Multiplicative Cipher

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While using Caesar cipher technique, encrypting and decrypting symbols involves converting the values into numbers with a simple basic procedure of addition or subtraction.

If multiplication is used to convert to cipher text, it is called a wrap-aroundsituation. Consider the letters and the associated numbers to be used as shown below −

The numbers will be used for multiplication procedure and the associated key is 7. The basic formula to be used in such a scenario to generate a multiplicative cipher is as follows −

(Alphabet Number * key)mod(total number of alphabets)

The number fetched through output is mapped in the table mentioned above and the corresponding letter is taken as the encrypted letter.

The basic modulation function of a multiplicative cipher in Python is as follows −

def unshift(key, ch):
   offset = ord(ch) - ASC_A
   return chr(((key[0] * (offset + key[1])) % WIDTH) + ASC_A)

Note − The advantage with a multiplicative cipher is that it can work with very large keys like 8,953,851. It would take quite a long time for a computer to brute-force through a majority of nine million keys.