# Information Bytes

Matthieu Bloch
School of Electrical and Computer Engineering
Georgia Institute of Technology

# Recent works

1. M. Tahmasbi and M. R. Bloch, “Covert Secret Key Generation with an Active Warden,” IEEE Transactions on Information Forensics and Security, vol. 15, pp. 1026–1039, Jun. 2020.

We investigate the problem of covert and secret key generation over a state-dependent discrete memoryless channel with one-way public discussion in which an adversary, the warden, may arbitrarily choose the channel state. We develop an adaptive protocol that, under conditions that we explicitly specify, not only allows the transmitter and the legitimate receiver to exchange a secret key but also conceals from the active warden whether the protocol is being run. When specialized to passive adversaries that do not control the channel state, we partially characterize the covert secret key capacity. In particular, the covert secret key capacity is sometimes equal to the covert capacity of the channel, so that secrecy comes “for free.”

@article{Tahmasbi2019,
author = {Tahmasbi, Mehrdad and Bloch, Matthieu R},
title = {Covert Secret Key Generation with an Active Warden},
journal = {IEEE Transactions on Information Forensics and Security},
year = {2020},
volume = {15},
pages = {1026-1039},
month = jun,
doi = {10.1109/TIFS.2019.2932906},
eprint = {1901.02044},
file = {:2020-Tahmasbi-IEEETransIFS.pdf:PDF},
groups = {Steganography and covert communications},
howpublished = {accepted to \emph{IEEE Transactions on Information Forensics and Security}}
}


2. M. Tahmasbi, A. Savard, and M. R. Bloch, “Covert Capacity of Non-Coherent Rayleigh-Fading Channels,” IEEE Transactions on Information Theory, vol. 66, no. 4, pp. 1979–2005, Apr. 2020.

The covert capacity is characterized for a non-coherent fast Rayleigh-fading wireless channel, in which a legitimate user wishes to communicate reliably with a legitimate receiver while escaping detection from a warden. It is shown that the covert capacity is achieved with an amplitude-constrained input distribution that consists of a finite number of mass points including one at zero and numerically tractable bounds are provided. It is also conjectured that distributions with two mass points in fixed locations are optimal.

@article{Tahmasbi2018a,
author = {Tahmasbi, Mehrdad and Savard, Anne and Bloch, Matthieu R},
journal = {IEEE Transactions on Information Theory},
title = {Covert Capacity of Non-Coherent Rayleigh-Fading Channels},
year = {2020},
issn = {0018-9448},
month = apr,
number = {4},
pages = {1979-2005},
volume = {66},
doi = {10.1109/TIT.2019.2956489},
eprint = {1810.07687},
file = {:2020-Tahmasbi-IEEETransIT.pdf:PDF},
groups = {Steganography and covert communications},
howpublished = {accepted to \emph{IEEE Transactions on Information Theory}}
}


3. H. Zivari-Fard, M. Bloch, and A. Nosratinia, “Keyless Covert Communication via Channel State Information.” submitted to IEEE Transactions on Information Theory, Mar. 2020.

@misc{ZivariFard2020,
author = {Zivari-Fard, Hassan and Bloch, Matthieu and Nosratinia, Aria},
howpublished = {submitted to \emph{IEEE Transactions on Information Theory}},
month = mar,
title = {Keyless Covert Communication via Channel State Information},
year = {2020}
}


4. M. Tahmasbi, M. R. Bloch, and A. Yener, “Learning an Adversary’s Actions for Secret Communication,” IEEE Transactions on Information Theory, vol. 66, no. 3, pp. 1607–1624, Mar. 2020.

Secure communication over a wiretap channel is investigated, in which an active adversary modifies the state of the channel and the legitimate transmitter has the opportunity to sense and learn the adversary’s actions. The adversary has the ability to switch the channel state and observe the corresponding output at every channel use while the encoder has causal access to observations that depend on the adversary’s actions. A joint learning/transmission scheme is developed in which the legitimate users learn and adapt to the adversary’s actions. For some channel models, it is shown that the achievable rates, defined precisely for the problem, are arbitrarily close to those obtained with hindsight, had the transmitter known the actions ahead of time. This initial study suggests that there is much to exploit and gain in physical-layer security by learning the adversary, e.g., monitoring the environment.

@article{Tahmasbi2018,
author = {Tahmasbi, Mehrdad and Bloch, Matthieu R. and Yener, Aylin},
journal = {IEEE Transactions on Information Theory},
title = {Learning an Adversary's Actions for Secret Communication},
year = {2020},
month = mar,
number = {3},
pages = {1607--1624},
volume = {66},
doi = {10.1109/TIT.2019.2940960},
eprint = {1807.08670},
file = {:2020-Tahmasbi-IEEETransIT-a.pdf:PDF},
howpublished = {accepted to \emph{IEEE Transactions on Information Theory}}
}