[whispersystems] stolen phone

[Jeff Burdges]

I noticed WhatsApp says "WhatsApp can only be activated with one phone
number on one device at a time." here :
https://www.whatsapp.com/faq/general/24460358

Is this really correct? I presume Signal works like this?

How is this enforced? Is there only one copy of the ratchet allowed per
contact? Or is there only one copy of a user's long-term key allowed
per account? The second offers a stronger security assurance.

It makes perfect sense because using a third party identification method
like phone number is incompatible with multi-device chat :

If a phone is stolen, then either (a) WhatsApp/Signal must provide a
method for deauthorizing that device, or (b) you cannot even keep the
same WhatsApp/Signal account, and hence cannot even keep your phone
number. It's obvious that (a) is preferable to (b), but the question
remains : Should deauthorizing a device be a power user feature? Or
should everyone get that assurance by limiting the system to one device
per account? That's actually a false question because even the power
user needs their inexperienced contacts to deauthorize them correctly.
Ergo, no multi-device support for messengers that use phone numbers.

Anyways, I'm pleasantly surprised this is the case, just curious how
strong the assurances go.

Jeff

[Johan Wevers]

Post by Jeff Burdges
I noticed WhatsApp says "WhatsApp can only be activated with one phone
https://www.whatsapp.com/faq/general/24460358
Is this really correct? I presume Signal works like this?

Yes.

Post by Jeff Burdges
How is this enforced?

Not strong. If you change phone, the official way is to reinstall signal
and re-register. New keys will be created then.

If you make a backup with Titanium or similar and restore the backup on
another phone, it's a gamble which one will receive the messages. Often
it will be the last one that was switched on. The keys on the other
device will be then out of sync of course, so you'll have to reset all
secure sessions affected.

Post by Jeff Burdges
It makes perfect sense because using a third party identification method

The phone number is only used for registration, if it is changed
thereafter Signal will keep working with the old number. Using a
temporary number I could even register Signal on a phone, make a backup,
restore the backup on a tablet without phone functionality and use
Signal on that tablet (only the encrypted messages of course, not the
sms function).

Post by Jeff Burdges
If a phone is stolen, then either (a) WhatsApp/Signal must provide a
method for deauthorizing that device,

You can do that via a website:
https://whispersystems.org/textsecure/unregister/

You need access to the phone number for that (otherwise everyone could
unregister anyone) but if I loose my phone or it gets stolen I can walk
in the providers shop, identify myself and have a new simcard registered
before I walk out of the shop again. For pay as you go requesting a new
simcard might take longer, and if you insist of not having it tied to
your name you're out of luck.

I don't know how this is organised in other countries than The
Netherlands though.

Post by Jeff Burdges
Ergo, no multi-device support for messengers that use phone numbers.

Well, the solution for that is the way it is done via the Browserplugin
and WhatsApp web. You trade some ease of use for security.

However, I wonder how Viber does it. They claim e2e encryption in the
new 6.x versions, and I had to re-register my PC and tablet installs
with my phone.