Substitution ciphers are probably the most common form of cipher. They work by replacing each letter of the plaintext (and sometimes puntuation marks and spaces) with another letter (or possibly even a random symbol).



A monoalphabetic substitution cipher, also known as a simple substitution cipher, relies on a fixed replacement structure. That is, the substitution is fixed for each letter of the alphabet. Thus, if a is encrypted to R , then every time we see the letter a in the plaintext, we replace it with the letter R in the ciphertext.-Substitution ciphers are probably the most common form of cipher. They work by replacing each letter of the plaintext (and sometimes puntuation marks and spaces) with another letter (or possibly even a random symbol).



A monoalphabetic substitution cipher, also known as a simple substitution cipher, relies on a fixed replacement structure. That is, the substitution is fixed for each letter of the alphabet. Thus, if　a　is encrypted to　R , then every time we see the letter　a　in the plaintext, we replace it with the letter　R　in the ciphertext.