Phillips Cipher

The Phillips cipher is a polyalphabetic substitution cipher applied in blocks. In its standard form, it uses eight 5×5 grids derived from a keyword.

Philips Encryption uses an initial grid of 5x5 (or keyword to generate the grid).

The first step consists of generating the other 7 grids. Grid 1 is the initial grid. To obtain grids 2 through 5, the row numbered 1 is successively shifted down by one position, starting from grid N-1. To obtain grids 6 through 8, the row numbered 2 is then successively shifted down by one position. The line orders are therefore: 1-2-3-4-5, 2-1-3-4-5, 2-3-1-4-5, 2-3-4-1-5, 2-3-4-5-1, 3-2-4-5-1, 3-4-2-5-1, 3-4-5-2-1.

The Phillips cipher splits the text into blocks of size T characters (by default T=5 letters, in which case the blocks are called pentagrams). At the Nth block is associated the grid N (if there are more blocks than grids, the 9th block is again associated with grid 1, and so on). The last block may contain fewer than T characters.

Each letter of a block is then located in the associated grid, and corresponds to a letter encoded according to a shift on the grid of 1 downwards and 1 to the right (offset 1,1). (If this cell does not exist, it is necessary to imagine a continuity of the grid by its opposite side).

The Phillips decryption is identical to the encryption, except for the shift in the grid which is reversed. Instead of moving one square to the right and one square down, the decryption performs the reverse path, moving one square to the left and one square to the top (with cyclic continuity at the edges).

In its standard form, the text is processed in blocks of five letters using a cycle of eight grids, resulting in a period of 40 characters.

Its overall index of coincidence is generally lower than that of a monoalphabetic substitution and typically ranges from 0.04 to 0.05 for sufficiently long texts.

With a 5×5 grid containing 25 symbols, the ciphertext cannot contain more than 25 distinct symbols.

A reference to a Phillips screwdriver (the name given to the cross-head screwdriver) may serve as an intentional nod within a puzzle.

Several variants are possible:

— An alternative method of generating the 8 grids, or even not limited to 8 grids.

— The shift of (+1, +1) can very well be modified by any pair (+n, +m)

The block size T may be different, or even vary according to a given split rule.