[guardian-dev] Blog post: VoIP Security Architecture

Michael Rogers michael at briarproject.org
Sun Nov 24 09:45:43 EST 2013

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Hi Lee,

Sorry, my thought process in that last email was really garbled! As
you guessed I was thinking of a direct connection, where someone
monitoring either endpoint would be able to see the IP address of the
other endpoint. I take your point that proxying would conceal the
other endpoint's IP address.

I still have some concerns about metadata though. First, do all SIP
servers proxy all calls? I thought some servers tried to set up direct
connections between the endpoints to save bandwidth. Maybe what I'm
asking is, are you writing about SIP in general or just Ostel?

Second, if the endpoints are using the same server, someone monitoring
the server can use the start and end times of calls to tell which
users are communicating.

Third, regardless of whether the endpoints are using the same server,
someone monitoring both endpoints can use the start and end times of
calls to tell whether they're communicating.

These seem like realistic threats that reveal metadata. I'm not saying
that VoIP is insecure, or that people shouldn't use it, or that I have
a better idea - but I think you should avoid strong statements like
"nor can metadata of sessions in progress be spied on".


On 22/11/13 20:23, Lee Azzarello wrote:
> Hello Michael,
> Could you describe in more detail the threat model in question? 
> Consider that much of the metadata required to establish a SRTP
> stream is contained in the payload of the encrypted SIP packets and
> the RTP stream. Also consider that the backend to support most
> VoIP implementations proxies encrypted traffic and since federated
> calls between domains works by default with a properly implemented
> SIP/RTP proxy the full network path can be arbitrarily complex.
> One detail I left out in the 500 word article is that a call's 
> endpoints use the SDP protocol to negotiate the media stream
> metadata. This includes endpoint IP addresses, codecs, etc. This
> information is encrypted within the SIP dialog.
> Regards, Lee
> On Fri, Nov 22, 2013 at 5:21 AM, Michael Rogers 
> <michael at briarproject.org> wrote: On 22/11/13 00:11, Lee Azzarello
> wrote:
>>>> Once the endpoints agree on a shared secret, the ZRTP session
>>>> ends and the SRTP session begins. When established, all audio
>>>> and video content going over the network is encrypted. Only
>>>> the two peer endpoints who established a session with ZRTP
>>>> can decrypt the media stream. This is the part of the
>>>> conversation that cannot be wiretapped nor can metadata of
>>>> sessions in progress be spied on.
> Hi Lee,
> Recently 'metadata' has been used a lot to refer to information
> about who's communicating, when they're communicating, and where
> they are at the time. SIP-TLS, ZRTP and SRTP don't conceal the IP
> addresses of the endpoints, which can often be mapped to identities
> and/or locations, so perhaps someone thinking of the recent usage
> of 'metadata' would misunderstand "nor can metadata of sessions in
> progress be spied on" to mean something stronger than it does?
> Cheers, Michael
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