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Four Square Cipher

Tool to decrypt / encrypt with 4 squares. The four-square encryption uses 4 5-by-5 grids combined 2 to 2 to extract letters.

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Four Square Cipher -

Tag(s) : Cryptography, Polygrammic Cipher

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Four Square Cipher

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Tool to decrypt / encrypt with 4 squares. The four-square encryption uses 4 5-by-5 grids combined 2 to 2 to extract letters.

Answers to Questions

How to encrypt using Four Squares cipher?

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

Grid 1Grid 2
Grid 3Grid 4

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

\12345
1JOHNA
2BCDEF
3GIKLM
4PQRST
5UVWXY
\12345
1GEORA
2BCDFH
3IJKLM
4NPQST
5UVWXY
\12345
1PAULB
2CDEFG
3HIJKM
4NOQRS
5TVWXY
\12345
1RINGO
2ABCDE
3FHJKL
4MPQST
5UVWXY

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 (bigramhref), 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

\12345
1JOHNA
2B
3IKLM
4QRST
5VWXY
\12345
1GEORA
2H
3IJKL
4NPQS
5UVWX
\12345
1AULB
2C
3HIJKM
4NOQRS
5TVWXY
\12345
1RING
2E
3FHJKL
4MPQST
5UVWXY

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

How to decrypt Four Squares cipher?

4 Squares decryption is almost identical to the encryption. The difference is that it is necessary to position the letters of the encrypted bigramhref 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.

\12345
1JOHNA
2B
3IKLM
4QRST
5VWXY
\12345
1GEORA
2H
3IJKL
4NPQS
5UVWX
\12345
1AULB
2C
3HIJKM
4NOQRS
5TVWXY
\12345
1RING
2E
3FHJKL
4MPQST
5UVWXY

The message HCASMFSX is decrypted BEATLESX

How to recognize a 4 Squares ciphertext?

Four Squares is a polygrammic encryption, so it is advisable to perform an analysis of the bigramshref 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.

What are the variants of the 4-Squares cipher?

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

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

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Source code

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