Substitution Cipher and Transposition Cipher

• A substitution technique is one in which the letters of plain text are replaced by other letters or number or symbols. If plain text is viewed as a sequence of bits then substitution involves replacing plain text bit patterns with cipher text bit patterns.

i. Monoalphabetic Ciphers: A character in plaintext is always changed to the same character in the ciphertext regardless of its position. Example of this is the Ceaser cipher which involves replacing each letter of the alphabet with the letter standing three places further down the alphabet.

Text – meet me after the toga party

Ciphertext- PHHW PH DIWHU WHK WRJD SDUWB

ii. Polyalphabetic Ciphers: Each occurrence of a character may have a different substitute. The relationship between characters in plaintext to a character in ciphertext is one-to-many. Example of this is Playfair Cipher cipher which treats diagrams in the plain text as single units and translates these units into ciphertext diagrams.

• Transposition cipher does not substitute one symbol for another instead it changes the location of the symbols. This technique encrypts plaintext by moving small pieces of the message around.

i. Keyless Transportation Ciphers: These are simple ones which are keyless. In first method the text is written into a table column by column and then row by row. In the second method the text is written into the table row by row and then transmitted column by column.

ii. Keyed Transportation ciphers: The plaintext is divided into groups of predetermined size called blocks and then uses a key to permute the characters in each block separately.