My exact use case is that I'm starting a new job, and I need to transmit sensitive personal documents to support my application. However, the same applies for applying for a loan, buying a house etc etc. In each case, certain sensitive documents need to be transmitted to a certain party, and then possibly sent onwards by them to others. Utility bills, bank statements, etc.
We all know that sensitive documents should not be sent by insecure email. So, I could put all the files in a zip, and secure it with a password. I could provide the password with a phone call or SMS etc. Great, but once the files have got to my recipient, I have lost control. He can forward them on unsecured.
Same with Dropbox (or similar). I can provide a link to the documents, and ask that the link is used for onward transmission. But I really have no control - the first recipient could re-transmit them any way he wished.
Anyone know of a way to continue to maintain the security of sensitive documents in such a scenario? Is there a file format I could use that would prevent the document being sent unsecured? I've never heard of one.