  Recently, an ingenious protocol called Algorand has been proposed to overcome
these limitations. Algorand uses an innovative process - called cryptographic
sortition - to securely and unpredictably elect a set of voters from the
network periodically. These voters are responsible for reaching consensus
through a Byzantine Agreement (BA) protocol on one block per time, guaranteeing
an overwhelming probability of linearity of the blockchain.
  In this paper, we present a security analysis of Algorand. To the best of our
knowledge, it is the first security analysis as well as the first formal study
on Algorand. We designed an attack scenario in which a group of malicious users
tries to break the protocol, or at least limiting it to a reduced partition of
network users, by exploiting a possible security flaw in the messages
validation process of the BA. Since the source code or an official simulator
for Algorand was not available at the time of our study, we created a simulator
(which is available on request) to implement the protocol and assess the
feasibility of our attack scenario. Our attack requires the attacker to have a
trivial capability of establishing multiple connections with targeted nodes and
costs practically nothing to the attacker. Our results show that it is possible
to slow down the message validation process on honest nodes, which eventually
forces them to choose default values on the consensus; leaving the targeted
nodes behind in the chain as compared to the non-attacked nodes. Even though
our results are subject to the real implementation assumption, the core concept
of our attack remains valid.
