Add a checkbox to not store key IDs in the encrypted data.

[ ] use anonymized recipients [i]

[i] The identities of the recipient certificates are not stored in the encrypted document.
    A recipient therefore cannot identify other recipients.
    This option is suitable for competitive bidding processes.

I would suggest to add this button below the list of recipients.

This is useful i.e. for tenders and whistle blowers.

German Info Text:

Die Identität der Empfänger-Zertifikate wird in dem verschlüsseltem Dokument nicht gespeichert.
Ein Empfänger kann daher andere Empfänger nicht erkennen.
Diese Option eignet sich für Ausschreibungsverfahren.