In the world of technology, all want a safe personal space to communicate. Cryptography is the practice and study of methods for safe communication in the presence of friendly behaviour.

Have you ever heard the term substitution cipher? If not, don't worry. We will cover this in this article.

Welcome back to yet another exciting article name 'Cryptanalysis of Substitution Cipher.'

Let's start with the introduction of Cryptanalysis.

Cryptanalysis

It is the analysis of ways to decode encoded data without access to the hidden information generally needed. Knowing how the process works and obtaining a private key are usually required.

Another name for cryptanalysis is cracking the code or codebreaking. Since the ciphertext is typically the component of a cryptosystem that is simplest to access, it is crucial to cryptanalysis.

As the name signifies, substitution encryption alternates (replaces) one element with another. In terms of a text, it is a matter of replacing the message's characters (often letters) with others.

For a substitution to be correct, it is required for the same component or element to be substituted by only one other (correct in both chances, for encryption and decryption) so that there is no chance of more than one option for encryption or decryption.

Example:

Here's a brief summary of the simple substitution cipher's encryption and decryption steps. The text we will encrypt is 'welcome to coding ninjas'.

The simple substitution cipher's keys normally consist of 26 letters (as compared to the caeser cipher's single number). Here's an example key:

Plaintext: welcome to coding ninjas.
Ciphertext: ldicorm cf lsienr oskwrs.

Historical Ciphers

Classical cyphers are also used to refer to historical pen and paper cyphers that were formerly in use. They consist of the following:

Cipher for Transposition

The plaintext is permuted into the ciphertext.

Cipher of Substitution

The ciphertext is used to replace the plaintext units.

Permutation Cipher

The transposition cypher with a permutational key.

Cipher using Polyalphabetic Substitution

A substitution cipher that uses several substitution alphabets

People are more vulnerable to attack, and historical cyphers are rarely used as a stand-alone cryptography solution. Traditional cyphers are vulnerable to brute force attacks or analysis of the only ciphertext other than the one-time pad.

Cryptanalysis of Substitution Cipher

Cryptanalysis of Substitution Cipher is also known as Frequency analysis.

Simple substitution cyphers (mono and poly-alphabetic) can be processed using a fast algorithm based on a method in which an initial key guess is improved over several iterations.

The plaintext equal to the current key is evaluated in each step, and the result is used to test how close we are to finding the correct key.

It turns out that only knowledge of the ciphertext's diagram distribution and the expected plaintext diagram distribution is required to solve the cypher.

The algorithm only needs to calculate the distribution matrix once. The next plaintext evaluation is completed using this matrix rather than decrypting the ciphertext and reparsing the resulting plaintext in each iteration.

Below are the steps for the cryptanalysis of the Substitution Cipher:

Suppose we have a long ciphertext. The task is to decipher it.

It will let us know that the text was in English and encrypted with a monoalphabetic substitution cipher.

Searching for all possible keys is impractical due to the keyspace's size of 26.

Analyze the ciphertext in question and determine the frequency of each letter.

If the most common letter in the ciphertext is, say, J, this is likely a substitution for E.

If P is the second most common letter in the ciphertext, it is most likely a substitution for t, and so on.

However, language commonnesses, such as relative frequency, can be used.

Frequency analysis usually requires logical reasoning, intuition, adaptability, and guesswork.

Frequently Asked Questions

What do you mean by "cryptanalysis"?

Cryptanalysis is the study of methods for decrypting encrypted data without access to the secret information that is generally required.

Name some cryptography algorithms.

These are some of the cryptography algorithms - AES(Advanced Encryption Standard), DES(Data Encryption Standard), Blowfish, and RSA(Rivest-Shamir-Adleman).

Can someone hack encrypted data?

Yes, attackers can hack encrypted data. However, depending on the encryption level applied, the difficulty level increased.

Why is encryption necessary?

It aims at protecting private and sensitive data. It also helps in the security of communication between clients and servers.

What are the different types of cryptanalysis?

There are three types of cryptanalysis based on what the cryptanalyst knows: ciphertext only understood ciphertext/plaintext pairs and plaintext or ciphertext of choice.

Conclusion

In this blog, we discussed the Cryptanalysis of Substitution Cipher. We also explored the main types and features of cryptography.

For more information on cryptography, you may refer to the following articles: