Four Square Cipher

Question 1

How to encrypt using Four Squares cipher?

Answer

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

Grid 1	Grid 2
Grid 3	Grid 4

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

\	1	2	3	4	5
1	J	O	H	N	A
2	B	C	D	E	F
3	G	I	K	L	M
4	P	Q	R	S	T
5	U	V	W	X	Y

\	1	2	3	4	5
1	G	E	O	R	A
2	B	C	D	F	H
3	I	J	K	L	M
4	N	P	Q	S	T
5	U	V	W	X	Y

\	1	2	3	4	5
1	P	A	U	L	B
2	C	D	E	F	G
3	H	I	J	K	M
4	N	O	Q	R	S
5	T	V	W	X	Y

\	1	2	3	4	5
1	R	I	N	G	O
2	A	B	C	D	E
3	F	H	J	K	L
4	M	P	Q	S	T
5	U	V	W	X	Y

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

\	1	2	3	4	5
1	J	O	H	N	A
2	B	→	→	→	→
3	↓	I	K	L	M
4	↓	Q	R	S	T
5	↓	V	W	X	Y

\	1	2	3	4	5
1	G	E	O	R	A
2	→	→	→	→	H
3	I	J	K	L	↑
4	N	P	Q	S	↑
5	U	V	W	X	↑

\	1	2	3	4	5
1	↓	A	U	L	B
2	C	←	←	←	←
3	H	I	J	K	M
4	N	O	Q	R	S
5	T	V	W	X	Y

\	1	2	3	4	5
1	R	I	N	G	↑
2	←	←	←	←	E
3	F	H	J	K	L
4	M	P	Q	S	T
5	U	V	W	X	Y

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

Question 2

How to decrypt Four Squares cipher?

Answer

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.

\	1	2	3	4	5
1	J	O	H	N	A
2	B	←	←	←	←
3	↑	I	K	L	M
4	↑	Q	R	S	T
5	↑	V	W	X	Y

\	1	2	3	4	5
1	G	E	O	R	A
2	←	←	←	←	H
3	I	J	K	L	↓
4	N	P	Q	S	↓
5	U	V	W	X	↓

\	1	2	3	4	5
1	↑	A	U	L	B
2	C	→	→	→	→
3	H	I	J	K	M
4	N	O	Q	R	S
5	T	V	W	X	Y

\	1	2	3	4	5
1	R	I	N	G	↓
2	→	→	→	→	E
3	F	H	J	K	L
4	M	P	Q	S	T
5	U	V	W	X	Y

The message HCASMFSX is decrypted BEATLESX

Question 3

How to recognize a 4 Squares ciphertext?

Answer

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.

Question 4

What are the variants of the 4-Squares cipher?

Answer

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.

Four Square Cipher

Four Squares Decoder

Four Squares Encoder

Answers to Questions

How to encrypt using Four Squares cipher?

How to decrypt Four Squares cipher?

How to recognize a 4 Squares ciphertext?

What are the variants of the 4-Squares cipher?

Source code

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