Substitution cipher | Algorithm, Encryption & Decryption (2024)

FAQs

How to solve substitution cypher? ›

The Playfair cipher is thus significantly harder to break since the frequency analysis used for simple substitution ciphers does not work with it. The frequency analysis of bigrams is possible, but considerably more difficult.

We can see that the Substitution cipher is not perfectly secret, as a ciphertext with repeated letters (obtainable from a message with repeated letters) cannot be obtained from a message with distinct letters. However, it is perfectly secure, if a randomly generated key is used for each letter of the message.

Table 3.1 shows an example of a substitution cipher. Because some letters appear more often and certain words are used more than others, some ciphers are extremely easy to decrypt and can be deciphered at a glance by more practiced cryptologists.

Decryption with Caesar cipher

In practice, a good encryption system ensures that the plaintext cannot be obtained from the ciphertext without the key, i.e. it can be decrypted but not cracked.

In cryptography, a Caesar cipher, also known as Caesar's cipher, the shift cipher, Caesar's code, or Caesar shift, is one of the simplest and most widely known encryption techniques.

AES ‍ One of the hardest codes to crack is arguably the US government's Advanced Encryption Standard (aka Rijndael or AES) which the Americans use to protect top-secret information. AES is considered unbreakable by even the most sophisticated hackers.

The explanation for this weakness is that the frequency distributions of symbols in the plaintext and in the ciphertext are identical, only the symbols having been relabeled. In fact, any structure or pattern in the plaintext is preserved intact in the ciphertext, so that the cryptanalyst's task is an easy one.

' Another—somewhat cryptographically stronger—example of a monoalphabetic substitution cipher is to use an arbitrary permutation of the alphabet, rather than shifting by a certain number. Rather than only 25 possible keys, we have 26!

One of the most common ways to crack a substitution cipher is to use frequency analysis. This means counting how often each letter appears in the ciphertext and comparing it to the expected frequency of each letter in the language of the plaintext.

What is used as an aid to breaking substitution ciphers? ›

In cryptography, frequency analysis is the study of the frequency of letters or groups of letters in a ciphertext. The method is used as an aid to breaking substitution ciphers (e.g. mono-alphabetic substitution cipher, Caesar shift cipher, Vatsyayana cipher).

That's because for substitution cipher the key is defined as a unique replacement of each letter with another one, e.g. A -> D, B -> M, C -> Y, etc.

Write down the alphabet in order from A to Z. Replace each letter of the message with the corresponding letter from the new alphabet. For example, if the shift value is 3, the word “hello” would become “khoor”. To decrypt the message, shift each letter back by the same amount.

To decipher it, the recipient has to work out the column lengths by dividing the message length by the key length. Then, write the message out in columns again, then re-order the columns by reforming the key word.