Mono-alphabetic Substitution

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:

The substitution involves a replacement 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 single alphabetical substitution (the same letter can be used only once in the alphabet).

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

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.

The interactive tool provided by dCode allows a semi-automatic decryption of messages encrypted by substitution ciphers.

The main technique is to analyze the frequencies of letters and find the most likely bigrams.

The known plaintext attack makes it 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: AZERTYUIOPQSDFGHJKLMWXCVBN
NBVCXWMLKJHGFDSQPOIUYTREZA
QWERTYUIOPASDFGHJKLZXCVBNM
MNBVCXZLKJHGFDSAPOIUYTREWQ
AQWZSXEDCRFVTGBYHNUJIKOLPM
ZYXWVUTSRQPONMLKJIHGFEDCBA
'AEIOUYBCDFGHJKLMNPQRSTVWXZ

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