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Top1. Introduction
With the rapid development of networks, how to achieve secure communication in an open environment is becoming increasingly important. So far, lots of research effort has been made in this field (Yang, 2004). Particularly in recent years, a new alternative technique for secure communications, called steganography, has attracted increasing attention, of which the target is to conceal secret data into public covers (e.g. images, audios, videos and texts) to achieve covert communications (Provos et al., 2003, Zielinska et al., 2014). In contrast with traditional techniques based on cryptographies, whose purpose are to hide the content of secret messages being exchanged between two communicating parties, the goal of steganography is to conceal not only the content but also its very existence. Thus, steganography can provide better protection for secret data in a sense.
Hitherto, a great number of researches on steganography have been carried out, and target steganographic covers have been also extended from initial images to almost all multimedia (Zielinska et al., 2014, El-Eman et al., 2015, Li et al., 2015, Ramalingam et al., 2015, Satir et al., 2014). Recently, a novel dynamic steganographic cover, Voice over Internet Protocol (VoIP, also named IP telephony), has attracted increasing interest (Huang et al., 2011a, Lubacz et al., 2010, Mazurczyk et al., 2013, Tian et al., 2011, 2012, 2015a, 2015b), which has three main advantages over traditional storage media (Tian et al., 2011, 2012a, 2015a, 2015b). First, the real-time nature of VoIP provides better security for secret messages by virtue of its instantaneity, because it gives eavesdroppers almost no time to discover possible abnormity owing to concealed messages. Second, VoIP can be considered a multi-dimensional carrier in that both the packet protocol headers and the payload data can be employed to conceal information. Third, the duration of a VoIP conversation is dynamic and variable, so it can offer adequate cover data according to the requirement of covert communication. In virtue of the above advanced characteristics, VoIP-based steganography can provide another sophisticated solution for secure communication.