Substitution Cipher

Substitution encryption consists, as its name suggests, of substituting (replacing) one element with another. In the case of a text, it is a question of replacing the characters (often letters) of the message by others.

For a substitution to be correct, it is necessary for the same element to be substituted by only one other (valid in both cases, for encryption and decryption) so that there is never more than one possibility of encryption or decryption.

The condition for a successful substitution decryption is to know the correspondence lookup table used.

If the substitution is a simple transcription, then the frequency analysis can quickly recognize the language of the plain message.

If the substitution is more complex, it may become impossible to recognize the encryption.

The notions of substitutes, replacements and changes are clues.

Substitution cipher is one of the most basic cryptography methods. Many variations are possible:

— Ciphers by mono-alphabetic substitution, with a disordered alphabet, one letter replaces another.

— Encryptions by poly-alphabetic substitution, with several alphabets.

— Encryptions by homophonic substitution, the same element can be substituted by several others.

— Substitution by dictionary, with words, n-grams substituted by others.

It is likely that substitution encryption appeared short after the invention of writing.