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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.

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    An untrusted counterpart and a sensitive document are mutually exclusive and incompatible. Either you trust them (and send the docs), or you don't (and don't send the docs). – Deer Hunter Oct 1 '14 at 9:56
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    You won't be able to stop the other side of resending/forwarding documents. Make sure you only send document to trusted parties. The best thing you can do is to provide them with an alternative solution, maybe based on a cloud solution. Here's an article on that subject: entrepreneur.com/article/223467 – sir_k Oct 1 '14 at 9:58
  • You are looking for a Digital Rights Management (DRM) scheme. Unfortunately DRM doesn't work unless all(!) involved devices are trustworthy. Information simply can not be controlled. – Philipp Oct 1 '14 at 10:41
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There is no way to be sure that the resipient won't retransmit your documents unsecured to others.

You could use password secured PDF files, theoretically they sould be safe if you forbit to print and edit them. But even in this case, one could easily take a screenshot and get the crititcal Information out of the PDF. Or someone resends your files in the same mail together with the password.

As long as you don't trust the recipient you can't really be sure what happens to your files. But the same logic applies to a normal letter or each other form of communication as this is not really a technical problem but a problem with humans ;)

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