Question

Describe how polyalphabetic substitution works

Answer 1

Answer:---

A polyalphabetic cipher is any cipher based on substitution, using multiple substitution alphabets

Vigenere cipher is one of the best known example of polyalphabetic substitution.

Polyalphabetic substitution is based on the substitution, using different substitution alphabets.

The first known polyalphabetic cipher was the Alberti cipher inverted by Leon Battista.

He used a mixed alphabet to encrypt the plaintext, but at random points he would change to a different mixed , indicating the change with an uppercase letter in the ciphertext. In order to utilize this cipher , alberti used cipherdisc

A number in the plaintext would encrypt to a letter in the cipher text.

When this was decrypted, the number would be revealed and the disc would be moved so that the ciphertext letter was the new key letter

Duo to the polyalphabetic nature of the Alberti cipher,

It was a very secure cipher when it was invented.

However, there are lots of hints with in the workings of the cipher as to how it, work, and

although frequency analysis on the whole message will not work,

You can do frequency analysis on the bits between the letters that indicate a change of the disc.

Alberti actual cipher disc was slightly differ and use for the number

for example of a polyalphabetic cipher

this was invented by Johannes Trithemius

Rather than switching alphabets randomly, and indicating it with an uppercase letter, the Trithemius

Cipher has the sender change the ciphertext alphabet after each letter was encrypted.

This was the first example of a progressive key cipher, and he used a tabula recta to show all the different alphabets.

This is the Tabula Recta for the English alphabet.

Trithemius' idea was to start at the column headed by "A", find the plaintext letter down the far left column, and encrypt this to the ciphertext letter in the first column. You would then move to the next column, and so on.

For example, the plaintext "johannes trithemius" would be encrypted as follows. The "j" would be found down the left column, and mapped to the letter in the column headed by A (shown in red below).

This gives "J". The "o" is found down the left column, and traced to the ciphertext in the B column, which is "P" (shown in blue).

The "h" (shown in green) gives "J", the "a" (shown in purple) gives "D", and the "n" (shown in pink) gives "R".

Continuing in this way we get "JPJDRSKZ BASETRAXKJ".

This is the Tabula Recta shows he encrypton of the first 5 letters of the pailntext “johannes trithemius”

A slightly more secure version of this cipher is to choose two ciphertext alphabets before hand, and alternate between which one you use for plaintext.

This adds an element of a key to the cipher