Can Skype be wiretapped by the authorities?

[Enzo Michelangeli]

----- Original Message ----- 
From: "Bill Stewart" <bill.stewart at pobox.com>
Sent: Sunday, May 09, 2004 12:44 PM
Subject: Re: Can Skype be wiretapped by the authorities?


[...]
> >BUT, unfortunately, the implementation is closed source, so there
> >are no guarantees that the software is not GAKked.
>
> Also no guarantee that it's not implemented sufficiently
> incompetently that the Authorities can't crack it if they want.
> Somebody else's message confirmed that there's a competence problem,
> though there may not be exploits.

Or, not exploits we're aware of...

[...]
> Skype uses a supernode structure to implement reflector service,
> so it doesn't have the same centralization problems.

Right, that's precisely my point. Skype is showing us the way to go,
although the security of the product may not be good enough (and being
closed source, it's automatically untrusted).

> They don't document it well enough to know if it's possible to
> wiretap a message by using a corrupt supernode as MITM, but perhaps.
> It's frustrating that they use proprietary protocols for everything.

That's understandable considering their business model. But I see Skype as
a proof of feasibility for the "real thing": an opensource application
built on sound bases.

> Their audio codec, however, is developed by a reputable company
> (brain spacing out on their name, but I'd seen them before.)

I've read that Skype uses an iLBC codec implemented by Global IP Sound.
There is also an opensource implementation of it (www.ilbcfreeware.org),
although its license contains weaselspeak clauses that I don't like very
much: http://www.globalipsound.com/legal/licenses.php .

> Most of that company's codec designs are intended for boring
> telephony-style 4khz mono audio, 64kbps uncompressed,
> something small compressed, with really good loss/noise resistence,
> rather than doing 7kHz or 11kHz audio or stereo sound,
> but I don't know which codecs they've chosen.

>From what I've seen, Speex (www.speex.org) would represent a better
choice, and is totally unencumbered.

I believe that we are finally close to the point where all the bits and
pieces for a secure, multiplatform, decentralized, opensource Internet
phone + text IM are available, and it would only take some coding effort
to put them to work together:

- Codec: Speex (www.speex.org)
- Portable audio interface layer: Portaudio (www.portaudio.com)
- Bulk encryption and authentication: SRTP, now a standard-track protocol
(RFC3711) and with an opensource reference implementation available at
srtp.sourceforge.net .
- Key exchange: authenticated D-H (how to perform the authentication, as I
said, should be discussed: biometric is not viable if only the text chat
feature is used, and multy-party conferencing calls for suitable
extensions to the basic D-H scheme)
- Directory and presence: any good P2P content-addressable scheme.
Preserving some sort of interoperability with file-sharing applications
would solve the bootstrapping problem (hundreds of thousands of nodes are
already up and running), but the most popular networks (eMule, Overnet and
ReverseConnect) are based on Kademlia, which is a Distributed Hash Table
algorithm and therefore doesn't allow sorted access (useful, e.g., to
locate the reflector with the largest available bandwidth). I recently
discovered a few tree-based distributed algorithms which would allow just
that:

P-trees:
http://techreports.library.cornell.edu:8081/Dienst/UI/1.0/Display/cul.cis/TR2004-1926

SkipGraphs: http://www.cs.yale.edu/homes/shah/html/pubs/skip-graphs.html

P-Grid: http://www.p-grid.org

Enzo

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