Difference between Monoalphabetic Cipher and Polyalphabetic Cipher

Have you ever wondered how our data is transferred and stored securely? It is done using different types of encryption. But, what is encryption? In simple words, it is defined as the process of converting data from one form to another form that the sender and receiver will only know. In this article, let us discuss two types of encryptions −

Monoalphabetic cipher and Polyalphabetic cipher.

Monoalphabetic Cipher

Consider the word "India." There are five letters in this word. Now, consider each letter of this word replaced by other letters in a specific pattern. For example, in a Caesar cipher, each alphabet is replaced by another alphabet three down the alphabetic series. So, the word "India '' becomes "Lqgld." Interesting, isn't it?

We might think that it will be tough for hackers to break by seeing the encrypted word. But, it is not. If we look closely at the encrypted word, we can see that the same letter "l" is repeated in the word "Lqgld". Also, the number of alphabets in the code is fixed and the same as the original word. These problems make it vulnerable to many types of attacks.

This type of encryption can be easily broken down using the "Brute Force Algorithm". This algorithm tries to decrypt the message by trying all the possible combinations. Thus, to prevent this type of attack, the words should be long enough, which is impossible for every word in a sentence.

Therefore, this type of cipher is not used in many cases.

Polyalphabetic Cipher

We saw that the disadvantage of Monoalphabetic Cipher is that it keeps the frequency of alphabets similar to the original word. How can we overcome this? It can be done quickly by using a Polyalphabetic cipher.

The alphabets for a particular letter in the original world won't be the same in this cipher type. Thus, the people trying to decrypt the message by using frequency analysis or brute force algorithms won't be able to do so that easily.

Vigenere cipher is a type of polyalphabetic cipher. In this cipher type, a person can write a repeating word in a row and write the message to be encrypted in the next row. Next, the letters in the message are mapped to the letter above it. This method ensures that the frequency of a letter is not similar to the message. And also, the space between every word is encrypted. This type of method was thought to be uncrackable for almost 300 years! However, as technology and programming evolved, people found the algorithm to crack this cipher too!

Monoalphabetic Cipher Vs. Polyalphabetic cipher

The following table highlights the major differences between Monoalphabetic Cipher and Polyalphabetic Cipher −

Monoalphabetic CipherPolyalphabetic Cipher
Less secureMore secure than a Monoalphabetic cipher.
Contains frequency of letters same as the message.It does not contain the same frequency of letters as in the message.
The space between words is left blank.The space between the words are also mapped to some letters.
Brute force algorithm can decrypt it.Brute force algorithm cannot decrypt it.
It is not used nowadays.It is used more frequently than Monoalphabetic cipher.
The same alphabet in the message will be made up of similar code letters.The same alphabet in the code may or may not be made of similar code letters.

Updated on: 07-Sep-2021

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