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[liberationtech] Is it possible to send secret messages via Skype using silence? (The Next Web)

Yosem Companys companys at
Mon Jan 7 22:07:21 PST 2013

Skype is used by millions of people every day, all of whom do not want
anyone eavesdropping on their conversations. Naturally, interested parties
will do everything they can to listen in, especially when there are lives
or sensitive information at stake. Yet users of the service may soon have a
way to send more secure Skype messages with less chance that anyone will be
able to listen in, according to New

Wojciech Mazurczyk <>, a professor at the Institute of
Telecommunications <> in Warsaw, Poland, says he has
developed a new technique for tracking and embedding secret data during a
Skype call. With his colleagues Maciej Karaś and Krysztof Szczypiorski, he
analysed Skype data traffic during calls and discovered an opportunity in
the way Skype “transmits” silence.

Skype sends 130-bit data packets for speech but, instead of sending no data
between spoken words, the service sends 70-bit-long data packets to
represent silence. Mazurczyk and his team saw this as an opportunity to
send extra information during a call.

Using a tool they build called SkypeHide, the team says they can hijack the
silence packets, inject encrypted message data into some of them, and then
decode it on the other end. They claim they were able to transmit secret
text, audio, or video at a rate of almost 1 kilobit per second alongside
Skype calls.

Normally, when Skype receives 70-bit data packets representing silence, it
just ignores them. If both parties have SkypeHide installed, however, they
can encode and decode secret messages sent between each other during a
Skype call.

Mazurczyk insists that the secret data is indistinguishable from
silence-period traffic, making detection of SkypeHide very difficult. While
this is certainly possible, we’re sure interested parties would still find
a way to intercept the “silent” packets.

Nevertheless, the proposition is definitely a tempting one: anyone from
security researchers to global activists might be interested in using the
world’s highest profile VoIP app with an extra privacy layer for their
conversations. Mazurczyk and his team will be presenting their findings
this June at a steganography conference in Montpellier, France, so we can
expect more details then.
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