  Over the past twenty years, the number of devices connected to the Internet
grew exponentially. Botnets benefited from this rise to increase their size and
the magnitude of their attacks. However, they still have a weak point in their
Command & Control (C&C) system, which is often based on centralized services or
require a complex infrastructure to keep operating without being taken down by
authorities. The recent spread of blockchain technologies may give botnets a
powerful tool to make them very hard to disrupt. Recent research showed how it
is possible to embed C&C messages in Bitcoin transactions, making them nearly
impossible to block. Nevertheless, transactions have a cost and allow very
limited amounts of data to be transmitted. Because of that, only messages from
the botmaster to the bots are sent via Bitcoin, while bots are assumed to
communicate through external channels. Furthermore, for the same reason,
Bitcoin-based messages are sent in clear. In this paper we show how, using
Bitcoin Testnet, it is possible to overcome these limitations and implement a
cost-free, bidirectional, and encrypted C&C channel between the botmaster and
the bots. We propose a communication protocol and analyze its viability in real
life. Our results show that this approach would enable a botmaster to build a
robust and hard-to-disrupt C&C system at virtually no cost, thus representing a
realistic threat for which countermeasures should be devised.
