### Scholarly works

###### Submitted
1. M. Tahmasbi and M. R. Bloch, “Towards Undetectable Quantum Key Distribution over Bosonic Channels.” submitted to IEEE Journal of Selected Topics in Information Theory, Feb. 2020.

@misc{Tahmasbi2020,
author = {Tahmasbi, Mehrdad and Bloch, Matthieu R.},
howpublished = {submitted to \emph{IEEE Journal of Selected Topics in Information Theory}},
month = feb,
title = {Towards Undetectable Quantum Key Distribution over Bosonic Channels},
year = {2020}
}


2. M. Tahmasbi and M. R. Bloch, “Steganography Protocols for Quantum Channels.” submitted to Journal of Mathematical Physics, Feb. 2020.

@misc{Tahmasbi2020a,
author = {Tahmasbi, Mehrdad and Bloch, Matthieu R},
howpublished = {submitted to \emph{Journal of Mathematical Physics}},
month = feb,
title = {Steganography Protocols for Quantum Channels},
year = {2020}
}


3. M. Tahmasbi and M. R. Bloch, “Covert and secret key expansion over quantum channels under collective attacks.” submitted to IEEE Transactions on Information Theory, Jan. 2019.

We consider an enhanced measure of security for a quantum key distribution protocol, in which we require not only that the adversary obtains no information about the key but also remains unaware that a key generation protocol has been executed. When the adversary applies the same quantum channel independently to each transmitted quantum state, akin to a collective attack in the quantum key distribution literature, we propose a protocol that achieves covert and secret key expansion under mild restrictions. A crucial component of the protocol is a covert estimation stage, which is then combined with universal channel coding for reliability and resolvability in the covert regime.

@misc{Tahmasbi2019a,
author = {Tahmasbi, Mehrdad and Bloch, Matthieu R},
title = {Covert and secret key expansion over quantum channels under collective attacks},
howpublished = {submitted to \emph{IEEE Transactions on Information Theory}},
month = jan,
year = {2019}
}


4. I. A. Kadampot, M. Tahmasbi, and M. R. Bloch, “Multilevel-Coded Pulse-Position Modulation for Covert Communications over Binary-Input Discrete Memoryless Channels.” submitted to IEEE Transactions on Information Theory, Nov. 2018.

We develop a low-complexity coding scheme to achieve covert communications over binary-input discrete memoryless channels (BI-DMCs). We circumvent the impossibility of covert communication with linear codes by introducing non-linearity through the use of pulse position modulation (PPM) and multilevel coding (MLC). We show that the MLC-PPM scheme exhibits many appealing properties; in particular, the channel at a given index level remains stationary as the number of level increases, which allows one to use families of channel capacity- and channel resolvability-achieving codes to concretely instantiate the covert communication scheme.

@misc{Kadampot2018a,
author = {Kadampot, Ishaque Ashar and Tahmasbi, Mehrdad and Bloch, Matthieu R},
title = {Multilevel-Coded Pulse-Position Modulation for Covert Communications over Binary-Input Discrete Memoryless Channels},
howpublished = {submitted to \emph{IEEE Transactions on Information Theory}},
month = nov,
year = {2018},
eprint = {1811.09695}
}


###### Accepted
1. M. Tahmasbi and M. R. Bloch, “Active Covert Sensing.” accepted to IEEE International Symposium on Information Theory, Mar. 2020.

@misc{Tahmasbi2020b,
author = {Tahmasbi, Mehrdad and Bloch, Matthieu R},
howpublished = {accepted to \emph{IEEE International Symposium on Information Theory}},
month = mar,
title = {Active Covert Sensing},
year = {2020}
}


###### Articles
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. 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.

@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}}
}


4. M. Tahmasbi and M. R. Bloch, “Framework for covert and secret key expansion over classical-quantum channels,” Physical Review A, vol. 99, no. 5, p. 052329, May 2019.

Covert and secret quantum key distribution aims at generating information-theoretically secret bits between distant legitimate parties in a manner that remains provably undetectable by an adversary. We propose a framework in which to precisely define and analyze such an operation, and we show that covert and secret key expansion is possible. For fixed and known classical-quantum wiretap channels, we develop and analyze protocols based on forward and reverse reconciliation. The crux of our approach is the use of information reconciliation and privacy amplification techniques that are able to process the sparse signals required for covert operation and the Shannon entropy of which scales as the square root of their length. In particular, our results show that the coordination required between legitimate parties to achieve covert communication can be achieved with a negligible number of secret key bits.

