What is a subliminal channel?
Many kleptographic attacks are based on the notion of a subliminal channel. The term subliminal channel refers to an information transmission channel that can be used to send information out of (or potentially into) a cryptosystem. A subliminal channel is a type of covert channel. However, covert channels are broader in scope since they are not specific to cryptosystems (covert channels are discussed in [An01]). A concrete example will go a long way to explain what a covert channel is. Suppose that Alice and Bob are connected to a computer that is running a multiuser operating system. In a secure operating system that can be used for sensitive (e.g., military) applications it should not be possible for a process that Alice is running to transmit information covertly to a process that Bob is running. But, suppose that a printer is connected to this machine. Each process can make an operating system call to print data. This call will return a result code indicating success or failure. Th
