[1] Covert channels and side channels are two types of information leakage channels. A covert channel uses mechanisms that are not intended for communications, e.g., writing and checking if a file is locked to convey a “1” or “0”. 

In a covert channel, an insider process leaks information to an outsider process not normally allowed to access that information. The insider (sending) process could be a Trojan horse program previously inserted stealthily into the computer. An outsider (receiver) process need only to be an unpriviledged process. 

[1] Covert and Side Channels due to Processor Architecture, by Zhenghong Wang and Ruby B. Lee, in ACSAC 2006