Skype likely to provide means of VoIP interception – eavesdropping by “state trojans” disproportionate

(German version)

The widely used voice-over-IP and video chat software Skype is likely to provide a way to monitor Skype-internal conversations. It seems that the answer to the lively debated question whether there is an alternative to the so-called “source-interception” (trojan-based eavesdropping) is “yes”, at least from a technical point of view. In order to avoid the infiltration of computer systems by monitoring software which infringes fundamental rights, enforcement agencies should enter into negotiations with Skype, now a Microsoft subsidiary, to strive for the practical implementation of less intrusive surveillance measures.

For years, rumors are circulating on the net that Skype software may include a monitoring interface for government agencies. These suspicions are underlined by the “small print” of the Skype Privacy Policy, which states (Section 3):

Skype, Skype’s local partner, or the operator or company facilitating your communication may provide personal data, communications content and/or traffic data to an appropriate judicial, law enforcement or government authority lawfully requesting such information. Skype will provide all reasonable assistance and information to fulfil this request and you hereby consent to such disclosure.

However a definitive proof for such a function has thus far not become known publicly.

Nevertheless, the system may already provide a technically simple way of listening to voice calls within the Skype network even without breaking the proprietary encryption of the audio data stream. The network operator, Skype, should be able to do this simply by secretly combining the “Skype In” and “Skype Out” features.

The “Skype In” and “Skype Out” call routing options are used to make calls from within the Skype network to the “normal” telephone network. For Skype-Out, the Skype user connects to an extension designated by a classic telephone number. With Skype-In things are just the other way round: A “normal” telephone number is assigned to a Skype user name, thus calls to this number are put through over the Skype network to a Skype client that has registered with the respective associated Skype username.

Both Skype-In and Skype-Out connections technically require the use of a gateway between the Skype network and the conventional telephone network. In detail two separate connections are established which are both linked to the gateway at one end, namely a Skype VoIP connection between the gateway and the Skype client as well as a normal telephone connection between the gateway and the called extension. The latter compound in turn requires that the encrypted Skype connection ends at the gateway: The – as far as is known, as such, secure – encryption therefore ends at the gateway to the traditional telephone network, where a plaintext audio signal can be retrieved (possibly digital, but certainly not encrypted).

It follows first that this setup allows for Skype-In and Skype-Out calls to be tapped at the gateway, at least from a technical perspective, with a “normal” telephone surveillance setup. From this finding, however, follows at once that even network-internal Skype conversations can be intercepted.

For this, the Skype network – always acting as a dispatcher between the clients involved while connecting two endpoints on the Skype network – just secretly instructs both clients not to establish a direct & encrypted connection between one another. Rather each one is forced to establish, without this fact being visible to the user, a connection directed to the regular telephone network gateway. This is technically pretty straightforward – the network only needs to indicate the gateway as the desired endpoint to the calling client instead of the actual addressee’s client. The called Skype client in turn is tricked into establishing a Skype-in ​​connection. Thirdly, both connections need to be switched together on the Skype gateway “in silence”, creating the impression that a highly secure direct connection had been established. In fact, however, the Skype operator sets up a a man-in-the-middle attack against Skype network internal connections, because at the intersection of the two “legs” of the connection, the Skype has full control over a plaintext audio signal. This signal can then by tapped by classic means of interception.

I have no information on whether Skype has already implemented this possibility. To me it seems logical, however, that if Skype-In and Skype-Out are considered as given, such eavesdropping could be achieved without any or at most minimal changes to the clients. The changes should essentially be to hide away any clues of Skype-In or Skype-Out from the user interface.

Technically speaking, this solution makes use of the Skype client being no open software but rather a “black box” from the user perspective. In particular, the user has no ways to independently authenticate the remote party. Thus Skype seems to be able to indicate arbitrary remote parties instead of the desired addressee as the endpoint of an outgoing connection – including the Skype Out / eavesdropping gateway.

Overall, it can therefore be assumed with great probability that the implementation of a monitoring interface for calls within the Skype network would require minimal changes at best, if such a function has not yet been implemented.

The fact that Skype already offers such a solution, or at least should easily be able to set up one, is also underlined by a patent granted to Microsoft in December 2009. The software maker, that has taken over Skype during the summer of 2011, filed a solution for the interception of voice-over-IP calls through targeted rerouting of the data stream via a “recording agent.” So it says in the abstract of the patent:

“Aspects of the subject matter Described start recording silently relate to communications. In aspects, data associated with a request to Establish a communication is modified to cause the communication to be established via a path that includes a recording agent.”

Just a coincidence?

I’m looking forward to your feedback about this!

If my thoughts are true, the use of trojans in order to monitor Skype traffic appears as disproportionate. Security authorities are rather required to set up technical and administrative details with Skype to put such tapping in place. Skype’s willingness to cooperate is rather likely as it’s now a Microsoft subsidiary, and German public administration spends many millions of euros each year on Microsoft software.