Can Skype be wiretapped by the authorities?

[Enzo Michelangeli]

----- Original Message ----- 
From: "Axel H Horns" <axel.h.horns at gmx.net>
To: <cryptography at metzdowd.com>
Sent: Wednesday, April 28, 2004 4:49 AM
Subject: Can Skype be wiretapped by the authorities?


> Is something known about the details of the crypto protocol within
> Skype? How reliable is the encryption?
>
> See e.g.
>
> http://www.financialcryptography.com/mt/archives/000076.html
>
> Can Skype be wiretapped by the authorities? With collaboration of the
> Skype operator? Without?

What do you mean with "operator"? AFAIK, the system is fully peer-to-peer
(http://www.skype.com/skype_p2pexplained.html ).

Regarding the crypto, at http://www.skype.com/help_faq.html#Technical they
say:

 What type of encryption is used?

 Skype uses AES (Advanced Encryption Standard) - also known as Rijndel
 - which is also used by U.S. Government organizations to protect
 sensitive, information. Skype uses 256-bit encryption, which has a
 total of 1.1 x 1077 possible keys, in order to actively encrypt the
 data in each Skype call or instant message. Skype uses 1536 to 2048
 bit RSA to negotiate symmetric AES keys. User public keys are
 certified by Skype server at login.

OK, so "Rijndael" is misspelled and the RSA-based key exchange does not
provide forward secrecy, but apart from that it doesn't smell like snake
oil. Not too bad, at least.

BUT, unfortunately, the implementation is closed source, so there are no
guarantees that the software is not GAKked. And yes, definitely an
opensource (and multiplatform) alternative would be a cool thing to have.
A message I posted a while ago to the list p2p-hackers was reposted by
Eugene Leitl to cypherpunks
(http://www.mail-archive.com/cypherpunks@minder.net/msg81814.html ) but
the couple of followups it elicited didn't seem to center the issues I
raised. I didn't reply then because I'm not a subscriber of cypherpunks
any longer, so I'd like to take this occasion for doing it here now:

Major Variola (ret) commented (indented lines, followed by my comment):
[...]
 >Skype claims to use RSA-based key exchange, which is good for
 >multi-party conferencing but does not preserve forward secrecy.
 >Maybe some variant of ephemeral D-H authenticated by RSA
 >signatures, with transparent renegotiation every time someone
 >joins the conference, could do the job better.

 RSA (ie persistant keys) may be an option but MUST NOT be
 required, for secrecy reasons as mentioned.  (At worst RSA keys
 can be used once, then discarded.  Lots of primes out there :-)

Well, I don't see why RSA signatures (only for authentication of the key
exchange) could endanger forward secrecy.

 Also, this is *voice*, ie biometric auth,
 so public-key-web-o-trust verislime scam is
 unnecessary at best.

It's not only voice, it's also IM-style text chat. And even with voice,
biometric authentication becomes awkward to use with conference calls.

[...]
 >One could always implement a brand new
 >network, using Distributed Hash Table algorithms such as Chord or
 >Kademlia,

 We don't give a flying fuck as to which shiny new algorithm you use,
 although were we a graph theory wonk, we might care.

The issue here is that DHT algorithms allow to implement a fully
distributed directory, which means one much more resistant to attacks
(especially from institutional attackers) than implementations based on
centralized servers (see, in a related fild, the different destinies of
Napster and its distributed successors such as Overnet or the less
efficient Gnutella). Still, a full search takes O(log(n)) steps, making
them practical for implementing directory/presence services.

[...]
 >but it would be much easier to rely from the very beginning upon
 >a large number of nodes (at least for directory and presence
 >functionality, if not for the reflectors which require specific
 >UDP code).

 What the NAT world (yawn) needs is free registry services
 exploitable by any protocol.  Those NAT-users with RSA-clue can
 sign their registry entry.

Not only that: NATted agents cannot be "called" unless they first register
with some reflector on the open Internet. And centralized reflectors are,
again, easy to attack, and also expensive to operate, as the bandwidth
requirements are substantial (all the traffic flows through them): see
e.g. John Walker's analysis of the reasons that led him to abandon
SpeakFreely at http://www.fourmilab.ch/speakfree/ .

Thomas Shaddack suggested to leverage on Jabber, but:

1. Jabber uses TCP as transport, and therefore can't be efficiently used
as transport for telephony, i.e. using encapsulation of the voice packets
in the Jabber protocol in order to traverse NAT devices.

2. Jabber is based on a client-server paradigm similar to e-mail. Running
a Jabber server requires an always-on machine with its own domain name;
and, although dynamic DNS can help, the model again tend to be
hierarchical, easy to attack etc. That pretty much rules it out also for
session initiation, directory/presence etc.

The beauty of Skype, encryption aside, is that it's based on an overlay
network solely based on P2P servents, relies (if their FAQ tells the
truth) upon NO central registry for presence and directory services, and
each client that runs non-NATted can transparently act as reflector
supporting NATted users. Plus, all this (including, besides voice,
text-based instant messaging) works with zero configuration with an
idiotproof UI.

Enzo

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