Covert channel is one of the most interesting topics in the computer networks security. Covert channel designers are seeking to discover weaknesses in the communications algorithms to use them as the medium for covert transmission. Broadcast nature of wireless channels has provided a favorable environment for the design of the hidden channels. CSMA/CA is used to control channel access in IEEE 802.11 network. Random features of this algorithm can be used to create timing covert channel. The statistical distribution of the free time intervals' duration is used in this paper for covert channel establishment. Hidden messages are sent via manipulating the timing of the overt packets' transmission. Hidden transceiver senses the wireless channel continuously to be adapted to the dynamic network condition and to be less detectable. To increase the covert channel's accuracy, some free interval durations are not used that leads to security degradation. This problem is also covered by a gap covering method that is based on the summation of the channel sensed free time interval distribution and a normal one. Hidden nodes also estimate the number of active nodes and adapt their behavior accordingly to keep their compatibility with the network. The statistical Kolmogorov-Smirnov and regularity tests are used to assess the security of the covert channel. Simulation results show that the proposed covert channel have a high bit rate along with high security.

Covert channel is one of the most interesting topics in the computer networks security. Covert channel designers are seeking to discover weaknesses in the communications algorithms to use them as the medium for covert transmission. Broadcast nature of wireless channels has provided a favorable environment for the design of the hidden channels. CSMA/CA is used to control channel access in IEEE 802.11 network. Random features of this algorithm can be used to create timing covert channel. The statistical distribution of the free time intervals' duration is used in this paper for covert channel establishment. Hidden messages are sent via manipulating the timing of the overt packets' transmission. Hidden transceiver senses the wireless channel continuously to be adapted to the dynamic network condition and to be less detectable. To increase the covert channel's accuracy, some free interval durations are not used that leads to security degradation. This problem is also covered by a gap covering method that is based on the summation of the channel sensed free time interval distribution and a normal one. Hidden nodes also estimate the number of active nodes and adapt their behavior accordingly to keep their compatibility with the network. The statistical Kolmogorov-Smirnov and regularity tests are used to assess the security of the covert channel. Simulation results show that the proposed covert channel have a high bit rate along with high security.

Steganography and Covert channels, their design and detection has been one of the most important issues in security systems and computer networks. One of the media that is used for steganography is JPEG images. The purpose of many steganography techniques in pictures is to insert more covert information in image's pixels with few changes.One of methods in JPEG steganography is EMD (Exploiting Modification Direction). EMD tries to increase embedding efficiency which means embed more hidden bits for each change. On the other hand, due to widespread use of wireless networks, designing covert channels in these networks has also attracted a lot of attention. Local wireless networks have a high degree of randomness in the selection of back off times in their collision avoidance algorithm. This feature can be used to create covert channels. In this article, the idea ofEMD method is discussed to create a covert channel in wireless networks. The goal is to achieve high throughput with high degree of secu rity.