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Physical-Layer Security

Physical-Layer Security

Due to an increase in the use of wireless transmissions, a lot of attention has been directed towards improving our ability to secure these transmissions. One of the areas being looked into is physical-layer security, which aims to utilize the non-uniformity of the physical world to secure transmission. One of the most tantalizing benefits of physical-layer security is that it provides information theoretic security, which is mathematically guaranteed. This technique could be used alone or in conjunction with other methods of securing data (such as cryptography) to provide a more comprehensive defense against people trying to steal data. However, much of the research into physical-layer security has been highly theoretical and relies on a large number of assumptions, which has prevented its use in real settings.

Picture of the 7 layers of the OSI Model
Photo by https://www.bmc.com/blogs/osi-model-7-layers/

The OSI Model:

When data is being sent between different systems, there is a standardized procedure that is followed. A visual representative of this model, known as the Open Systems Interconnection (OSI) model, can be seen to the side. Each of the seven layers of the OSI model is responsible for a different function.

Cryptography, the most common method for securing data transmission today, typically is applied to the top few layers of the model. However, because of the relative independence of each layer, methods that provide security on separate layers can be combined to maximize their efficiency. The physical layer, which comprises the materials being used to transmit information (things like cables, computers, and any walls that may be in between the transmitter and receiver) as well as the physical world that the signals travel through, has been relatively untouched.

The Wiretap Channel Model:

A model that is often used for this type of communication is known as the Wiretap Channel model and was proposed by Wyner in 1975. This model includes three participants: (1) Alice, the transmitter; (2) Bob, the legitimate recipient; and (3) Eve, an eavesdropper. Alice is trying to transmit a message to Bob, and this message travels across a channel known as the main channel. Eve is in a different location than Bob, and therefore the signal must go through a different channel, known as the eavesdropper's channel. The goal is to make sure that Bob is able to get the correct information while preventing Eve from doing so. This is down by taking advantage of the differences between the main channel and the eavesdropper's channel.

The Multipath Effect:

Example of a multipath signal bouncing off of buildings and trees

We have stated that physical-layer security utilizes the non-uniformity of the physical world to provide advantages to one user (Bob) over another (Eve). There are many different phenomena that come into play, but one good example is something known as the Multipath Effect. This effect occurs when a transmitted signal reflects off of surrounding objects towards a receiver, resulting in multiple copies of the transmit signal at different phases arriving at the receiver simultaneously. These signals can either interfere constructively, resulting in a stronger signal, or destructively, resulting in a weaker signal and potentially preventing the extraction of the signal information.

In a Wiretap Channel environment, this means that the signals could combine in a way to give Bob an advantage over Eve.

Our Contribution:

We focus on looking at various assumptions made in the theoretical analysis of physical-layer security systems and what we can do to map them to a more practical, real-world system. We also look at new challenges that arise as we move towards actually implementing these systems.

Relevant Publications:

M. Rice, B. Clark, D. Flanary, B. Jensen, N. Nelson, K. Norman, E. Perrins, W. K. Harrison, “Physical-Layer Security for Vehicle-to-Everything Networks,” IEEE Vehicular Technology Magazine, submitted July 1, 2019 (Under Review).

D. Flanary, B. Jensen, B. Clark, K. Norman, N. Nelson, M. Rice, W. K. Harrison, “Manufacturing an Erasure Wiretap Channel from Channel Sounding Measurements,ISIT 2019 IEEE International Symposium on Information Theory (ISIT), Paris, France, 2019. 10.1109/ISIT.2019.8849638

B. Jensen, B. Clark, D. Flanary, K. Norman, M. Rice and W. K. Harrison, "Physical-Layer Security: Does it Work in a Real Environment?," ICC 2019 - 2019 IEEE International Conference on Communications (ICC), Shanghai, China, 2019, pp. 1-7. 10.1109/ICC.2019.8761418.