On the Subject of the Transposed Halved Polybius CM

Create a key using the keyword on screen 1 and the Boolean Expression on screen A. Take the resulting key and enter it into the Halved Polybius Square, in reading order, filling the left half 1st. The Halved Polybius Square is laid out like the diagram below:

1234512345
1
2
3####
LR

Convert each letter of the encrypted word into 3 coordinates and write them out top to bottom in the order Half, Row, Column and to the right of any previous coordinates. This will be referred to as the Transposed Matrix.

If the Submit Button is white, follow the instructions under Encrypt Instructions. Otherwise follow the instructions under Decrypt Instructions.

Encrypt Instructions

  • For each letter in backwards order of the keyword on screen 2, take the Nth letter of the keyword and find it in the Halved Polybius Square, converting it into 3 coordinates.
  • If N is odd, shift the row of that coordinate in the Transposed Matrix to the right by the column coordinate. When doing this, make sure to not move any 3s in the 2nd row and any 4s/5s in the 3rd row.
  • Otherwise, start at the column of the Transposed Matrix equal to the coordinate's column, wrapping around if necessary (Column A). Then continue counting columns to the right equal to the coordinate's row, wrapping around if necessary (Column B). Swap Column A with Column B.
  • After processing each letter of the keyword, convert each coordinate back into a letter using the Transposed Matrix to get a decrypted word.

Decrypt Instructions

  • For each letter of the keyword on screen 2, take the Nth letter of the keyword and find it in the Halved Polybius Square, converting it into 3 coordinates.
  • If N is odd, shift the row of that coordinate in the Transposed Matrix to the left by the column coordinate. When doing this, make sure to not move any 3s in the 2nd row and any 4s/5s in the 3rd row.
  • Otherwise, start at the column of the Transposed Matrix equal to the coordinate's column, wrapping around if necessary (Column A). Then continue counting columns to the right equal to the coordinate's row, wrapping around if necessary (Column B). Swap Column A with Column B.
  • After processing each letter of the keyword, convert each coordinate back into a letter using the Transposed Matrix to get a decrypted word.

Example 1

Encrypted Word: OFOECB
Key: CDFGHIJKLMOPQRSUVWXYZBEATN
Keyword: FIDDLE
Using Encrypt Rules

1234512345
1CDFGHRSUVW
2IJKLMXYZBE
3OPQ##ATN##
LR
LLLRLR
313212
131514
LLLRLR
113232
131514
LLLRLR
113232
131514
LLLRLR
131232
113514
LRLLLR
131232
113514
LRLLLR
131232
311514
LLRLRL
131232
311514

Decrypted Word: FORMAL

Example 2

Encrypted Word: CWSHV
Key: ABCDEFGHJLMNOPQTUVWXYZRISK
Keyword: MASTERY
Using Decrypt Rules

1234512345
1ABCDEPQTUV
2FGHJLWXYZR
3MNO##ISK##
LR
LRRLR
12321
31235
LRRLR
12321
12335
RLRLR
21321
21335
RLRLR
21321
33215
RLLRR
21231
33125
RLLRR
21231
33125
RRLRL
21231
35123
RRLRL
12132
35123

Decrypted Word: TRASH