The paper concerns available steganographic techniques that can be used for creating covert channels for VoIP (Voice over Internet Protocol) streams. Apart from characterizing existing steganographic methods we provide new insights by presenting two new techniques. The first one is network steganography solution which exploits free/unused protocols' fields and is known for IP, UDP or TCP protocols but has never been applied to RTP (Real-Time Transport Protocol) and RTCP (Real-Time Control Protocol) which are characteristic for VoIP. The second method, called LACK (Lost Audio Packets Steganography), provides hybrid storage-timing covert channel by utilizing delayed audio packets. The results of the experiment, that was performed to estimate a total amount of data that can be covertly transferred during typical VoIP conversation phase, regardless of steganalysis, are also included in this paper.
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