Re: Scope for self-destructing email?

Christian Huitema <huitema@xxxxxxxxxxx> · Wed, 16 Aug 2017 23:28:25 -0700



    On 8/16/2017 9:43 PM, Phillip Hallam-Baker wrote:

    
          On Wed, Aug
            16, 2017 at 11:47 PM, Deen, Glenn (NBCUniversal) <Glenn.Deen@xxxxxxxxxx>
            wrote:

          
              The
                key issue is that no method currently in existence can
                defeat a camera taking a picture of the screen.  That
                will always let an authorized receiver make a permanent
                copy of the email and do what they want with it.  Yes,
                you may be throwing away repudiation and signature
                information, but they will have the contents of the
                mail.

              
                  Absolutely true. And really serious for some
                  applications. But consider the Game of Thrones hack, a
                  screen capture would be serious, yes. But nowhere near
                  as serious as having the final cut leak in HD.
                

                  The kids are using snapchat because their principle
                  security concern is that the receiver will forget to
                  delete the pics they send and they will end up with
                  the parents. They understand full well that they can
                  photograph the screen of one phone with another.
              
              
              The
                perfect is the enemy of the good. 
               
            
    Yes. The threat model here is not that the recipient will somehow
    divulge the message. Of course they can. The problem is that copies
    of the message are kept in multiple places. 

    
    If the recipient is cooperating, that is not an unsolvable problem.
    Suppose that the recipient's copy is encrypted with a short lived
    public key, and that the recipient voluntarily discards the
    corresponding private key after some time. There may well be copies
    of the bits around in many places, but nobody would be able to
    decrypt them. Of course, this requires that the sender acquires an
    ephemeral key from the recipient before sending the message. That
    may be hard to do in practice. But it is certainly doable.

    
    -- 
Christian Huitema