Polyalphabetic Cipher
The Polyalphabetic Cipher is a substitution cipher.
With a shift value of n a letter is replaced with the letter that is n positions further forward
Decryption
With a shift value of n a letter is replaced with the letter that is n positions further back.
Alternatively, this is the equivalent of replacing each letter with the letter that is (26 - n)
positions further back
Example
The grid below shows the substution values for a shift value of 3
| plaintext | a | b | c | d | e | f | g | h | i | j | k | l | m | n | o | p | q | r | s | t | u | v | w | x | y | z |
| ciphertext | d | e | f | g | h | i | j | k | l | m | n | o | p | q | r | s | t | u | v | w | x | y | z | a | b | c |
Using different shift values
The polyalphabetic cipher has different shift values for different letter positions
In this activity, plaintext encrypted with a cipher that has three alphabets,
uses shift 1 for the 1st, 4th, 7th ... letters, shift 2 for the 2nd, 5th, 8th ... letters and shift 3 for the 3rd, 6th, 9th ... letters
