Frequency Analysis

However, Vigenère Cipher not entirely secure as the key could be repeated thus make it vulnerable to Frequency Analysis if there is too many repetitions. Another way to attack it is by guessing the number of alphabets being used, and then attack by brute force. If the key is too short, then it is easy to break with Kasiski Method#.

Polyalphabetic Cipher is a Substitution Cryptographic #algorithm where the same plaintext letter could be replaced by several ciphertext letters depending on its position in plaintext. The relationship between a character in the plaintext and a character in the ciphertext is one-to-many, thus make it hard to crack using Frequency Analysis# and more secure than Monoalphabetic Cipher.

Playfair Cipher is a #Monoalphabetic Cipher #algorithm invented by Charles Wheatstone in 1854, that encrypts two letters (digram) at once instead of encrypts only one letter. It is resilient against Frequency Analysis since there are 676 digrams to analyse (\(26 \times 26\)) therefore more ciphertext needed to crack. However, given enough amount of letters, it could be easily cracked especially after computer was invented since it still retains much of plaintext structure.

A Cryptanalytic Attack done on Permutation/Transposition Cryptographic algorithm is by first guessing period, try all possible permutation on the period (Brute Force Attack), search for common patterns and then use lists of common diagrams and trigrams (Frequency Analysis#).

Permutation/Transposition Cryptography is a #Cryptographic method that encrypt a message by rearranging the plaintexts. This includes changing Encoding# system of the plaintexts. Since it doesn’t alter the frequency of letters from the original plaintext, it is vulnerable to Frequency Analysis#. It is even more dangerous to group the ciphertext in group that match the key size used by the cipher.

Monoalphabetic Cipher is a #Substitution Cryptographic #algorithm that shuffle the letters arbitrarily (random) which has a key size of 26 letters. The total number of keys are \(26! = 4.03 \times 10^{26}\). However, since in Monoalphabetic Cipher the relationship between the letter in a plaintext to the letter in a ciphertext is always one-to-one, it is vulnerable to Frequency Analysis# due to the fact the frequency of the letters is not altered.

Kasiski Method is a #Cryptanalytic Attack, developed by Baddage and Kasiski, attempts to break #Polyalphabetic Cipher by deducing the length of the keyword. The repetition in the ciphertext, especially in Vigenère Cipher gives clues to the key length by examining the period apart between multiple identical ciphertext patterns and find out the common factor# of the distances (excluding 1 as a possible key length). After that, we could easily do a Frequency Analysis on the ciphertext.

Autokey Cipher is an improvement to #Vigenère Cipher that not only use keyword but also plaintext as the part of the key generation which is as long as the plaintext. By knowing the keyword, we can recover the rest of the message as the plaintext is also part of the key. This makes it rather resilent to Kasiski Method#, however due to its inclusion of plaintext as part of the key, it is not entirely safe from Frequency Analysis#.