Three Squares Cipher

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

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

Split the plain message into bigrams (pairs of two letters L1 and L2). Find L1 in grid 1 and L2 in grid 2. Then note the intersection in grid 3 of the line of L1 in grid 1 with the column of L2 in the grid 2.

Example: For the bigram ME, M is in position (line 3, column 5) in grid 1, and E is in position (line 2, column 3) in the grid 2. The intersection in grid 3 is the letter K (line 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 line 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 line' 2 ( CDEFG ') of the grid' 2 '
The corresponding encrypted trigram is' TKD'.
The final encrypted message is TKDGNVSAFRAV.

Decryption by three squares is done with three grids.

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

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 (line 5, column 1) in grid 1, D is in position (line 2, column 3) in grid 3, and B is in position (line 1, column 5) in grid 2.

Find the 2 plain letters:
Plain letter 1: intersection of the line 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 line of the letter L3 in grid 2.

Example: The first plain letter is C, intersection of line 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 line' 1 of B 'in grid' 2 '.
Finally the complete plain message is' CODE'.

The text is theoretically composed of maximum 25 distinct characters.

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

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

There are several ways to encrypt and note the letters:

The first letter of the bigram is searched in grid 1 and the second letter in grid 2 (encryption 1-2) but it is possible to invert (encryption 2-1)

The trigram is then generally noted as:
- a letter taken randomly in the same column as the letter in the grid 1
- the letter of the grid 3
- a letter taken randomly in the same line as the letter of the grid 2
This encryption is said (1-3-2). It is of course possible to encrypt with a different order.