  Computer network is unpredictable due to information warfare and is prone to
various attacks. Such attacks on network compromise the most important
attribute, the privacy. Most of such attacks are devised using special
communication channel called "Covert Channel". The word "Covert" stands for
hidden or non-transparent. Network Covert Channel is a concealed communication
path within legitimate network communication that clearly violates security
policies laid down. The non-transparency in covert channel is also referred to
as trapdoor. A trapdoor is unintended design within legitimate communication
whose motto is to leak information. Subliminal channel, a variant of covert
channel works similarly except that the trapdoor is set in a cryptographic
algorithm. A composition of covert channel with subliminal channel is the
"Hybrid Covert Channel". Hybrid covert channel is homogenous or heterogeneous
mixture of two or more variants of covert channels either active at same
instance or at different instances of time. Detecting such malicious channel
activity plays a vital role in removing threat to the legitimate network. In
this paper, we present a study of multi-trapdoor covert channels and introduce
design of a new detection engine for hybrid covert channel in transport layer
visualized in TCP and SSL.
