  Smart meter data aggregation protocols have been developed to address rising
privacy threats against customers' consumption data. However, these protocols
do not work satisfactorily in the presence of failures of smart meters or
network communication links. In this paper, we propose a lightweight and
fault-tolerant aggregation algorithm that can serve as a solid foundation for
further research. We revisit an existing error-resilient privacy-preserving
aggregation protocol based on masking and improve it by: (i) performing changes
in the cryptographic parts that lead to a reduction of computational costs,
(ii) simplifying the behaviour of the protocol in the presence of faults, and
showing a proof of proper termination under a well-defined failure model, (iii)
decoupling the computation part from the data flow so that the algorithm can
also be used with homomorphic encryption as a basis for privacy-preservation.
To best of our knowledge, this is the first algorithm that is formulated for
both, masking and homomorphic encryption. (iv) Finally, we provide a formal
proof of the privacy guarantee under failure. The systematic treatment with
strict proofs and the established connection to graph theory may also serve as
a starting point for possible generalizations and improvements with respect to
increased resilience.

Cryptography and Security