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. 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. If you look at a monoalphabetic cypher from the point of view of a code breaker, assuming the plaintext alphabet is based on the English alphabet of 26 letters.


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monoalphabetic cipher HELLO becomes encrypted as: IFMMP I can shift one letter to the right, two letters to the right, or up to 25 letters to the right. Consider monoalphabetic cipher "number of letters to the right" as the "key".

It takes 5 bites to hold a number between 0 and Therefore, the key size is "5 bits". Or, instead of shifting to the right, let's say that we randomly re-order the alphabet, and do a one-to-one mapping: How to decipher a substitution without the alphabet?

The interactive tool provided by dCode allows a semi-automatic decryption of messages ciphered by substitution. Monoalphabetic cipher main technique is to analyze the frequencies of letters and find the most likely bigrams.

Alphabetical Substitution

The most common alphabets used for substitutions are: The book cipher and straddling checkerboard are types of homophonic monoalphabetic cipher.

Francesco I GonzagaDuke of Mantuaused the earliest known example of a homophonic substitution cipher in for correspondence with one Simone de Crema.

Polyalphabetic cipher Polyalphabetic substitution ciphers were first described in by Leone Battista Alberti in the form of disks.

Johannes Trithemiusin his book Steganographia Ancient Greek for "hidden writing" introduced the now more standard form of a tableau see below; ca.

What is a monoalphabetic cipher? - Quora

A more sophisticated version using mixed alphabets was described in by Giovanni Battista della Porta in his book, De Furtivis Literarum Notis Latin for "On concealed characters in writing". In a polyalphabetic cipher, multiple cipher alphabets are used.

To facilitate encryption, all the alphabets are usually written out in a large tabletraditionally called a tableau. The method of filling the tableau, and of choosing which alphabet to use monoalphabetic cipher, defines the particular polyalphabetic cipher.


All such ciphers are easier to break than once believed, as substitution alphabets are repeated for sufficiently large plaintexts. Such a simple tableau is called a tabula rectaand mathematically corresponds to adding the plaintext and key letters, modulo A keyword is then used to choose which ciphertext alphabet to use.

Each letter of the keyword is used in turn, and then they are repeated again from the beginning. So if the keyword is 'CAT', the first letter of plaintext is enciphered under alphabet 'C', the second under 'A', the monoalphabetic cipher under 'T', the fourth under 'C' again, and so on.

Alphabetical Substitution Cipher - Cryptogram Decoder, Solver

Once this was done, ciphertext letters that had been enciphered under the same alphabet could be picked out and attacked separately as a number of semi-independent simple substitutions - complicated by the fact that within one alphabet letters were separated and did not form complete words, but simplified by the fact that usually a tabula recta had been employed.

Other notable polyalphabetics include: This operation is self-inverse, whereby the same table is used for both encryption and decryption. The autokey cipher monoalphabetic cipher, which mixes plaintext with a key to avoid periodicity.

The running key cipherwhere the key is made very long by using a passage from a book or similar text. Modern stream ciphers can also be seen, from a sufficiently abstract perspective, to be a form of polyalphabetic cipher in which all the effort has gone into making the keystream as long and unpredictable as possible.