Tool to decrypt monoalphabetical substitution and find each letter of a substituted message with a deranged alphabet (monoalphabetic cipher).

Mono-alphabetic Substitution - dCode

Tag(s) : Substitution Cipher

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An alphabetic substitution is a substitution cipher where the letters of the alphabet are replaced by others according to a 1-1 correspondence (a plain letter always corresponds to the same cipher letter).

The substitution is said to be monoalphabetic because it uses only one alphabet, this alphabet is said to be disordered.

The monoalphabetical substitution consists in using a mixed alphabet (with the letters in an unusual order) and replacing the letters of the alphabet normal by it.

__Example:__ `NBAJYFOWLZMPXIKUVCDEGRQSTH` is a totally random alphabet with the 26 letters of the Latin alphabet.

To understand, write the alphabet over the classic alphabet:

Plain alphabet | ABCDEFGHIJKLMNOPQRSTUVWXYZ |
---|---|

Substitution alphabet | NBAJYFOWLZMPXIKUVCDEGRQSTH |

The substitution involves a replacement in the plaintext of all the letters of the first row with the letters associated with the second row.

__Example:__ All `A` become `N`, all the `B` remain `B`, all the `C` become `A`, etc.

__Example:__ With this substitution `DCODE` is encrypted as `JAKJY`.

Any deranged alphabet can be used to create a mono alphabetic substitution provided it respects the criterion of an alphabet: not containing the same letter several times.

Decryption requires knowing the alphabet mixed used and the inverse substitution encryption.

Substitution Alphabet | NBAJYFOWLZMPXIKUVCDEGRQSTH |
---|---|

Plain Alphabet | ABCDEFGHIJKLMNOPQRSTUVWXYZ |

The substitution involves replacing in the ciphertext all the letters of the first row with the letters associated with the second row.

__Example:__ The encrypted message `JAKJY` has for plain message `DCODE`.

The ciphered message has an index of coincidence identical to the language of the plain text.

English speakers call this encryption *aristocrat* (if there are spaces) or *patristocrat* (if there are no spaces between words).

The MCMC technique (used by dCode) is one of the most effective for finding the most probable plain text and proposing a replacement alphabet.

dCode additionally adds an interactive tool to manually decrypt substitution-encrypted messages.

Another possibility, the known plaintext attack that makes possible to deduce some letters of the alphabet via the knowledge or the preliminary guess of certain portions of the plain text.

__Example:__ The most common alphabets used for substitutions are: `QWERTYUIOPASDFGHJKLZXCVBNM``MNBVCXZLKJHGFDSAPOIUYTREWQ`

QAZWSXEDCRFVTGBYHNUJMIKOLPAZERTYUIOPQSDFGHJKLMWXCVBN

MCMC (Markov Chain Monte Carlo) is the name given to a statistical method that applies very well to mono-alphabetical substitutions.

1 - Initialization: use a random substitution alphabet (but it is possible to carry out an analysis of letter frequencies to obtain a first plain alphabet - encrypted alphabet correspondence table).

2 - Evaluation: calculation of the probability that the current substitution alphabet produces plain text (score generally based on the frequencies of appearance of bigrams in the target language).

3 - Modification of the alphabet: randomly exchange certain plain letter-cipher letter correspondences

4 - Repeat steps 2 and 3 as long as the plausibility score of the message obtained increases and the alphabet selected offers the most probable plain message.

First, some substitution use specific alphabets, such as Atbash that takes the alphabet backwards `ZYXWVUTSRQPONMLKJIHGFEDCBA` or the Caesar cipher which uses a shifted alphabet `DEFGHIJKLMNOPQRSTUVWXYZABC` that is shifted by 3.

There are also substitutions that use several alphabets, alphabet that changes depending on an algorithm defined by encryption (e.g. Vigenere uses 26 alphabets).

In game-play journals, substitution games / exercises are often called cryptograms.

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Cite as source (bibliography):

*Mono-alphabetic Substitution* on dCode.fr [online website], retrieved on 2024-10-07,

- Monoalphabetic Substitution Decoder
- Monoalphabetic Substitution Encoder
- Custom Deranged Alphabet Generator
- What is a (mono-)alphabetical substitution? (Definition)
- How to encrypt using an alphabetical substitution?
- How to decrypt using an alphabetical substitution?
- How to recognize a mono alphabetical substituted text?
- How to decipher a substitution without the alphabet?
- What is the MCMC technique?
- What are the variants of the substitution cipher?

substitution,monoalphabetical,alphabet,attack,known,plaintext,cryptogram,aristocrat,patristocrat

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