Nihilist Cipher

The Nihilist cipher is an over-encryption of the Polybius square. It is first a transposition cipher into numbers over which is added an addition which transforms the cipher into a fractionated polyalphabetic cipher, in a certain way, similar to the Vigenere cipher, but with numbers.

The nihilist's cipher uses a grid (usually 5x5 = 25 cells) that is filled with letters of the alphabet (often a deranged alphabet). For a 5x5 grid and the 26-letter latin alphabet, choose a letter to omit, often the J, V or W are omitted. The grid has digit headers for its rows and columns (typically 1 to 5).

To encrypt a text, it is necessary to replace each letter of the initial text, by its coordinates in the grid. Generally, the coordinates [row, column] (and more rarely [column, row]) are used. A numerical code consisting of pairs of digits is obtained.

The particularity of the Nihilist cipher in relation to the Polybius cipher is its over-encryption. The nihilists use a key that is added for each couple of digits previously created.

Decryption requires to know the grid and the over-encryption key.

The message can have the form of a list of numbers (2 or 3 digits) or of a large series of digits, in the second case, separate them into pairs of 2 digits.

To decrypt, the coded key is subtracted from each pair of 2 digits and each number obtained is replaced by the corresponding letter with these coordinates in the grid.

In the general case of using a 5x5 grid with coordinates from 1 to 5, the message is composed of numbers with the following properties:

— If the encryption is without a separator, the numbers are between 00 and 99 and the message is composed of an even number of digits.

— If the encryption is with separating spaces, the numbers are between 22 and 110

— In all cases, the numbers 11,12,13,14,15,16,17,18,19,20,21,31,41,51,61,71,81,91 can never appear by addition because they contain only one ten or only one unit.

Any reference to Russia (USSR), tsars or Russians traditions is a clue.

The word nihilist is generally associated with a philisophic doctrine, but in Russian the word нигилизм has a Latin root meaning nothing.

dCode analyzes the pairs of digits of the message to extract the potential additions that formed them. It is then possible to find all the possible combinations and to deduce the potential keys with the grid.

Assuming that the over-encryption key is known, it is possible to decode normally with a basic / random grid. The result will then be a mono-alphabetic substitution of the original message. Use dCode's monoalphabetic substitution tool to decrypt the message almost automatically.

It is possible to make several variants:

— Inversion of coordinates: rather than using [row, column], it is possible to use [column,row].

— Change of coordinates names: the digits from 1 to 5 can very well be mixed or replaced by other digits.

— Multiple keys, it is possible to use several keys, summed successively, but this only extends the process time and does not complicate a brute-force attack.

The Russian nihilist movement is described in the history textbooks as between 1855 and 1885.