate Moderate High
Frequency
Analysis
Vulnerable Partially Resistant Resistant More Resistant
Comparative STUDY

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The choice of substitution cipher technique depends on your specific needs. For educational
purposes or low-security applications, simple ciphers like Caesar or Vigenère might suffice.
For moderate security, consider homophonic or polygram ciphers. However, for strong
security and confidentiality, modern and complex encryption methods are far superior to
classic substitution ciphers.
CONCLUSION

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Thank You