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What are the substitution techniques in information security?
Substitution technique is a classical encryption approach where the characters present in the initial message are restored by the other characters or numbers or by symbols. If the plain text (original message) is treated as the string of bits, thus the substitution technique would restore bit pattern of plain text with the bit pattern of cipher text.
There are various types of substitution ciphers which are as follows −
Monoalphabetic Cipher − In monoalphabetic substitution cipher, a character in a plaintext is always restored or changed to the similar character in the ciphertext indifferent of its position in the text.
For instance, if a letter A in the plaintext is changed to G then each appearance of A in the plaintext will be restored by G.
Plaintext : hello
Ciphertext : IFMMP
This is a monoalphabetic cipher as both 1’s are encrypted as ‘M’.
Polyalphabetic cipher − In polyalphabetic substitution, each appearance of a character in the plaintext can have a different substitution character in the ciphertext.
The relationship among a character in plaintext and a character in ciphertext is one to many. For instance, letter ‘A’ can be restored by the letter ‘C’ and the similar letter ‘A’ can be restored by ‘N’ later in the ciphertext.
In polyalphabetic cipher, frequencies of plaintext letter are not reflected in the ciphertext. Therefore, breaking of polyalphabetic cipher is complex than monoalphabetic cipher as statistical analysis cannot be used on it.
The main feature of polyalphabetic substitution cipher are the following −
- It is needed a set of associated monoalphabetic substitution rules.
- It need a key that decides which rule is used for which transformation.
- It can hiding the letter frequency of the underlying language including Playfair Cipher, Vigenere Cipher, and Hill Cipher.
One-Time Pad − The one-time pad cipher recommend that the key length must be as long as the plain text to avoid the repetition of key. Along with that, the key must be used only once to encrypt and decrypt the individual message after that the key must be discarded.
Caesar Cipher − In this substitution technique, it can encrypt the plain text, each alphabet of the plain text is restored by the alphabet three places further it and it can decrypt the cipher text each alphabet of cipher text is restored by the alphabet three places before it.
Playfair Cipher − The playfair cipher is also known as Playfair Square. It is a cryptographic technique used for manual encryption of information. This scheme was developed by Charles Wheatstone in 1854.
The Playfair cipher was used by the British army in World War I and by the Australian in World War II. This was applicable because the playfair cipher is perfectly fast to use and does not demand some specific equipment to be used.
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