You can reach this dialog via the menu entry Crypt/Decrypt \ Symmetric(classic) \ Playfair.
The 5x5 Matrix and 6x6 Matrix radio buttons are used to select whether the Playfair algorithm should be run with a 5x5 or a 6x6 matrix. (For further information, see explanation on the Playfair encryption algorithm.)
If you want to encrypt the document with the key you just click the Encrypt button. If you wish to decrypt the document, click the Decrypt button.
A key that is stored into the key memory could be inserted by the button (if there is no key stored, the button is deactivated and appears in grey).
In the Key field you can enter your choice of password.
The option ignore duplicates within the key phrase affects the manner in which the password is entered in the matrix. If this is enabled, double letters in the password are simply ignored, otherwise in each case the next letter in alphabet is used. The boxes below show the matrix generated from the input key.
The Pre-format text option is used to determine whether the text should be confined to the alphabet used (depending on the size of the matrix). Under this option in the 5x5 matrix all J‚s are transformed into I‚s. In addition, as mentioned in the explanation for the Playfair encryption algorithm, double letters are separated by the insertion of an X (a double X being separated by insertion of a Q). The text thus changed is displayed in addition to the encrypted text.
After you have entered the key, you can encrypt the document by clicking on the Encrypt button or decrypt the document by clicking Decrypt.