Re: [MLS] multiple devices per user?

Rich Persaud <> Mon, 26 March 2018 09:16 UTC

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From: Rich Persaud <>
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Date: Mon, 26 Mar 2018 05:16:30 -0400
Cc: Daniel Kahn Gillmor <>,
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To: Eric Rescorla <>
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Subject: Re: [MLS] multiple devices per user?
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On Mar 24, 2018, at 18:42, Eric Rescorla <> wrote:
>> On Sat, Mar 24, 2018 at 10:32 PM, Daniel Kahn Gillmor <> wrote:
>> Furthermore, it's not clear what a group conversation participant can
>> *do*, security-wise, in the event of recieving such a message from
>> another participant -- is this actually a new phone, or is it a wiretap
>> injection?  should i ask the user about it?  should i take action?  what
>> ction?

In Wire, if Alice has marked each of Bob's device fingerprints as "verified", then the addition of a new device by Bob results in an immediate message to Alice.  The recommended action is to verify the new fingerprint out of band, then Alice can mark Bob's new device as "verified". 

> Generally, I wouldn't expect them to take any action at all. It's a user's
> responsibility to ensure that the right number of devices are registered
> to their account, just as its common for the number of Web browsers
> one has attached to ones Gmail account.

Here is a scenario on iOS:

(1) User is logged into Facebook via Chrome.  User is not logged into Facebook in Safari, to avoid Facebook tracking on non-Facebook websites visited in Safari.

(2) User receives FB email notification in Apple email.  If user clicks on a link in the email, it opens in Safari and Facebook *automatically logs the user in without a password*.

(3) User then receives FB email notification that they have added a new device.

(4) User must now manually log out of Facebook, then click on a tiny close button on account icon to delete the cookie associated with that user, to avoid Facebook tracking on non-Facebook websites.  That removes (?) the browser as a known device.

Each of the individual UX decisions above could make sense in some contexts, but when combined on iOS where the user cannot choose the browser in which email links are opened, it leads to an undesirable UX where the user must invest repeated effort to ensure that the right number of devices are registered to their account.