The compound wiretap channel provides a general framework for studying secrecy communication under channel uncertainty. Characterizing the secrecy capacity of nondegraded compound wiretap channels is a challenging problem in information theory. This paper considers the class of parallel Gaussian compound wiretap channels with only one possible channel realization for the legitimate receiver and characterizes the secrecy capacity. (Such parallel Gaussian compound wiretap channels are generally nondegraded.) Moreover, it is shown that the proposed coding scheme strictly outperforms the best known single-letter scheme with Gaussian codebooks.
The compound wiretap channel provides a general framework for studying secrecy communication under channel uncertainty. Characterizing the secrecy capacity of nondegraded compound wiretap channels is a challenging problem in information theory. This paper considers the class of parallel Gaussian compound wiretap channels with only one possible channel realization for the legitimate receiver and characterizes the secrecy capacity. (Such parallel Gaussian compound wiretap channels are generally nondegraded.) Moreover, it is shown that the proposed coding scheme strictly outperforms the best known single-letter scheme with Gaussian codebooks.
This paper considers the compound wiretap channel, which generalizes Wyner's wiretap model to allow the channels to the (legitimate) receiver and to the eavesdropper to take a number of possible states. No matter which states occur, the transmitter guarantees that the receiver decodes its message and that the eavesdropper is kept in full ignorance about the message. The compound wiretap channel can also be viewed as a multicast channel with multiple eavesdroppers, in which the transmitter sends information to all receivers and keeps the information secret from all eavesdroppers. For the discrete memoryless channel, lower and upper bounds on the secrecy capacity are derived. The secrecy capacity is established for the degraded channel and the semideterministic channel with one receiver. The parallel Gaussian channel is further studied. The secrecy capacity and the secrecy degree of freedom ( ) are derived for the degraded case with one receiver. Schemes to achieve the for the case with two receivers and two eavesdroppers are constructed to demonstrate the necessity of a prefix channel in encoder design. Finally, the multi-antenna (i.e., MIMO) compound wiretap channel is studied. The secrecy capacity is established for the degraded case and an achievable is given for the general case.
Strong secrecy capacity of compound wiretap channels is studied. The known lower bounds for the secrecy capacity of compound finite-state memoryless channels under discrete alphabets are extended to arbitrary uncertainty sets and continuous alphabets under the strong secrecy criterion. The conditions under which these bounds are tight are given. Under the saddle-point condition, the compound secrecy capacity is shown to be equal to that of the worst-case channel. Based on this, the compound Gaussian multiple-input multiple-output wiretap channel is studied under the spectral norm constraint and without the degradedness assumption. First, it is assumed that only the eavesdropper channel is unknown, but is known to have a bounded spectral norm (maximum channel gain). The compound secrecy capacity is established in a closed form and the optimal signaling is identified. The compound capacity equals the worst-case channel capacity and thus establishing the saddlepoint property; the optimal signaling is Gaussian and on the eigenvectors of the legitimate channel and the worst-case eavesdropper is isotropic. The eigenmode power allocation somewhat resembles the standard water-filling but is not identical to it. More general uncertainty sets are considered and the existence of a maximum element is shown to be sufficient for a saddle-point to exist, so that signaling on the worst-case channel achieves the compound capacity of the whole class of channels. The case of rank-constrained eavesdropper is considered and the respective compound secrecy capacity is established. Subsequently, the case of additive uncertainty in the legitimate channel, in addition to the unknown eavesdropper channel, is studied. Its compound secrecy capacity and the optimal signaling are established in a closed form as well, revealing the same saddle-point property. When a saddle-point exists under strong secrecy, strong and weak secrecy compound capacities are equal.
The compound wiretap channel generalizes the classical problem in broadcast information-theoretic secrecy by allowing a class of potential eavesdroppers. This represents uncertainty in the eavesdropper channel and the characterization of its secrecy capacity is an open question. In this paper we present a new coding scheme that generalizes known approaches to this problem. The scheme prefixes an artificial multiple access channel to the transmission scheme in order to design a structured transmit codebook. The idea is that such a structure can potentially increase the perfect secrecy rate for the legitimate users in the presence of the class of eavesdroppers. We develop the achievable secrecy rate of this scheme and provide examples where this scheme is optimal.
We study the degraded compound multi-receiver wiretap channel, which consists of two groups of users and a group of eavesdroppers. We consider two different communication scenarios. In both scenarios, the transmitter sends two confidential messages, one for each group of users. In the first scenario, both messages need to be kept confidential from the eavesdroppers. For this scenario, we assume that there is only one eavesdropper. We obtain the secrecy capacity region for the general discrete memoryless channel model, the parallel channel model, and the Gaussian parallel channel model. For the Gaussian multi-input multi-output (MIMO) channel model, we obtain the secrecy capacity region when there is only one user in the second group. In the second scenario, the message sent to the first group of users needs to be kept confidential from both the second group of users and eavesdroppers, whereas the message sent to the second group of users needs to be kept confidential only from the eavesdroppers. For this scenario, we do not put any restriction on the number of eavesdroppers. We find the secrecy capacity region for the general discrete memoryless channel model, the parallel channel model, and the Gaussian parallel channel model. For the Gaussian MIMO channel model, we obtain the secrecy capacity region when there is only one user in the second group.
The secrecy capacity of compound wiretap channels is studied. Using the known lower bounds for non-degraded discrete memoryless channels and establishing the converse, the secrecy capacity is established under the less capable and noisier conditions. The compound capacity is shown to be equal to the worst-case channel capacity when there exists a saddle- point in the mutual information difference. Earlier results on the secrecy capacity of compound Gaussian MIMO wiretap channels under the spectral norm constraint are extended to the case of rank-deficient eavesdroppers (which may model a massive MIMO base station) without the degradedness assumption. Its compound secrecy capacity is established in a closed form and the optimal signaling is identified: the compound capacity equals the worst-case channel capacity thus establishing the saddle-point property; the optimal signaling is Gaussian and on the eigenvectors of the legitimate channel and the worst-case eavesdropper is omnidirectional (i.e. isotropic over the sub-space spanned by the active eigenmodes of the legitimate channel). The case of additive uncertainty in the legitimate channel, in addition to the unknown eavesdropper channel, is also studied and its compound secrecy capacity is established.
We study the security of communication between a single transmitter and many receivers in the presence of an eavesdropper for several special classes of broadcast channels. As the first model, we consider the degraded multireceiver wiretap channel where the legitimate receivers exhibit a degradedness order while the eavesdropper is more noisy with respect to all legitimate receivers. We establish the secrecy capacity region of this channel model. Secondly, we consider the parallel multireceiver wiretap channel with a less noisiness order in each subchannel, where this order is not necessarily the same for all subchannels, and hence the overall channel does not exhibit a less noisiness order. We establish the common message secrecy capacity and sum secrecy capacity of this channel. Thirdly, we study a class of parallel multireceiver wiretap channels with two subchannels, two users and an eavesdropper. For channels in this class, in the first (resp., second) subchannel, the second (resp., first) receiver is degraded with respect to the first (resp., second) receiver, while the eavesdropper is degraded with respect to both legitimate receivers in both subchannels. We determine the secrecy capacity region of this channel, and discuss its extensions to arbitrary numbers of users and subchannels. Finally, we focus on a variant of this previous channel model where the transmitter can use only one of the subchannels at any time. We characterize the secrecy capacity region of this channel as well.