SkypeHide system, steganography to secure communications on Skype

Pierluigi Paganini January 08, 2013

In this period there is an intense debate on the wiretapping of every communication channel, governments are increasing the monitoring of internet, of social network platforms and VOIP conversations in many cases with supports of the companies that provide those services. In a recent post I discussed on the control ordered by Indian Governments that ,for homeland security, decided to control every user’s activity on-line exactly as many other authorities in the world.

Skype is one of the most diffused VOIP client used by millions of persons for business such as in the free time, simple and efficiency it has catch a large piece of the market also because in the past it was considered a secure tool to speak avoiding law enforcement interception due its architecture. With the purchase of Skype made by Microsoft, every think is changed, from graphical layout of the application to the possibility of being intercepted also thanks to VOIP interception patent, dubbed Legal Intercept,  registered by Microsoft few days after the acquisition. In the hacker communities many voices sustain the Skype architecture has been modified to enable “lawful interception” of calls.

Analyzing Skype’s privacy policy appears clear that the company is able spy on user communications communicating them to authorities under explicit request.

Under Section 3 of the privacy policy, it is stated that

“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 fulfill this request and you hereby consent to such disclosure..”

Under Section 12 it stated that

“Your instant messaging (IM) communications-content may be stored by Skype (a) to convey and synchronise your messages and (b) to enable you to retrieve the messages and history where possible. IM messages are currently stored for a maximum of 30 days unless otherwise permitted or required by law. Voicemail messages are currently stored for a maximum of 60 days unless otherwise permitted or required by law. Skype will at all times take appropriate technical and security measures to protect your information. By using this product, you consent to the storage of your IM communications as described above.”

With 663 million registered users, Skype represents a precious mine of information, a great concentrator of communications exposed to interception. But technology could reveal sensational surprises like the news that Skype could still be used to transfer information in secure way avoiding wiretapping.

The popular VOIP client uses a 256-bit advanced encryption for is calls by default, but many researchers are studying the way to make more secure Skype,  Wojciech Mazurczyk professor at the Warsaw University of Technology seems have found the way to do it.

“There are concerns that Skype calls can be intercepted and analysed,”

says Mazurczyk at the Institute of Telecommunications in Warsaw. The idea is very ingenious, the researcher has discovered that is possible to exploit the silence period during a common conversation to transfer secretly data.

Wojciech Mazurczyk has demonstrated that is possible to insert data in 70-bit service packets that Skype sends by default when it’s detecting silence, when the user is mute rather than send no data between spoken words Skype sends 70-bit-long data packets instead of the 130-bit ones that normally carry user’s voice.

The Mazurczyk team, that include the colleagues Maciej Karaś and Krzysztof Szczypiorski, has designed the SkypeHide system that allow users to hide extra non-chat messages during a call. The researchers have inserted encrypted message data into those packets noting that client in reception ignores the cyphered data.

“The secret data is indistinguishable from silence-period traffic, so detection of SkypeHide is very difficult,” declared  Mazurczyk.

The technique allows the transmission of text messages but also of video and audio at a rate of almost 1 kilobit per second alongside phone calls. The SkypeHide will be presented in June at a steganography conference in Montpellier, France.

Steganography is certainly not a new concept, for decades researchers exploring new channels to convey information in a secure way and the study conducted on Skype protocol demonstrates that it is possible to use control data to hide the information, the technique discovered by the researchers could also be improved from the use of string cyphering mechanism for the data to transfer, of course the optimal solution is represented by the compromise between the level of security requested and the performance obtained.

Pierluigi Paganini

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