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Crypto boffins smuggle secret messages in silent Skype calls

Masquerades as normal VoIP traffic

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Polish security researchers have come up with a cunning method to transmit hidden messages using the silence packets transmitted during a Skype call.

The VoIP service transmits voice data in 130-byte packets, and silences in 70-byte packets, a difference that creates a potential means to conceal a hidden encrypted message in the latter. The novel form of steganography was devised by Wojciech Mazurczyk, Krysztof Szczypiorski and Maciej Karaœ, researchers at the Institute of Telecommunications of the Warsaw University of Technology.

The crypto boffins developed an application, dubbed SkypeHide (or SkyDe), that embeds an encrypted message using structured sequences of silent packets. The same software running on a receiving computer is used to extract the concealed message.

Hidden messages can contain text, audio or video content, although the maximum transmission rate of 1kbps would more or less preclude the practical transmission of video clips. Packets generated by SkypeHide Would be difficult to distinguish from normal Skype traffic, Trusted Third Party (via Google Translate) reports.

The latest technique builds on earlier research by Mazurczyk and Szczypiorski into steganography using VoIP streams. Four years ago the researchers developed techniques for using unused fields in the RTCP (Real-Time Control Protocol) and RTP (Real-Time Transport Protocol) VoIP protocols to transmit hidden messages.

The researchers hope to present more about the technology at 1st ACM Workshop on Information Hiding and Multimedia Security conference in Montpellier, France, in June. ®

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