
LOCAL GRAPH-HOMOMORPHIC PROCESSING FOR PRIVATIZED DISTRIBUTED
SYSTEMS
Elsa Rizk, Stefan Vlaski, Ali H. Sayed
ABSTRACT
We study the generation of dependent random numbers in a dis-
tributed fashion in order to enable privatized distributed learning by
networked agents. We propose a method that we refer to as local
graph-homomorphic processing; it relies on the construction of par-
ticular noises over the edges to ensure a certain level of differen-
tial privacy. We show that the added noise does not affect the per-
formance of the learned model. This is a significant improvement
to previous works on differential privacy for distributed algorithms,
where the noise was added in a less structured manner without re-
specting the graph topology and has often led to performance dete-
rioration. We illustrate the theoretical results by considering a linear
regression problem over a network of agents.
Index Terms—distributed systems, distributed learning, differ-
ential privacy, random number generator
1. INTRODUCTION AND RELATED MATERIAL
Distributed systems consist of a network of agents that collaborate
to achieve a common goal. Some examples include distributed com-
puting [1] when components of a software are shared over a network,
or distributed machine learning [2] where the goal is to fit a global
model to the data dispersed at different computing locations. Dur-
ing collaboration in such systems, communication between neigh-
bours is necessary. However, the shared information might be sensi-
tive, such as in distributed systems handling health or financial data.
Thus, there is a need to privatize communication channels. One way
to achieve secure communications is through cryptographic meth-
ods [3–6], while another is by adding random noise to make the
communication differentially private [7–11].
In the standard implementations, agents add independent noise
to their shared messages. This property degrades the performance of
the learned model since the noises propagate over the graph through
cooperation, as already shown in Theorem 1 of [12]. In order to
endow agents with enhanced privacy with minimal effect on perfor-
mance, it is necessary for the additional noise sources to be mindful
of the graph topology [11]. However, this information is not avail-
able globally and, therefore, one needs to devise a scheme to gen-
erate graph-dependent random noise sources in a distributed man-
ner and without assuming any global information about the graph
structure. Motivated by this observation, we develop in this work a
scheme that constructs privacy perturbations in a manner that their
negative effect on performance is canceled out. One solution was
suggested in [3] for the case of federated learning. Pairs of agent
collaborate to add noise that cancels out at the server. However, the
suggested method generates pseudo-random numbers, which is less
School of Engineering, ´
Ecole Polytechnique F´
ed´
erale de Lausanne (e-
mail:{elsa.rizk, ali.sayed}@epfl.ch).
Department of Electrical and Electronic Engineering, Imperial College
London (e-mail: s.vlaski@imperial.ac.uk).
secure than true random numbers [13] and without any guarantees
of differential privacy.
The objective of this work is therefore to generate dependent
random numbers in a distributed manner across a graph. The prob-
lem is challenging for at least two reasons. Firstly, generating ran-
dom numbers is usually difficult without enforcing beforehand some
distribution for the random process. In practice, random number
generators exploit a variety of entropy sources in a computer such
as mouse movements, available memory, or temperature [14]. Sec-
ondly, it is not evident how agents should exploit independent en-
tropy sources to generate dependent random numbers. Most avail-
able solutions [15–21] rely on a central orchestrator or consider a
fully connected network. A truly distributed method does not appear
to exist.
2. LOCAL GRAPH-HOMOMORPHIC PROCESS
2.1. Problem Setup
We consider a network of Kagents connected by some graph topol-
ogy (Fig. 1). We let amk >0denote the weight attributed to the
message sent by neighbour mto agent kand let A= [amk]denote
the corresponding combination matrix. We assume Ais symmetric
and doubly-stochastic, i.e.:
1TA=1TA1=1.(1)
We further denote the neighbourhood of agent kby Nk; it consists
of all agents connected to kby an edge.
Fig. 1: Illustration of a network of agents.
We consider problems where agents aggregate the received mes-
sages from their neighbours. In other words, if we let ψmk,i denote
arXiv:2210.15414v1 [cs.CR] 26 Oct 2022