What is the difference between Substitution Cipher and Transportation Cipher? Explain: Additive Cipher and double transportation cipher with an example.

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Paramters	Substitution Cipher	Transportation Cipher
Definition	A substitution technique is one in which the letters of plain text are replaced by other letters or number or symbols.	Transposition cipher does not substitute one symbol for another instead it changes the location of the symbols
Type	Monoalphabetic and Polyalphabetic substitution cipher.	Keyless and Keyed transportation cipher.
Changes	Each letter retains its position changes its identity	Each letter retains its identity but changes its position
Disadvantage	The last letters of the alphabet which are mostly low frequency tend to stay at the end.	Keys very close to the correct key will reveal long sections of legible plaintext
Example	Ceaser Cipher	Rail fence Cipher

Additive cipher:

The simplest mono-alphabetic cipher is the additive cipher. This cipher is sometimes called ashift cipher and sometimes Caesar cipher, but the term additive cipher better reveals its mathematical nature.
- Assume that the plain text consists of lowercase letters and ciphertext of uppercase letters. For mathematical operations on plaintext and ciphertext assign numerical values to each letter.
- $C= (P+k)mod26$ for encryption and P=(C-k)mod26 for decryption.
- When the cipher is additive, the plaintext, ciphertext, and key are integers in Z26

Plain text:

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Cipher text:

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Value :

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Encrypt the message “hello” using additive cipher with key = 15

Apply the encryption algorithm to the plain-text, character by character

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Double transportation can make the job of the cryptanalyst difficult. It designates the letters in the original plaintext message by the numbers designating their position.
- First the plaintext is written into an array of a given size and then permutation of rows and columns is done according to the specified permutations.

Plaintext: attack at four

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First transportation: permute rows from (1, 2, 3) to (3, 2, 1)

Middle text: fourckatatta

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Second transportation: permute columns from (1, 2, 3, 4) to (4, 2, 1, 3)

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Ciphertext: ROFUTKCAATAT

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