What Is Substitution Cipher Technique?

In cryptography (the science of writing, analyzing and deciphering codes), a substitution cipher is a method of encryption by which units of plaintext are replaced with other symbols or groups of symbols in accordance with a fixed set of rules. These plaintext units may be individual letters or characters, letter pairs, triplets or other combination.

Substitution cipher technique may only replace the letters of the standard alphabet with ciphertext or apply substitution to spaces and punctuation marks as well.

There are two main approaches that have been employed with substitution ciphers to lessen the extent to which structure in the plaintext-primarily single-letter frequencies-survives in the ciphertext. One approach is to encrypt elements of plaintext consisting of two or more symbols e.g digraphs and trigraphs. The other is to use several cipher alphabets. When this approach of polyalphabetic substitution is carried to its limit, it results in onetime keys or pads.

What You Need To Know About Substitution Cipher

In substitution cipher technique, the letters of plain text are replaced by other letters or number or symbols.
Monoalphabetic , polyalphabetic substitution cipher, Playfair cipher are some of the algorithms which use the substitution technique.
In substitution cipher technique, the letter with low frequency can detect plain text.
In substitution cipher technique, character’s identity is changed while its position remains unchanged.
The last letters of the alphabet which are mostly low frequency tend to stay at the end.
Caesar cipher is a good example of substitution cipher.

What Is Transposition Cipher Technique ?

Transposition cipher is a method of encryption by which the positions held by units of plaintext (which are commonly characters or group of characters) are shifted according to a regular system, so that the ciphertext constitutes a permutation of the plaintext. That is, the order of the units is changed (the plaintext is reordered).

What You Need To Know About Substitution Cipher

Transposition cipher technique does not substitute one symbol for another instead it changes the location of the symbol.
Keyless and keyed transpositional cipher are the main forms of transposition cipher.
In transposition cipher technique, the keys which are nearer to correct key can disclose plain text.
In transposition cipher technique, the position of the character is changed but character’s identity is not changed.
Keys very close to the correct key will reveal long sections of legible plaintext.
Reil Fence Cipher is a good example of transposition cipher.

Also Read: Difference Between Monoalphabetic And Polyalphabetic Cipher

Difference Between Substitution And Transposition Cipher Technique In Tabular Form

BASIS OF COMPARISON	SUBSTITUTION CIPHER TECHNIQUE	TRANSPOSITION CIPHER TECHNIQUE
Description	In substitution cipher technique, the letters of plain text are replaced by other letters or number or symbols.	Transposition cipher technique does not substitute one symbol for another instead it changes the location of the symbol.
Form	Monoalphabetic , polyalphabetic substitution cipher, Playfair cipher are some of the algorithms which use the substitution technique.	Keyless and keyed transpositional cipher are the main forms of transposition cipher.
Disadvantage	In substitution cipher technique, the letter with low frequency can detect plain text.	In transposition cipher technique, the keys which are nearer to correct key can disclose plain text.
Alterations	In substitution cipher technique, character’s identity is changed while its position remains unchanged.	In transposition cipher technique, the position of the character is changed but character’s identity is not changed.
Position	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	Caesar cipher is a good example of substitution cipher.	Reil Fence Cipher is a good example of transposition cipher.

Also Read: Difference Between Block Cipher And Stream Cipher