Skip to main content

Home Substitution cipher

Substitution cipher

Substitution cipher definition

A substitution cipher is an encryption method in which each letter in the message is replaced by another letter or symbol according to a secret key. Substitution ciphers are some of the simplest and oldest forms of cryptography, dating back thousands of years.

In modern day, substitution ciphers are rarely used outside of puzzles or learning environments. They have been supplanted by more sophisticated algorithms (like the Advanced Encryption Standard (AES) and Rivest-Shamir-Adleman (RSA) encryption) that provide greater resilience against cryptanalysis.

See also: block cipher, ciphertext, 128-bit encryption, AES encryption, cryptanalysis, cryptographic algorithm, cryptographic key, decryption, encryption key

How substitution ciphers work

In a substitution cipher, each letter of the alphabet is mapped to a corresponding replacement letter or symbol using a secret key. The substitution process turns plaintext (the unencrypted text) into ciphertext (the encrypted message visible to others). To decrypt the ciphertext and obtain the original plaintext, you need to reverse the substitution process using the same key.

Breaking substitution ciphers

Because they rely on fixed and consistent patterns, substitution ciphers are relatively weak and easy to break. Due to their vulnerability to attacks, substitution ciphers are mainly used for learning purposes, brain teasers, or as historical examples of early encryption methods.

One common method used to crack substitution ciphers is frequency analysis — by analyzing the frequency of the letters in the ciphertext, cryptanalysts can make educated guesses about the corresponding letters in the plaintext.