Substitution Cipher

Substitution ciphers are a cryptographic method in which each symbol in the plaintext (most often a letter) is replaced by another symbol according to a fixed lookup table called a key.

In its most common form (monoalphabetic substitution), each letter of the alphabet is associated with a unique, different letter, and this association remains the same throughout the entire text.

For a simple monoalphabetic substitution to be correct, the correspondence between letters must be bijective: each letter in the plaintext corresponds to one and only one letter in the ciphertext, and vice versa. This guarantees the uniqueness of the encryption and decryption.

To decrypt by substitution, the user must either possess the lookup table (the key) or be able to retrieve it through cryptographic analysis.

Decryption then consists of applying the inverse combination of the one used for encryption.

If the substitution is monoalphabetic, frequency analysis often allows us to identify the language of the plaintext, because the letters retain their statistical distribution.

The presence of systematic correspondences between symbols (substitutes) and recurring patterns are typical indicators of a substitution cipher.

The concepts of substitutes and changes/replacements 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 (or book cipher), with words, n-grams substituted by others.

There is no single date for its invention, but attested examples of substitution ciphers appear as early as antiquity: the Atbash cipher is documented in ancient Hebrew, and the Caesar cipher is attested in Latin texts from the 1st century BCE. It is likely that substitution ciphers appeared shortly after the invention of writing.