Four Square Cipher

The 4-squares encryption is made, as its name suggests, with four square grids (possibly generated from a keyword) positioned in a square:

Example: 4 grids formed from the first names JOHN, GEORGE, PAUL and RINGO (the letter Z is omitted)

The first step is to split the plain message into pairs of two letters (completed with a neutral letter if necessary).

Example: The plain message BEATLES is decomposed into 'BE, AT, LE, SX'

For each pair (bigram), locate the first letter in grid 1 and the second letter in grid 4. Intersect the rows and columns of the letters found in the two adjacent grids and write down these 2 new letters.

Example: Bigramme BE: B is positioned (row 2, column 1) in grid 1 E is positioned (row 2, column 5) in grid 4
The extension of their rows and Columns in grids 2 and 3 leads to 2 intersections: H, positioned (row 2, column 5) in grid 2 and C, positioned (row 2, column 1) in grid 3

The encrypted text consists of the letters found at the intersections: first the letter in the grid 2 and then the letter in the grid 3.

Example: The final encrypted message is HCASMFSX

4 Squares decryption is almost identical to the encryption. The difference is that it is necessary to position the letters of the encrypted bigram in grids 2 and 3, then read the light letters in grids 1 and 4.

Example: (See above) the bigramme HC is positioned in grids 2 and 3 and deciphers BE in grids 1 and 4.

Four Squares is a polygrammic encryption, so it is advisable to perform an analysis of the bigrams if the text is long enough.

Also, the presence of 4 keywords that can generate the grids is a clue.

Grids with 25 letters, there is normally at least one missing letter from Latin alphabet in the encrypted text.

Variation 1: reverse the order of the bigram obtained. Rather than taking the encrypted letters in the grid 2 then the grid 3, it is possible to reverse this bigram and take the grid 3 then the grid 2.

Option 2: Switch the position of grids 1, 2, 3 and 4.