  The following submission constitutes a guide and an introduction to a
collection of articles submitted as a Ph.D. dissertation at the University of
Gda\'nsk. In the dissertation, we study the fundamental limitations within the
selected quantum and supra-quantum cryptographic scenarios in the form of upper
bounds on the achievable key rates. We investigate various security paradigms,
bipartite and multipartite settings, as well as single-shot and asymptotic
regimes. Our studies, however, extend beyond the derivations of the upper
bounds on the secret key rates in the mentioned scenarios. In particular, we
propose a novel type of rerouting attack on the quantum Internet for which we
find a countermeasure and benchmark its efficiency. Furthermore, we propose
several upper bounds on the performance of quantum (key) repeaters settings. We
derive a lower bound on the secret key agreement capacity of a quantum network,
which we tighten in an important case of a bidirectional quantum network. The
squashed nonlocality derived here as an upper bound on the secret key rate is a
novel non-faithful measure of nonlocality. Furthermore, the notion of the
non-signaling complete extension arising from the complete extension postulate
as a counterpart of purification of a quantum state allows us to study
analogies between non-signaling and quantum key distribution scenarios.
