## Doctoral Dissertations

Dissertations that have an embargo placed on them will not be available to anyone until the embargo expires.

N/A

#### Document Type

Campus-Only Access for Five (5) Years

#### Degree Name

Doctor of Philosophy (PhD)

#### Degree Program

Electrical and Computer Engineering

2019

#### Month Degree Awarded

February

Prof. Dennis Goeckel

Prof. Don Towsley

#### Subject Categories

Digital Communications and Networking | Electrical and Computer Engineering | Systems and Communications

#### Abstract

This dissertation focuses on covert communication in channels where the communication takes place by the transmission of packets. Consider a channel where authorized transmitter Jack sends packets to authorized receiver Steve according to a Poisson process with rate $\lambda$ packets per second for a time period $T$. Jack's transmitted packet visit Alice, Willie, Bob and Steve, respectively. Suppose that covert transmitter Alice wishes to communicate information to covert receiver Bob without being detected by a watchful adversary Willie. We consider three sets of assumptions for this channel. For each set of assumptions, we present a technique for establishing covert communication and analyze its fundamental limits: (1) if Willie cannot verify the sources of the packets but he is aware of the channel's packet rate $\lambda$, Alice can insert packets of her own in the channel covertly and Bob can successfully extract them (packet insertion); (2) if Willie can verify the sources of the packets but Alice and Bob can share a codebook and Bob receives the packets after they pass through an $M/M/1$ queue, Alice can establish covert communication with Bob by embedding information in packet arrival timings (timing channel); and (3) if packets have available space and Willie knows the probability density function (pdf) of the packet sizes, Alice can insert her own bits in the packets covertly and Bob can extract them successfully (bit insertion). We discuss the extensions of the first and second techniques to packet channels whose packet timings are governed by a renewal process.

Finally, we consider the use of timing channels for network fingerprinting which is a traffic analysis method applicable to breaking the anonymity of networks. Assume Alice and Bob have access to the input and the output links of a network, respectively, where the timings of the flows are governed by independent Poisson point processes. The connections between the input and output links are hidden. We show the degree to which Alice can use flow identifiers (fingerprints) and transmit them via a timing channel to Bob to reveal the input-output connections without being detected by Willie who protects the network. We model the network as parallel $M/M/1$ queues where each queue is shared by a flow from Alice to Bob and other flows independent of the flow from Alice to Bob.

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