Vigenère Cipher

Vigenère Cipher is a #Polyalphabetic Cipher which is basically multiple interwoven Shift Cipher# using a long key with the length of the list of alphabets used.

The encryption process works like this:

Write the plaintext out.
Write the keyword repeated until it match the length of the plaintext, which will be the key for the Cipher.
Use each key letter as a Shift Cipher# key.
Substitute the letter with its corresponding position in the current Cipher key and proceed until we encrypt all the plaintext letters.

The decryption is simply reversing the whole encryption process.

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#.

Links to this page

TAC3121 Chapter 3: Classic Ciphers

Vigenère Cipher
Polyalphabetic Cipher

Vigenère Cipher#
Kasiski Method

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

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#.

#cryptography