@article{Tahmasbi2018b,
author = {Tahmasbi, Mehrdad and Bloch, Matthieu R.},
title = {Framework for covert and secret key expansion over classical-quantum channels},
journal = {Physical Review A},
year = {2019},
volume = {99},
pages = {052329},
month = may,
doi = {10.1103/PhysRevA.99.052329},
eprint = {1811.05626},
file = {:2019-Tahmasbi-PRA.pdf:PDF},
groups = {Quantum comms},
issue = {5},
numpages = {11},
publisher = {American Physical Society}
}


5. M. Tahmasbi and M. R. Bloch, “First and Second Order Asymptotics in Covert Communication,” IEEE Transactions on Information Theory, vol. 65, no. 4, pp. 2190–2212, Apr. 2019.

We study the first- and second-order asymptotics of covert communication over binary-input DMC for three different covertness metrics and under maximum probability of error constraint. When covertness is measured in terms of the relative entropy between the channel output distributions induced with and without communication, we characterize the exact first- and second-order asymptotics of the number of bits that can be reliably transmitted with a maximum probability of error less than εand a relative entropy less than δ. When covertness is measured in terms of the variational distance between the channel output distributions or in terms of the probability of missed detection for fixed probability of false alarm, we establish the exact first-order asymptotics and bound the second-order asymptotics. PPM achieves the optimal first-order asymptotics for all three metrics, as well as the optimal second-order asymptotics for relative entropy. The main conceptual contribution of this paper is to clarify how the choice of a covertness metric impacts the information-theoretic limits of covert communications. The main technical contribution underlying our results is a detailed expurgation argument to show the existence of a code satisfying the reliability and covertness criteria.

@article{Tahmasbi2017,
author = {Tahmasbi, Mehrdad and Bloch, Matthieu R},
title = {First and Second Order Asymptotics in Covert Communication},
journal = {IEEE Transactions on Information Theory},
year = {2019},
volume = {65},
number = {4},
pages = {2190 --2212},
month = apr,
doi = {10.1109/TIT.2018.2878526},
eprint = {1703.01362},
file = {:2019-Tahmasbi-IEEETransIT.pdf:PDF},
groups = {Steganography and covert communications, Quantum information theory}
}


###### Conference proceedings
1. M. Tahmasbi and M. R. Bloch, “Covert Communication with Unknown Code at the Warden,” in Proc. of Allerton Conference on Communication, Control and Computing, Monticello, IL, Sep. 2019, pp. 1060–1067.

In covert communication, a transmitter attempts to send a message over a channel while avoiding detection from a warden. We investigate here how the warden’s knowledge of the code used by the transmitter and the receiver influences the optimal number of bits that can be covertly and reliably sent. We formulate the problem in three scenarios: the warden completely knows the code, the warden knows only the rate and an upper-bound on the probability of error at the decoder, and the warden has access to previous samples of its channel when the code was used. The first scenario corresponds to the standard model in covert communication. For the second scenario, we characterize the number of bits that can be transmitted reliably subject to a covertness constraint up to the first-order of asymptotics. Finally, for the third scenario, we provide lower-and upper-bound on the number of bits that can be transmitted reliably subject to a covertness constraint for two scalings of the number of observed samples.

@inproceedings{Tahmasbi2019d,
author = {Tahmasbi, Mehrdad and Bloch, Matthieu R.},
booktitle = {Proc. of Allerton Conference on Communication, Control and Computing},
title = {Covert Communication with Unknown Code at the Warden},
year = {2019},
month = sep,
pages = {1060--1067},
doi = {10.1109/ALLERTON.2019.8919792},
file = {:2019-Tahmasbi-Allerton.pdf:PDF},
groups = {Steganography and covert communications},
isbn = {978-1-7281-3152-8},
keywords = {Reliability, Transmitters, Receivers, Zinc, Decoding, Standards, AWGN channels},
location = {Monticello, IL, USA}
}


2. I. Kadampot, M. Tahmasbsi, and M. R. Bloch, “Forward Reconciliation for Covert Key Generation,” in Proc. of IEEE Information Theory Workshop, Visby, Sweden, Aug. 2019, pp. 1–5.

