This paper investigates secure communication over a broadcast channel in the presence of an unreliable cooperation link between the two decoders. Two messages are sent over the channel. One receiver aims to decode both messages while ensuring that the second message remains confidential from the other receiver. The second receiver is only interested in the first message, decoding either a part of it when the cooperation link fails or the entire message when the link is operational. A communication scheme is proposed that ensures reliability, confidentiality, and robustness against potential link failures. The capacity regions are characterized for both the discrete memoryless and the Gaussian version of the channel. Additionally, several notable special cases of the problem are examined.

An upper bound to the identification capacity of discrete memoryless wiretap channels is derived under the requirement of semantic effective secrecy, combining semantic secrecy and stealth constraints. A previously established lower bound is improved by applying it to a prefix channel, formed by concatenating an auxiliary channel and the actual channel. The bounds are tight if the legitimate channel is more capable than the eavesdropper’s channel. An illustrative example is provided for a wiretap channel that is composed of a point-to-point channel, and a parallel, reversely degraded wiretap channel. A comparison with results for message transmission and for identification with only secrecy constraint is provided.

Deterministic identification over K-input multiple-access channels with average input cost constraints is considered. The capacity region for deterministic identification is determined for an average-error criterion, where arbitrarily large codes are achievable. For a maximal-error criterion, upper and lower bounds on the capacity region are derived. The bounds coincide if all average partial point-to-point channels are injective under the input constraint, i.e. all inputs at one terminal are mapped to distinct output distributions, if averaged over the inputs at all other terminals. The achievability is proved by treating the MAC as an arbitrarily varying channel with average state constraints. For injective average channels, the capacity region is a hyperrectangle. The modulo-2 and modulo-3 binary adder MAC are presented as examples of channels which are injective under suitable input constraints. The binary multiplier MAC is presented as an example of a non-injective channel, where the achievable identification rate region still includes the Shannon capacity region.

We study the problem of identification over a DMC wiretap channel under effective secrecy. In identification, due to the fact that single messages are compared to each other, all conditions are inherently semantic, and thus we are forced to consider semantic effective secrecy. We show that even effective secrecy “comes for free” by giving an achievability theorem for stealth identification. We use two concatenated transmission codes, the first one is a resolvability transmission code. The second code is an effective-secrecy transmission code. An achievable rate is derived for the problem.