Timing channel codes for covert communication and network flow tracing

The goal of this thesis is to achieve covert communication, i.e. to covertly relay information using resources that are traditionally unintended for data transmission. In a covert channel, only the sender and receiver are aware of the communication and capable of reading the transmitted message, and any other party would not be aware of the existence of this transmission. The timing channel has been studied as a medium for covert communication in a computer network. It offers the means to communicate a message by relaying information encrypted within the timings formed between consecutive packets. This thesis covers two schemes that utilize the timing channel for covert communication -- to send a message and to trace network flows. The former, is the design of the first spyware circuit that covertly leaks spied information using an efficient encoder and for a network employing the CSMA/CA. This protocol modifies the inter-packet delays of the network flow using an exponential back-off rule. The second scheme does not relay any message but adds a signature called a watermark to enable flow tracing through the network with dependent deletion and substitution errors, and unlike earlier schemes, it is capable of withstanding desynchronization caused by packet drops. We will verify that our schemes are detectable by the receiver, and robust to the noise introduced by the network. In addition to their reliability, they are both difficult to detect and remain invisible to an observer