In this document we explain our implementation of the MONET Hub; in particular the mechanism that dictates who can participate in the consensus system, and how to make participants accountable for their actions. Before deliberating on an implementation, it is important to have a clear picture of the desired outcome. So we will start by reiterating the role of the Hub in MONET, and outline its principal requirements. We then visit the spectrum of potential implementations before explaining our choice of a permissioned Byzantine Fault Tolerant (BFT) consensus algorithm coupled to the Ethereum Virtual Machine (EVM). Lastly we weigh up the pros and cons of Proof of Stake (PoS), and explain our decision to implement Proof of Authoriry (PoA) for the time being.

MONET and the MONET Hub

MONET’s mission is to boost the adoption of peer-to-peer architectures by enabling mobile devices to connect directly to one another in dynamic ad-hoc networks. We believe that a new generation of applications will emerge from this technology. The real force behind MONET, which makes it original and disruptive, is the concept of Mobile Ad-Hoc Blockchains, and the open-source software which implements it; particularly Babble, the powerful consensus algorithm which is suitable for mobile deployments due to its speed, bandwidth efficiency, and leaderlessness.

We anticipate that many MONET applications will require a common set of services to persist non-transient data, carry information across ad-hoc blockchains, and facilitate peer-discovery. So we set out to build the MONET Hub, an additional public utility that provides these services. In the spirit of open architecture, MONET doesn’t rely on any central authority, so anyone is free to implement their own alternative, but the MONET Hub is there to offer a reliable, fast, and secure solution to kickstart the system.

As such, the qualitative requirements of the Hub are:

  • Speed: It should support thousands of commands per second, with latencies under one second.
  • Finality: Results from the hub should be definitive, without the possibility of being arbitrarily overridden in the future.
  • Availability: It should provide a continuous service in the face of network failures or isolated disruptions.
  • Cost: As we want to lower the barrier to entry for developers, using the Hub should be cheaper than rolling out one’s own solution.
  • Security: The hub should provide a trusted source of data and computation, with measures guarding against information loss, data manipulation, or censorship.
  • Governance: The set of entities controlling this utility should be transparent, with a mechanism to add or remove participants, and keep them accountable for their actions.
  • Flexibility: It should be possible and relatively easy to update the software, recover from failures, and adapt to changes.

Spectrum of possible Implementations

From a simple web-service hosted on a privately-owned server, to a public global blockchain like Ethereum, there are many potential ways to implement this service. However, given our requirements, a simple server scores pretty low in all categories (except perhaps speed and flexibility), and global public blockchains are too slow, too hard to update, and usually provide only probabilistic finality, which is not acceptable.

Somewhere in the middle lies a category of distributed systems consisting of relatively small clusters of servers maintaining identical copies of an application via sophisticated communication routines and consensus algorithms. Within this category, there are instances where the entire cluster is controlled by a single entity, and others where each replica is controlled by a different entity.

Modern blockchain projects, including cryptocurrencies like Facebook’s Libra and the Cosmos Atom, adopt the second variant, where nodes are controlled by different entities. A naive implementation would render them vulnerable to malicious actors trying to subvert the system; hence they require strong consensus algorithms, commonly referred to as Byzantine Fault Tolerant (BFT), and a reputation system to incentivize good behavior and punish malicious actors.

Given the requirements stated in the previous section, we believe that the MONET Hub falls in the same category, and requires a permissioned BFT system.

Ethereum with Babble Consensus

We have developed the Monet Toolchain, a complete set of software tools for setting up and using the MONET Hub. This includes monetd, the software daemon that powers nodes on the MONET Hub.

To build monetd, we used our own BFT consensus algorithm, Babble, because it is fast, leaderless, and offers finality. For the application state and smart-contract platform, we use the Ethereum Virtual Mahcine (EVM) via EVM-Lite, which is a stripped down version of Go-Ethereum.

The EVM is a security-oriented virtual machine specifically designed to run untrusted code on a network of computers. Every transaction applied to the EVM modifies the State which is persisted in a Merkle Patricia tree. This data structure allows to simply check if a given transaction was actually applied to the VM and can reduce the entire State to a single hash (merkle root) rather analogous to a fingerprint.

The EVM is meant to be used in conjunction with a system that broadcasts transactions across network participants and ensures that everyone executes the same transactions in the same order. Ethereum uses a Blockchain and a Proof of Work consensus algorithm. EVM-Lite makes it easy to use any consensus system, including Babble.

The remaining question is how to govern the validator-set, and what to use as a reputation system to punish or incentivise participants to behave correctly.

PoS and PoA

A BFT consensus algorithm ensures that a distributed system remains available and consistent in adversarial conditions, with some nodes exhibiting arbitrary failures or malicious behavior, as long as a majority of participants are functioning correctly (actually ⅔). Any trust in the system therefore depends on the ability to legitimise this assumption. What is needed is a mechanism to ensure, with a high degree of confidence, that at least two thirds of participants in the consensus system are functioning correctly at all times. The problem is two-fold: who gets to be a participant, and how are participants incentivised to behave correctly? Not surprisingly, the most convincing answers revolve around money or reputational risk.

In a Proof of Stake (PoS) arrangement, participants are required to lock a significant portion of their assets (usually the blockchain’s built-in token), and respect an extended un-bonding period when they want to leave. At any given time, the validator set is defined by the top N stakers, where N is the desired size of the validator-set. If they are caught undermining the network, this deposit is destroyed. Hence, participants are deterred from cheating. Additionally, participants are usually programmatically compensated for actively participating in securing the network. Hence they are incentivised to act correctly. A nice feature of PoS is that, being a very capitalistic model, it is relatively open; anyone can participate without asking for permission, as long as they put up a stake.

In Proof of Authority (PoA), the stake is tied to reputational risk. It relies on the natural aversion of most humans to tarnish their own reputation. The list of allowed validators is governed by a whitelist. The whitelist is amended through a voting process among existing whitelisted entities. This scheme is less anonymous or open than PoS but has deep roots. The trust of a PoA system rests on the initial group of participants because any amendment to the list has to gather consensus from them; so the trust (or distrust) is carried over as the validator-set evolves. In a system like Babble, the most serious offence consists in signing two different blocks at the same height. Evidence of this can be packaged into an irrefutable proof, and used to punish the guilty participants.

Proof of Stake opens exciting opportunities for a variety of stakeholders, and these economic incentives are excellent for the industry as they drive innovation. That being said, we are of the opinion that it is too early to ascertain the resilience of PoS in the face of decisive attacks, as current production deployments are very recent, and the theoretical arguments alone are not sufficiently convincing (although they sound quite reasonable). We are keeping an eye on PoS systems, hoping that they withstand the test of time. In the meantime, we have opted to implement PoA, to roll out a reliable version of the MONET Hub, with an eye on extending to PoS in a coordinated software update later down the road.


The MONET Hub is a pivotal utility that facilitates the creation of mobile ad-hoc blockchains, and the emergence of a new breed of decentralised applications. To maximise the performance, security, and flexibility of this system, we have opted to build the Monet Toolchain, a smart-contract platform based on the Ethereum Virtual Machine and a state-of-the-art BFT consensus algorithm, Babble. To govern the validator-set involved in the consensus algorithm, we have chosen to implement a Proof of Authority system, with the idea of extending to Proof of Stake when more evidence of its efficacy becomes available.