The Gaussian wiretap channel with rate-limited help available at the legitimate receiver (decoder) is studied under various channel configurations (degraded, reversely degraded and non-degraded). In all considered cases but one, the rate-limited help results in a secrecy capacity boost equal to the help rate. This holds irrespective of whether the help is secure or not, so that secure help does not provide any advantage over non-secure one. The secrecy capacity is positive for the reversely-degraded channel (where the no-help secrecy capacity is zero) and no wiretap coding is needed to achieve it. More noise at the legitimate receiver can sometimes result in higher secrecy capacity. The same secrecy capacity boost also holds if non-secure help is available to the transmitter (encoder), in addition to or instead of the receiver help.
The Gaussian wiretap channel with rate-limited help available at the legitimate receiver (decoder) is studied under various channel configurations (degraded, reversely degraded and non-degraded). In all considered cases but one, the rate-limited help results in a secrecy capacity boost equal to the help rate. This holds irrespective of whether the help is secure or not, so that secure help does not provide any advantage over non-secure one. The secrecy capacity is positive for the reversely-degraded channel (where the no-help secrecy capacity is zero) and no wiretap coding is needed to achieve it. More noise at the legitimate receiver can sometimes result in higher secrecy capacity. The same secrecy capacity boost also holds if non-secure help is available to the transmitter (encoder), in addition to or instead of the receiver help.
The Gaussian wiretap channel with rate-limited help, available at the legitimate receiver (Rx) or/and transmitter (Tx), is studied under various channel configurations (degraded, reversely degraded and non-degraded). In the case of Rx help and all channel configurations, the rate-limited help results in a secrecy capacity boost equal to the help rate irrespective of whether the help is secure or not, so that the secrecy of help does not provide any capacity increase. The secrecy capacity is positive for the reversely-degraded channel (where the no-help secrecy capacity is zero) and no wiretap coding is needed to achieve it. More noise at the legitimate receiver can sometimes result in higher secrecy capacity. The secrecy capacity with Rx help is not increased even if the helper is aware of the message being transmitted. The same secrecy capacity boost also holds if non-secure help is available to the transmitter (encoder), in addition to or instead of the same Rx help, so that, in the case of the joint Tx/Rx help, one help link can be omitted without affecting the capacity. If Rx/Tx help links are independent of each other, then the boost in the secrecy capacity is the sum of help rates and no link can be omitted without a loss in the capacity. Non-singular correlation of the receiver and eavesdropper noises does not affect the secrecy capacity and non-causal help does not bring in any capacity increase over the causal one.
The wiretap channel with additive (possibly non-Gaussian) noise and rate-limited help, available at the legitimate receiver (Rx) or/and transmitter (Tx), is studied under various channel configurations (degraded, reversely degraded and non-degraded) and power/amplitude constraints. For all channel configurations, the rate-limited Rx help results in a (weak or strong) secrecy capacity boost equal to the help rate. This holds irrespective of whether the help is secure or not, or whether the helper is aware of the message being transmitted or not; the secrecy of help or helper’s knowledge of the message does not provide any extra capacity boost. The secrecy capacity is positive for the reversely-degraded channel (where the no-help secrecy capacity is zero) and no wiretap coding is needed to achieve it under weak secrecy. The same capacity boost also holds if non-secure help is available to the transmitter (encoder), in addition to or instead of the same Rx help, so that, in the case of the joint Tx/Rx help, one help link can be omitted without affecting the capacity. If Rx/Tx help links are independent of each other, the capacity boost is the sum of help rates and no link can be omitted without loss in the capacity. Non-singular correlation of the receiver and eavesdropper noises does not affect the secrecy capacity and non-causal help does not bring in any capacity increase over the causal one. The choice of the secrecy criterion (weak/strong) affects the complexity of implementation but not the secrecy capacity. Stronger noise at the legitimate receiver can sometimes result in higher secrecy capacity.
