[guardian-dev] Blackphone

[Michael Rogers]

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On 17/01/14 11:10, Matej Kovacic wrote:
> Yes, but adversary can see when do you connect to OSTN server to
> place a call and see who responded to a call. Actually they can
> only see who is transferring the data and in what amount, but it is
> enough for correlation. So they can see who is communicating with
> who.

Yup, an adversary wiretapping both parties can see that they're
communicating. But the parties' OSTN servers can see that _without_
wiretapping them.

> A solution would be to use OSTN with Tor network (OK, it would not
> work, but you can use ChatSecure's voice messages), while both
> parties should also generate some additional fake traffic to Tor
> network to prevent correlation.
> 
> In that case you would have encrypted conversation, server
> administrator cannot see who is communicationg with who (actually,
> can only see some identity number like number 1000 is calling
> number 2000, but without real IP addresses), and adversary can only
> see that you have connected to Tor network and that you are
> transferring some data.
> 
> In that case you do not need to trust anyone. Except endpoint
> devices. :-)

Sounds like a good solution, as long as the OSTN accounts are
registered and used always through Tor.

IIRC, the Tor protocol allows clients to send padding frames to relays
- - but it doesn't allow clients to ask relays to generate padding. So
the Tor->Alice direction of the Alice->Tor connection, and the
Tor->Bob direction of the Tor->Bob connection, could still be
correlated. And of course, circuit setup and teardown times are quite
revealing even if the circuits are fully padded.

Cheers,
Michael

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