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Re: [MLS] multiple devices per user?

Justin Uberti <juberti@google.com> Tue, 27 March 2018 23:41 UTC

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From: Justin Uberti <juberti@google.com>
Date: Tue, 27 Mar 2018 23:40:58 +0000
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To: dkg@fifthhorseman.net
Cc: Eric Rescorla <ekr@rtfm.com>, mls@ietf.org
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Subject: Re: [MLS] multiple devices per user?
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On Tue, Mar 27, 2018 at 2:01 PM Daniel Kahn Gillmor <dkg@fifthhorseman.net>
wrote:

> On Sat 2018-03-24 22:52:48 +0000, Justin Uberti wrote:
> > Also, allowing exfiltration of keys in order to allow cross-device
> sharing
> > seems to introduce its own set of problems.
>
> it's not clear that any protocol can prevent key exfiltration from an
> endpoint that wants to exfiltrate keys outside of that protocol.  so i
> don't think it's a question of "allowing" here.
>

There are hardware architectures (e.g. Secure Enclave) where key material
can be used for encryption but the raw private key cannot be extracted. If
the protocol requires access to the key in order to enroll a new device,
this reduces the efficacy of the hardware mechanism (or may not work at
all).

Overall, I agree with Richard on this - it's an application decision. There
are some niceties associated with a single key per user, but this will not
be an option in all circumstances.

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