@inproceedings{Kadampot2019a,
booktitle = {Proc. of IEEE Information Theory Workshop},
title = {Forward Reconciliation for Covert Key Generation},
year = {2019},
month = aug,
pages = {1--5},
doi = {10.1109/ITW44776.2019.8989274},
howpublished = {accepted to \emph{IEEE Information Theory Workshop}}
}


3. M. Tahmasbi, M. Bloch, and A. Yener, “In-Band Sensing of the Adversary’s Channel for Secure Communication in Wireless Channels,” in Proc. of IEEE International Symposium on Information Theory, Paris, France, Jul. 2019, pp. 2184–2188.

We propose a model of secure communication over wireless channels in which the legitimate parties leverage Radio Tomographic Imaging (RTI) to learn the adversary. Specifically, we model the results of RTI as an "in band" sensing channel that provides causal information about the eavesdropper’s path-loss to the transmitter. This ability to learn the path-loss is exploited to achieve secrecy, even in presence of an eavesdropper that moves to optimize its path-loss and improves its eavesdropping. We show that the secrecy rates achieved are the same as those that would have been obtained with hindsight, had the transmitter known the average path-loss ahead of time.

@inproceedings{Tahmasbi2019c,
author = {Tahmasbi, Mehrdad and Bloch, Matthieu and Yener, Aylin},
title = {In-Band Sensing of the Adversary's Channel for Secure Communication in Wireless Channels},
booktitle = {Proc. of IEEE International Symposium on Information Theory},
year = {2019},
pages = {2184-2188},
month = jul,
doi = {10.1109/ISIT.2019.8849506},
file = {:2019-Tahmasbi-ISIT2.pdf:PDF},
howpublished = {accepted to \emph{IEEE International Symposium on Information Theory}}
}


4. I. A. Kadampot, M. Tahmasbi, and M. R. Bloch, “Codes for Covert Communication over Additive White Gaussian Noise Channels,” in Proc. of IEEE International Symposium on Information Theory, Paris, France, Jul. 2019, pp. 977–981.

We propose a coding scheme for covert communication over additive white Gaussian noise channels, which extends a previous construction for discrete memoryless channels. We first show how sparse signaling with On-Off keying fails to achieve the covert capacity but that a modification allowing the use of binary phase-shift keying for "on" symbols recovers the loss. We then construct a modified pulse-position modulation scheme that, combined with multilevel coding, can achieve the covert capacity with low-complexity error-control codes. The main contribution of this work is to reconcile the tension between diffuse and sparse signaling suggested by earlier information-theoretic results.

@inproceedings{Kadampot2019,
author = {Kadampot, Ishaque Ashar and Tahmasbi, Mehrdad and Bloch, Matthieu R},
title = {Codes for Covert Communication over Additive White Gaussian Noise Channels},
booktitle = {Proc. of IEEE International Symposium on Information Theory},
year = {2019},
pages = {977-981},
month = jul,
doi = {10.1109/ISIT.2019.8849662},
howpublished = {accepted to \emph{IEEE International Symposium on Information Theory}}
}


5. M. Tahmasbi and M. Bloch, “Steganography Protocols for Quantum Channels,” in Proc. of IEEE International Symposium on Information Theory, Paris, France, Jul. 2019, pp. 2179–2183.

@inproceedings{Tahmasbi2019b,
author = {Tahmasbi, Mehrdad and Bloch, Matthieu},
title = {Steganography Protocols for Quantum Channels},
booktitle = {Proc. of IEEE International Symposium on Information Theory},
year = {2019},
pages = {2179-2183},
month = jul,
file = {:2019-Tahmasbi-ISIT.pdf:PDF},
howpublished = {accepted to \emph{IEEE International Symposium on Information Theory}}
}


6. I. A. Kadampot, M. Tahmasbi, and M. R. Bloch, “Multilevel-Coded Pulse Position Modulation for Covert Communications,” in Proc. of IEEE International Symposium on Information Theory, Vail, CO, Jun. 2018, pp. 1864–1868.

We develop a low-complexity coding scheme to achieve covert communications over binary symmetric channels. We circumvent the impossibility of covert communication with linear codes by introducing non-linearity through the use of pulse-position modulation (PPM) and multilevel coding (MLC). We show that the MLC-PPM scheme exhibits many appealing properties, in particular, the channel at a given index level remains the same as the number of level increases, which allows one to use families of capacity- and resolvability-achieving codes to concretely instantiate the covert communication scheme.

