  Quantum computing had a profound impact on cryptography. Shor's discovery of
an efficient quantum algorithm for factoring large integers implies that many
existing classical systems based on computational assumptions can be broken,
once a quantum computer is built. It is therefore imperative to find other
means of implementing secure protocols. This thesis aims to contribute to the
understanding of both the physical limitations, as well as the possibilities of
cryptography in the quantum setting. In particular, we investigate several
questions that are crucial to the security of quantum protocols: How can we
find good uncertainty relations for a large number of measurement settings? How
does the presence of entanglement affect classical protocols? And, what
limitations does entanglement impose on implementing quantum protocols?
Finally, can we circumvent some of those limitations using realistic
assumptions?
