## On the Subject of the ElsieFour CM

A machine that knows no bounds when it comes to ciphers.

Create a key using the keyword on screen 1 and the Boolean Expression on screen A. Make sure to remove any Zs. Rearrange the key into a 5×5 matrix.

Take the letter displayed on screen B and label it the Marked Letter.

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

#### Encrypt Instructions

- Take the Marked Letter’s alphabetic position, minus 1 (A0Y24) and modulo it by 5. This will be called R.
- Take the Marked Letter’s alphabetic position, minus 1 (A0Y24) and divide it by 5, rounding down. This will be called D.
- Find the Nth letter of the encrypted word in the Matrix and go right an R amount of times and down an D amount of times, wrapping around if needed.
- The letter you end up on becomes your Nth decrypted letter.
- Shift the row to the right that contains the Nth decrypted letter.
- Shift the column down that contains the Nth encrypted letter.
- Take the encrypted letter's alphabetic position, minus 1 (A0Y24) and modulo it by 5. This will be called R.
- Take the encrypted letter's alphabetic position, minus 1 (A0Y24) and divide it by 5, rounding down. This will be called D.
- Find the Marked Letter in the Matrix and go right an R amount of times and down an D amount of times, wrapping around if needed.
- The letter you end up on becomes your new Marked Letter.