This paper explores the tradeoff between covert communication capacity and secret key requirements over discrete memoryless channels (DMCs). We focus on settings where under a covertness constraint both communication and key rates are measured as the number of bits per square root of the block-length. While previous work has identified the maximum covert communication rates and the corresponding minimum key rates needed to achieve them, our study characterizes the minimum key rates necessary for all of desired covert communication rates. In equivalent terms, we determine, for any given key rate, the set of achievable covert rates. This relationship defines what we call the covert capacity-key tradeoff.Our analysis reveals several new insights. In scenarios where only small key rates are available and the adversary has a stronger channel than the intended receiver, binary signaling is optimal—regardless of the specific channel characteristics or input alphabets. In these cases, the covert capacity increases linearly with the available key rate. In other cases and for larger key rates, the covert capacity-key tradeoff grows sublinearly.We also extend our findings to multi-access channels (MACs) with binary inputs.

This paper identifies the Stein exponent of two distributed detection (binary hypothesis testing) setups with limited communication over a discrete memoryless channel (DMC). In the first setup, the DMC can only be used k(n) times, where k(n) grows sublinearly in the length of the observations n. In the second setup, the DMC can be used n times, however a block-input cost constraint Cnis imposed and Cngrows sublinearly in n. The optimal Stein exponent coincides for both setups and depends on whether the DMC is partially-connected, i.e., one of the output symbols can only be induced by a strict subset of the input symbols, or fully-connected. For partially-connected DMCs, the optimal Stein exponent of our setups coincides with the optimal Stein exponent (identified by Han and by Shalaby and Papamarcou) for the scenario where the sensor can communicate a sublinear (in n) number of bits to the decision center and communication is over a noiseless link. In contrast, for fully-connected DMCs the optimal Stein exponent collapses and is given by the optimal Stein exponent of the local test at the decision center. In this case, the sensor and the DMC do not help in improving the Stein exponent. Our results hold for general independent and identically distributed sources.

The fundamental limit of Semantic Communications (joint source-channel coding) is established when the transmission needs to be kept covert from an external warden. We derive information-theoretic achievability and matching converse results and we show that source and channel coding separation holds for this setup. Furthermore, we show through an experimental setup that one can train a deep neural network to achieve covert semantic communication for the classification task. Our numerical experiments confirm our theoretical findings, which indicate that for reliable joint source-channel coding, the number of transmitted source symbols can only scale as the square-root of the number of channel uses.

In this paper, we caracterize the fundamental limits of a communication system with three users (i.e., three transmitters) and a single receiver where communication from two covert users must remain undetectable to an external warden. Our results show a tradeoff between the highest rates that are simultaneously achievable for the three users. They further show that the presence of a non-covert user in the system can enhance the capacities of the covert users under stringent secret-key constraints. To derive our fundamental limits, we provide an information-theoretic converse proof and present a coding scheme that achieves the performance of our converse result. Our coding scheme is based on multiplexing different code phases, which seems to be essential to exhaust the entire tradeoff region between the rates at the covert and the two non-covert users. This property is reminiscent of the setup with multiple non-covert users, where multiplexing is also required to exhaust the entire rate-region.

This paper studies the problem of distributed detection (binary hypothesis testing) over a discrete memoryless channel (DMC) under the constraint that an eavesdropping adversary should not be able to determine whether communication is ongoing or not, i.e., communication over the DMC has to remain covert. The main contribution of the paper is an upper bound on the largest possible Stein exponent, showing that it cannot exceed the largest exponent achievable under zero-rate communication over a noise-free link. In interesting special cases, the upper bound is achieved by a local test at the decision center that completely ig-nores the communication. In these cases, the covertness constraint renders communication useless for improving the Stein exponent.

This paper establishes the fundamental limits of a two-user single-receiver system where communication from User 1 (but not from User 2) needs to be undetectable to an external warden. Our fundamental limits show a tradeoff between the highest rates (or square-root rates) that are simultaneously achievable for the two users. Moreover, coded time-sharing for both users is fundamentally required on most channels, which distinguishes this setup from the more classical setups with either only covert users or only non-covert users. Interestingly, the presence of a non-covert user can be beneficial for improving the covert capacity of the other user.