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Three Squares Cipher

Tool to decrypt/encode with the three-square cipher which uses 3 grids to extract letters in rows or columns with a notion of randomness.

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Three Squares Cipher -

Tag(s) : Polygrammic Cipher, GRID_CIPHER

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# Three Squares Cipher

## Three Squares Decoder

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## Three Squares Encoder

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## Answers to Questions (FAQ)

### What is the Three Squares cipher? (Definition)

The 3-square cipher is a polygrammic cipher using 3 grids. From plain text bigrams, it generates ciphered trigrams according to the position of the letters in the three grids.

### How to encrypt using Three Squares cipher?

3-square encryption is done with three grids (possibly generated from a keyword or disordered alphabet)

Example: Encrypt MESSAGE with the keys ONE, TWO, THREE corresponding to the grids

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

Split the plain message into bigrams (pairs of two letters respectively noted L1 and L2). Complete with a neutral letter of the second grid if the message has an odd length.

Find L1 in grid 1 and L2 in grid 2. Then note the intersection in grid 3 of the row of L1 in grid 1 with the column of L2 in the grid 2.

Example: For the bigram ME, M is in position (row 3, column 5) in grid 1, and E is in position (row 2, column 3) in the grid 2. The intersection in grid 3 is the letter K (row 3, column 3).

Each bigram of the plain text is associated with 3 new letters: a letter taken randomly in the same column as the letter in the grid 1, the letter intersection of the grid 3 and a letter taken randomly in the same row as the letter of the grid 2. These 3 letters (a trigram) represent the coded text for the bigram.

Example: Take T: a random letter in the column 5 (BHMTY) of the grid 1
Take K: the intersection letter of the grid 3 previously found
Take ' D ': a random letter in the row 2 (CDEFG) of the grid 2
The corresponding encrypted trigram is TKD.
Repeat the process for each bigram. The final encrypted message is TKDGNVSAFRAV.

### How to decrypt Three Squares cipher?

Decryption by three squares is done with three grids.

Example: Decrypt UDBJDC with the keys ONE, TWO, THREE' corresponding to the grids

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

Split the message into trigrams (triplets of three letters L1, L2 and L3) and find L1 in the grid 1, L2 in the grid 3 and L3 in the grid 2.

Example: The first trigram is UDB, U is in position (row 5, column 1) in grid 1, D is in position (row 2, column 3) in grid 3, and B is in position (row 1, column 5) in grid 2.

Find the 2 plain letters:
Plain letter 1: intersection of the row of the letter L2 in the grid 3 with the column of the letter L1 in the grid 1
Plain letter 2: intersection of the letter L2 column in grid 3 with the row of the letter L3 in grid 2.

Example: The first plain letter is C, intersection of row 2 of D in grid 3 with column 1 of U in grid 1.
The second plain letter is O, intersection of column 3 of D in grid 3 with row 1 of B in grid 2.
Finally the complete plain message is CODE.

### How to recognize a 3 squares ciphertext?

The message has a length multiple of 3.

The final ciphertext is longer than the original of 33%.

The frequency analysis and the coincidence index is similar to an almost random text.

The text is theoretically composed of a maximum of 25 distinct characters if the grids are 5x5 and use the same letters of the alphabet.

### What are the variants of the Three Squares cipher?

In the three-square cipher, several ways of ciphering and noting the letters are possible:

The first letter of the bigram is sought in grid 1 and the second letter in grid 2, noted 1-2 (by default) but it is possible to reverse, notation 2-1.

The trigram is then generally noted as follows:
- a letter taken randomly from the same column as the letter in grid 1
- the letter from grid 3
- a letter taken randomly from the same row as the letter of the grid 2
This cipher is indicated 1-3-2 (by default). It is also possible to encrypt with a different order.

Finally, it is possible to mix the grids, such as inverting grids 1 and 2, or other permutation.

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Three Squares Cipher on dCode.fr [online website], retrieved on 2024-09-14, https://www.dcode.fr/three-squares-cipher

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