Signal and Data Processing

Covert channel means communicating information through covering of overt and authorized channel in a manner that existence of channel to be hidden. In network covert timing channels that use timing features of transmission packets to modulating covert information, the appropriate encoding schema is very important. In this paper, a hybrid encoding schema proposed through combining "the inter-packets gap" and "the reordering packets" encoding schemas, emphasizing on improvement of capacity and stealthiness of covert channel. The capacity of proposed channel have computed and stealthness and robustness of channel have evaluated in experimental manner. Our results show that selecting 3 to 5 packet in a codword in accordance to normal situation of network traffic, the capacity is increased from 10% to 300% and stealthness is boosted up to acceptable value, and robustness is high enough.