  This paper provides a detailed analysis of the impact of quantum computing on
the security of 5G mobile telecommunications. This involves considering how
cryptography is used in 5G, and how the security of the system would be
affected by the advent of quantum computing. This leads naturally to the
specification of a series of simple, phased, recommended changes intended to
ensure that the security of 5G (as well as 3G and 4G) is not badly damaged if
and when large scale quantum computing becomes a practical reality. By
exploiting backwards-compatibility features of the 5G security system design,
we are able to propose a novel multi-phase approach to upgrading security that
allows for a simple and smooth migration to a post-quantum-secure system.
