This dialog window is shown if you press the Perform preparations button in the side-channel attack against the hybrid encryption protocol dialog.
In the given scenario Alice wants to send a hybrid encrypted file to Bob. Alice can either hybrid encrypt a file by herself or she can choose an already hybrid encrypted file.
Here you can find additional information on hybrid encryption.
Remark:
If you neither want to execute the hybrid encryption on your own nor choose an already hybrid encrypted message, a message integrated in CrypTool will be used.