@inproceedings{Kadampot2018,
author = {Kadampot, Ishaque Ashar and Tahmasbi, Mehrdad and Bloch, Matthieu R.},
title = {Multilevel-Coded Pulse Position Modulation for Covert Communications},
booktitle = {Proc. of IEEE International Symposium on Information Theory},
year = {2018},
pages = {1864--1868},
month = jun,
doi = {10.1109/ISIT.2018.8437587},
groups = {Steganography and covert communications},
howpublished = {accepted to \emph{IEEE International Symposium on Information Theory}}
}


7. M. Tahmasbi, M. R. Bloch, and V. F. Tan, “Error exponents covert communications,” in Proc. of IEEE Information Theory Workshop, Kaohsiung, Taiwan, Nov. 2017, pp. 304–308.

We define and study the error exponent of covert communications over binary-input Discrete Memoryless Channels (DMCs). Our main result consists of upper and lower bounds for the exponent, which match in a regime that we explicitly characterize. While our proofs follow standard techniques, the vanishing rate regime inherent to covert communications and the low-weight of codewords introduces specific technical challenges. In particular, the lower bound of the error exponent follows from a non-standard constant-composition ensemble instead of an independent and identically distributed (i.i.d.) ensemble, and the upper bound requires a careful treatment that does not appear in the traditional analysis of error exponent.

@inproceedings{Tahmasbi2017b,
author = {Tahmasbi, Mehrdad and Bloch, Matthieu R and Tan, Vincent F},
title = {Error exponents covert communications},
booktitle = {Proc. of IEEE Information Theory Workshop},
year = {2017},
pages = {304-308},
month = nov,
doi = {10.1109/ITW.2017.8278024},
file = {:2017-Tahmasbi-ITW.pdf:PDF},
groups = {Steganography and covert communications}
}


8. M. Tahmasbi and M. R. Bloch, “Covert secret key generation,” in Proc. of IEEE Conference on Communications and Network Security, Workshop on Physical-Layer Methods for Wireless Security, Las Vegas, NV, Oct. 2017, pp. 540–544.

We investigate the possibility of covert and secret key generation over a discrete memoryless channel model with one way public discussion. Protocols are required to conceal not only the key but also whether a protocol is being implemented. For some models, we show that covert secret key generation is possible and characterize the covert secret key capacity in special cases; in particular, the covert secret key capacity is sometimes equal to the covert capacity of the channel, so that secrecy comes ï¿½for free.ï¿½ Our main contribution is the analysis of a protocol that exploits the likelihood encoder to circumvent source coding with side information and privacy amplification.

@inproceedings{Tahmasbi2017c,
author = {Tahmasbi, Mehrdad and Bloch, Matthieu R.},
title = {Covert secret key generation},
booktitle = {Proc. of IEEE Conference on Communications and Network Security, Workshop on Physical-Layer Methods for Wireless Security},
year = {2017},
pages = {540-544},
month = oct,
doi = {10.1109/CNS.2017.8228681},
groups = {Steganography and covert communications, Secret key agreement},
keywords = {Communication networks;Communication system security;Conferences;Privacy;Protocols;Security;Throughput}
}


9. M. Tahmasbi, M. R. Bloch, and A. Yener, “Learning Adversary’s Actions for Secret Communication,” in Proc. of IEEE International Symposium on Information Theory, Aachen, Germany, Jun. 2017, pp. 2713–2717.

We analyze the problem of secure communication over a wiretap channel with an active adversary, in which the legitimate transmitter has the opportunity to sense and learn the adversary’s actions. Specifically, the adversary has the ability to switch between two channels and to observe the corresponding output at every channel use; the encoder, however, has causal access to observations impacted by adversary’s actions. We develop a joint learning/transmission scheme in which the legitimate users learn and adapt to the adversary’s actions. For some channel models, we show that the achievable rates, which we define precisely, are arbitrarily close to those obtained with hindsight, had the transmitter known the actions ahead of time. This suggests that there is much to exploit and gain in physical-layer security by monitoring the environment.

