# Substitution Techniques and Its Types | Cryptography

Cryptography | Substitution Techniques: In this tutorial, we will learn about the substitution techniques of Cryptography, and its different types. By Himanshu Bhatt Last updated : May 24, 2023

As we already discussed **what are the Substitution techniques and one of its type Ceasar Cipher**? So we are not discussing it here for that please refer to **Cryptography: CeasarCipher** here: Cryptography: Caesar Cipher and its Python Implementations

## Types of Substitution Techniques

### 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.

### 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.

### 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.

### 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 50^{th} 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:

- It uses a set of related mono-alphabetic substitution rules.
- The ruleused for transformations determined by the key it uses.

Related Tutorials

- Introduction to Cryptography
- Types of Cryptography: Symmetric and Asymmetric
- Cryptography Transposition Techniques
- Playfair Cipher in Cryptography
- Feistel Cipher in Cryptography
- Hill Cipher in Cryptography
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- Elliptic Curve Cryptography
- Message-Digest Algorithm 5 (MD5) in Cryptography
- Cryptography Security: Goals, Attacks, and Principles
- Difference Between Stream Ciphers and Block Ciphers
- Caesar Cipher in Cryptography
- Vigenère Cipher in Cryptography
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