[MLS] Fwd: Re: multiple devices per user?

Simon Friedberger <simon.tls@a-oben.org> Sun, 25 March 2018 10:54 UTC

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Date: Sun, 25 Mar 2018 12:54:48 +0200
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Subject: [MLS] Fwd: Re: multiple devices per user?
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(Resent because I had the wrong sender address.)


Hi Jon!


On 25.03.2018 01:48, Jon Millican wrote:
> I think this is likely application-specific, but in our case we surface all device list changes within E2E threads, and you can look at the specific device public keys if wish. If you see that I have a new device, you can thus - in theory - ask me out of band whether the key is legitimate. The relevant part of our threat model here is "you should know what endpoints you're sending messages to before you send the message", so this fulfils that requirement.
I think having the partners deal with new devices is the wrong approach.
They probably don't know if a user has new devices so adding trust to
devices should be handled by the users with the devices.


> In our use case, this is common. Whenever somebody logs in on a new device, there's a strong chance that they don't have access to the previous one at the time - and won't want to be blocked from using Messenger until they can authorise the new device - particularly if it's a replacement phone for example. As Facebook Messenger itself is not an E2E app in the general case, we wouldn't want to add additional barriers that discourage people from using Secret Conversations.
I think the cases of replacing devices and adding them should be treated
differently. In the latter case trust has already been established and
can be transferred. In the first case, which is the case you are
concerned with, there is no trust and it should be an implementation
detail to highlight this to the user so the protocol should not make
them equivalent.


> I think I addressed this above, but we let people know when new devices are added to their account. Those who wish to can go check the list of devices on their account at this point.
But that only works with some form of key transparency, right? Otherwise
the device list can easily be manipulated.


Best Regards,
Simon