@inproceedings{Tahmasbi2017a,
author = {Tahmasbi, Mehrdad and Bloch, Matthieu R and Yener, Aylin},
title = {Learning Adversary's Actions for Secret Communication},
booktitle = {Proc. of IEEE International Symposium on Information Theory},
year = {2017},
pages = {2713--2717},
month = jun,
doi = {10.1109/ISIT.2017.8007021},
file = {:2017-Tahmasbi-ISIT.pdf:PDF},
howpublished = {accepted to \emph{IEEE International Symposium on Information Theory}}
}


10. K. S. K. Arumugam, I. A. Kadampot, M. Tahmasbi, S. Shah, M. Bloch, and S. Pokutta, “Modulation recognition using side information and hybrid learning,” in Proc. IEEE Int. Symp. Dynamic Spectrum Access Networks (DySPAN), Piscataway, NJ, Mar. 2017, pp. 1–2.

Recent applications of machine learning to modulation recognition have demonstrated the potential of deep learning to achieve state-of-the-art performance. We propose to further extend this approach by using flexible time-space decompositions that are more in line with the actual learning task, as well as integrate side-information, such as higher order moments, directly into the training process. Our promising preliminary results suggest that there are many more benefits to be reaped from such approaches.

@inproceedings{Arumugam2017a,
author = {Arumugam, K. S. Kumar and Kadampot, I. A. and Tahmasbi, M. and Shah, S. and Bloch, M. and Pokutta, S.},
title = {Modulation recognition using side information and hybrid learning},
booktitle = {Proc. IEEE Int. Symp. Dynamic Spectrum Access Networks (DySPAN)},
year = {2017},
pages = {1--2},
month = mar,
doi = {10.1109/DySPAN.2017.7920750},
file = {:2017-Arumugam-Dyspan.pdf:PDF},
groups = {Steganography and covert communications},
keywords = {learning (artificial intelligence), modulation, telecommunication computing, deep learning, flexible time-space decompositions, higher order moments, machine learning, modulation recognition, side information, Cognitive radio, Convolution, Dynamic spectrum access, Machine learning, Modulation, Network architecture, Signal to noise ratio}
}


11. M. Tahmasbi and M. R. Bloch, “Second Order Asymptotics for Degraded Wiretap Channels: How Good Are Existing Codes?,” in 54th Annual Allerton Conference on Communication, Control, and Computing, Monticello, IL, Sep. 2016, pp. 830–837.

We develop three new results regarding the second-order asymptotics of secure communication over wiretap channels. We first establish the optimal second-order asymptotics for a class of degraded wiretap channels without feedback under an effective secrecy criterion. We then derive the optimal second-order asymptotics for degraded wiretap channels with feedback. We finally develop a new converse bound for channel resolvability with non-capacity achieving distributions, which we use to develop useful converse bounds for asymmetric degraded wiretap channels. Our results are illustrated with several numerical examples, and suggest that known coding techniques already achieve their best performance.

@inproceedings{Tahmasbi2016a,
author = {Tahmasbi, Mehrdad and Bloch, Matthieu R.},
title = {Second Order Asymptotics for Degraded Wiretap Channels: How Good Are Existing Codes?},
booktitle = {54th Annual Allerton Conference on Communication, Control, and Computing},
year = {2016},
pages = {830-837},
month = sep,
doi = {10.1109/ALLERTON.2016.7852319},
groups = {Wiretap channels},
keywords = {encoding;telecommunication security;asymmetric degraded wiretap channels;channel resolvability;converse bound;effective secrecy criterion;existing codes;known coding techniques;optimal second-order asymptotics;secure communication;Decoding;Encoding;Measurement;Monte Carlo methods;Receivers;Reliability;Transmitters}
}


12. M. Tahmasbi and M. R. Bloch, “Second-Order Asymptotics of Covert Communications over Noisy Channels,” in Proc. of IEEE International Symposium on Information Theory, Barcelona, Spain, Jul. 2016, pp. 2224–2228.

We consider the problem of covert communication over noisy binary input Discrete Memoryless Channels (DMCs). Covertness is measured with respect to an adversary in terms of the divergence between the channel output distribution induced with and without communication. We characterize the exact second order asymptotics of the number of bits that can be reliably transmitted with a probability of error less than epsilon and a divergence less than delta. The main technical contribution of this paper is a detailed analysis of how to expurgate a random code while maintaining its channel resolvability properties.

@inproceedings{Tahmasbi2016,
author = {Tahmasbi, Mehrdad and Bloch, Matthieu R.},
title = {Second-Order Asymptotics of Covert Communications over Noisy Channels},
booktitle = {Proc. of IEEE International Symposium on Information Theory},
year = {2016},
pages = {2224-2228},