You can reach this dialog via the menu entry Crypt/Decrypt \ Symmetric(classic) \ Substitution/Atbash.
This dialog is used to enter the key for monoalphabetic substitution, and accordingly for the special case Atbash.
For Atbash the key is fixed and is completely transferred to the key entry field.
For the general monoalphabetic substitution they key entry field has to be filled bei yourself.
When entering the key the following points should be noted:
There are two ways of entering the key:
You select Atbash. So a fixed key is chosen.
You enter a keyword from which the permutation of the alphabet is derived. An example here might be the keyword "RAILWAYS".
The resulting permutation is worked out as follows. The A is mapped to the first letter (in this case the R). Similarly, the B is mapped to A, the C to I, the D to L and the E to W. As the sixth letter of the keyword, the A, has already occurred it is passed over, since no two (different) letters can be mapped to the same letter. This continues until all the letters in the keyword have been mapped, i.e. F to Y and G to S. For all the other letters (in this case H to Z) there are two options how to map them,
If the entered keyword contains all 26 characters of the alphabet, then it doesn't matter, how to fill up.
The sequence of all the 26 different alphabet characters is also called a permutation of the alphabet.
After you have entered the key, you can encrypt the document by clicking on the Encrypt button or decrypt the document by selecting Decrypt.
The formatting (retention of the spaces, special characters and upper and lower case distinction) of the output of the encrypted or decrypted document can be set in the Text Options dialog.