Monoalphabetic Substitution Cipher, also known as a simple substitution cipher, relies (depends) on a fixed replacement structure. That is substitution is fixed for each letter of the alphabet. A simple example is where each letter is encrypted as the next letter in the alphabet: "a simple message" becomes "B TJNQMF NFTTBHF". Each letter can be encrypted to any symbol [&,%,?,/,*,{},@,~,+,-] not just another letter. 1.) Additive Cipher The Simplest Monoalphabetic cipher is an Additive cipher. This cipher is sometimes called Shift Cipher. When the Cipher is Additive, the Plain Text, Cipher Text and key are integers in Z(26). For Encryption: C = (p + k) mod 26 For Decryption: P = (c - k) mod 26 Where, C = Cipher Text P = Plain Text k = Key The secret key between Alice and Bob is also an integer value. The encryption Algorithm adds the key to the Plain text character. Decryption Algorithm Subtract the key from the ciphertext c
