As an additional note: The "Remove unencrypted files after encryption" option will be removed. We do not want to have that option as this might cause critical data loss in case we fail to detect an error. The responsibility for removal should be with the user who might also want to use a dedicated removal tool that ensures that the plaintext is really removed from disk.

I've tried this out a but. The Combo box with the line edit felt not good as it basically offers multiple places to select a certificate. To show the user that multiple files will be created when X509 and OpenPGP Certificates are selected and to make it possible to change the Output location I've moved the output file selection also in this dialog at the bottom.

Screenshots how it looked on Windows 10:

For the recipient selection we currently think it would be better to have it like the Address Entry in KMail. Just one Edit that autocompletes on Name / EMail / KeyID etc. and as soon as you enter something a second line shows up. When you remove the entered text the additional line is removed. An Icon behind the lineedit shows a status e.g. Ok for a fully valid certificate or a question mark if no certificate was found.
It will also offer a button to open the full blown certificateselectiondialog:

The idea is that the recipient selection behaves like recipient entry and the certificateselectiondialog is something like the Addressbook with a full view on all your certificates.

Current draft (Windows Screenshots)

The Idea is that the behavior is more like the recipients editor in a Mailcomposer. You add recipients with autocompletion or alternatively you can open an address book.

After File selection you start with this:

When entering something you get autocompletion on the name or e-mail.

When a certificate is selected you see a status indicator behind it to show the validity of the user id:

Tooltip with explanation:

The old certificateselection dialog can still be opened with the button on the right. If someone wants further searches. This is basically the addressbook.

For the case that someone wants to mix S/MIME and OpenPGP the Output area changes it's contents to indicate that multiple files will be created:

Some more changes are that the addressbook icon is gone now and the layouting will be a bit more ordered. The CertificateDialog now also allows multiple selection so that there should be no regression agains the old stuff.

A mock:

We also need some option to add archiving.

I'm planning to get it into a working enough state today to push it to master, with Applications 16.08 branched off we should have enough time to fix issues with the new dialog before the next release.

Groundwork and a first working version was pushed to master now ( 8466330425f7dea442084b12cbbb0dd0073f7594 ). Now on to improve, fix it.