Additive Shift Ceaser Cipher Technique

In this class, We discuss the Additive Shift Ceaser Cipher Technique.

The reader should have prior knowledge of Discrete mathematics basics. Click Here.

Additive Cipher:

The other names for additive cipher or, shift, or cease cipher.

For each character, we provide a number in additive cipher.

The alphabets A- Z are given numbers 0 – 25.

Example:

We take a plain text P = hello

We use the technique additive modulo 26.

h is assigned number 7

e is assigned number 4

l is assigned number 11

o is assigned number 14

Let’s take key K = 15

Encryption:

h is encrypted as (7 + 15) mod 26 = 22 = w

h is encrypted as w.

Similarly remaining alphabet.

e encrypted as t.

l encrypted as a

The final cipher text C = wtaad

The formulae during encryption is C = (p + k) mod 26

The formulae during decryption are P = (c – k) mod 26

Here, before we do encryption and decryption key has to be exchanged between two parties.

How to send the key safely?

The discussion on sending keys will be done at the end of the course.

Two important points one should always think about in cryptography.

1) Is it easy to find the plain text by looking at cipher text

2) Is it easy to find the key by looking at cipher text