[Cryptography] Usage models (was Re: In the face of "cooperative" end-points, PFS doesn't help)

Perry E. Metzger perry at piermont.com
Sun Sep 8 15:51:49 EDT 2013

On Sun, 8 Sep 2013 14:50:07 -0400 Jerry Leichter <leichter at lrw.com>
> Even for one-to-one discussions, these days, people want
> transparent movement across their hardware.  If I'm in a chat
> session on my laptop and leave the house, I'd like to be able to
> continue on my phone.  How do I hand off the conversation - and the
> keys?

I wrote about this a couple of weeks ago, see:


In summary, it would appear that the most viable solution is to make
the end-to-end encryption endpoint a piece of hardware the user owns
(say the oft mentioned $50 Raspberry Pi class machine on their home
net) and let the user interact with it over an encrypted connection
(say running a normal protocol like Jabber client to server
protocol over TLS, or IMAP over TLS, or https: and a web client.)

It is a compromise, but one that fits with the usage pattern almost
everyone has gotten used to. It cannot be done with the existing
cloud model, though -- the user needs to own the box or we can't
simultaneously maintain current protocols (and thus current clients)
and current usage patterns.

Perry E. Metzger		perry at piermont.com

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