A monoalphabetic cipher is any cipher in which the letters of the plaintext are mapped to ciphertext letters based on a single alphabet key.
Substitution ciphers work by replacing each letter of the plaintext with another letter. For this reason, a monoalphabetic cipher is also called a simple substitution cipher.
It relies on a fixed replacement structure, meaning the substitution is fixed for each letter of the alphabet. Thus, if the letter “a” is encoded as letter “Q”, then every time the letter “a” appears in the plaintext, it’s replaced with the letter “Q”.
Examples of monoalphabetic ciphers include:
Caesar cipher, where each letter is shifted based on a numeric key.
Atbash cipher, where each letter is mapped to the letter symmetric to it about the center of the alphabet.
There are many different monoalphabetic substitution ciphers, actually infinitely many, as each letter can be encrypted to any symbol, not just another letter.
What is the difference between a monoalphabetic and a polyalphabetic cipher
A monoalphabetic cipher mixes up letters of the alphabet and keeps that same arrangement for the entire message while a polyalphabetic cipher uses more that one monoalphabetic cipher with the same message.
To put it simply, polyalphabetic ciphers utilizes multiple, changing ciphertext alphabets to encrypt the plaintext while monoalphabetic ciphers use a single method throughout the entire message.
For example, if you are using a Vigenere disk and you have the two disks stationary while encoding a message, that’s a monoalphabetic cipher.
If on the other hand, you rotate the disks while encoding the same message, while using a keyword, then that becomes a polyalphabetic cipher.
Why are monoalphabetic ciphers easy to break
All monoalphabetic ciphers can be easily broken using a decryption method called letter frequency analysis, which was invented in the 1300s by Ibn ad-Duraihim.
This is done by studying the text in the language of the cipher, and the frequency of each letter can be determined. For example, in the English language, the letter “E” is the most frequent letter, followed by the letter “T”.
Here is the entire frequency in order:
Frequency order: ETAOINSHRDLCUMWFGYPBVKJXQZ.
By substituting the most frequent letter in the ciphertext with the letter “E”, the second most frequent with the letter “T”, and so on, you will end up with the original plaintext.
For example, try out this example:
Ciphertext: dtz fwj fbjxtrj.
You need to determine the type of cipher I used to encode it.