91Ó°ÊÓ

Frequency analysis is a technique used in cryptography to crack ciphers by studying the frequency of letters or groups of letters in a piece of ciphertext. In English, some letters such as 'E', 'T', 'A', and 'O' appear more frequently than others. Suppose you intercept a ciphertext message and find that the letter frequencies align closely with those in English.
This similarity is a big clue. It suggests that whatever cipher was used to encrypt the text, it didn't change the relative frequencies of the letters. Therefore, frequency analysis allows us to suspect that a simple substitution cipher was employed.
Using this principle, cryptanalysts count letter occurrences in the ciphertext. They compare these counts to known frequencies in the language. Deviations or consistencies can reveal patterns. Sometimes, even non-alphabetic elements, like spaces, can be analyzed to offer more clues.

Ciphertext is the result of encrypting plaintext through a cipher. This text appears scrambled and unreadable without the decryption key.
Let's quickly look at how you might get ciphertext: You start with a regular message; this is your plaintext. By applying a cipher to this plaintext, each letter or set of letters is transformed according to the rules of the cipher. The output is what we call ciphertext. For example, the plaintext 'HELLO' encrypted with a Caesar cipher might become 'IFMMP'.
Intercepting ciphertexts is common in cryptographic practices. However, without knowing the type of cipher or the key, decrypting that ciphertext can be challenging. This is where analysis techniques like frequency analysis become invaluable.

The Caesar cipher is one of the simplest and oldest forms of substitution ciphers. Named after Julius Caesar, it works by shifting each letter of the plaintext by a fixed number of positions down the alphabet. For example, with a shift of 3, 'A' becomes 'D', 'B' becomes 'E', and so on.
This shift wraps around at the end of the alphabet. So, with a shift of 3, 'X' would become 'A', 'Y' would become 'B', and 'Z' would become 'C'. When a message is encrypted with a Caesar cipher, the letter frequencies of the ciphertext remain the same as those in the plaintext, only shifted.
It's straightforward to decrypt if you know the key (the number of positions shifted). If not, it can still be cracked relatively easily through frequency analysis, as patterns in letter distributions can reveal the shift.

Monoalphabetic substitution is a type of substitution cipher where each letter of the plaintext is replaced with another fixed letter of the alphabet. Unlike the Caesar cipher, the substitution is not just a shift but a more complex and typically randomized mapping of letters.
For example, in a monoalphabetic substitution cipher, 'A' might be substituted with 'M', 'B' with 'N', 'C' with 'O', and so forth. The key here is the mapping itself. Once the mapping is defined, it consistently transforms the entire plaintext.
While this type of cipher maintains letter frequency, making it susceptible to frequency analysis, it's more secure than a Caesar cipher due to its non-linear letter substitution. However, complex this mapping may be, if we can determine letter frequencies accurately, we have a good chance of revealing the original message.