The Gaussian wiretap channel (WTC) with rate-limited help, available at the transmitter/encoder (Tx), in addition to or instead of the same help at the legitimate receiver, is studied under various channel configurations. For the degraded or reversely-degraded WTC, rate-limited non-secure Tx help results in a secrecy capacity boost equal to the help rate irrespective of whether the help is causal or not. For the non-degraded WTC, the secrecy capacity boost is lower bounded by the help rate. A capacity-achieving signaling is two-phase time sharing, where wiretap coding without help is used in Phase 1 and help without wiretap coding is used in Phase 2. The secrecy capacity with Tx help is positive for the reversely-degraded channel (where the no-help secrecy capacity is zero) and no Phase 1 is needed to achieve it. Unlike the no-help case, more noise at the legitimate receiver can sometimes result in higher secrecy capacity with Tx help. In the case of the joint Tx/Rx non-secure help, one help link can be omitted without affecting the capacity.
In physical-layer security, communication secrecy is commonly achieved by providing the legitimate receiver a physical-layer advantage over the eavesdropper or, equivalently, by making the eavesdropper's channel more degraded than the legitimate receiver's, a setting traditionally known as the degraded wiretap channel. There may be situations, however, where such advantages cannot be guaranteed. One example is the wireless channel where the legitimate receiver's channel may be more degraded than the eavesdropper's owing to a deep fade (outage), additive interference at the edge of a network cell, etc. To our knowledge, such scenarios have not been remedied in the related literature. Thus, we propose in this work the first physical-layer security scheme that achieves communication in a setting where the eavesdropper has the physical-layer advantage over the legitimate receiver, a scenario that we call the reversely-degraded wiretap channel. Precisely, we show that by constraining the transmitter's input to be discrete uniformly distributed (such as an on-off keying modulation) and by using nonlinear threshold-based signal detection, the legitimate receiver is guaranteed to have a positive secrecy capacity even though the eavesdropper has a better signal-to-noise ratio (SNR). The former requirement on the input distribution is to limit the capacity gains of the eavesdropper at high SNR. The latter requirement allows the communication channel to exhibit supra-threshold stochastic resonance, a phenomenon whereby noise improves signal detection and increases the mutual information. A numerical example is provided to confirm our claims.
In this paper, we consider the secrecy capacity of a wiretap channel in the presence of causal state information and secure rate-limited feedback. In this scenario, the causal state information from the channel is available to both the legitimate transmitter and the legitimate receiver. In addition, the legitimate receiver can send secure feedback to the transmitter at a limited rate Rf. We derive upper and lower bounds on the secrecy capacity and show that, when the channel to the eavesdropper is degraded, the bounds are tight and the secrecy capacity is completely characterized. The capacity achieving scheme is based on Wyner, Csiszár, and Körner wiretap coding and two steps of shared-key generation: one from the state information and one via the noiseless feedback. The upper bound is more involved and requires a nontrivial recursive lemma extending previous results in the literature to include both state and feedback. We conclude the paper by showing that a few interesting known results can be seen as special cases of the above, as well as discussing the case where the source of local randomness at the encoder is limited.
In this paper, we consider the secrecy capacity of a wiretap channel in the presence of causal state information and secure rate-limited feedback. In this scenario, the causal state information from the channel is available to both the legitimate transmitter and legitimate receiver. In addition, the legitimate receiver can send secure feedback to the transmitter at a limited rate Rf. We shown that the secrecy capacity is bounded by Cs ≥ max {p(u|v) p(x|u,v)max min{I(U;Y|v) - I(U;Z|V) + H(V|Z) + Rf, I(U;Y|V)},p(u)p(x|u,v)max min{H(V|Z,U) + Rf, I(U;Y|V)}}. Moreover, when the channel to the legitimate receiver is less noisy than the channel to the eavesdropper, the bound is shown to be tight. The capacity achieving scheme is based on both the Wyner wiretap coding and two steps of shared-key generation: one from the state information and one via the noiseless feedback. Finally, we consider several special cases. When state information is available only at the legitimate receiver, the analysis suggests that unlike previous results involving feedback, it is better to use the feedback to send the state information to the transmitter (when possible), rather than send a random key.