  Traditional electricity meters are replaced by Smart Meters in customers'
households. Smart Meters collects fine-grained utility consumption profiles
from customers, which in turn enables the introduction of dynamic, time-of-use
tariffs. However, the fine-grained usage data that is compiled in this process
also allows to infer the inhabitant's personal schedules and habits. We propose
a privacy-preserving protocol that enables billing with time-of-use tariffs
without disclosing the actual consumption profile to the supplier. Our approach
relies on a zero-knowledge proof based on Pedersen Commitments performed by a
plug-in privacy component that is put into the communication link between Smart
Meter and supplier's back-end system. We require no changes to the Smart Meter
hardware and only small changes to the software of Smart Meter and back-end
system. In this paper we describe the functional and privacy requirements, the
specification and security proof of our solution and give a performance
evaluation of a prototypical implementation.
