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11 minutes read

Security in Kadena’s Public Blockchain

Example of simple blockchain vs. two-chain configuration from Chainweb 2018 whitepaper (Martino, Quaintance)
Example of simple blockchain vs. two-chain configuration from Chainweb 2018 whitepaper (Martino, Quaintance)

As awareness of Kadena and our blockchain protocol Chainweb grows, I see the same misunderstandings about how braided Proof of Work (PoW) functions, leading to repeated earnest but incorrect critiques. This issue is understandable as scalability research and analysis is still in its infancy, and we are still inventing a shared language for describing Byzantine Fault Tolerant (BFT) at scale systems.

People commonly raise one or more security issues from a set of scenarios that on the face seem different, but underneath share a core property. Examples I’ll discuss in this post include:

  • It’s easy for me as a miner to dedicate all my hash power to one chain, overwhelm / make all the blocks on a single chain, and thereby corrupt the network with censorship / bad blocks / etc.

  • It’s easy for me with control of one chain to censor that chain and thereby halt the network

Both of these scenarios start with the same primary assumption: that by dedicating hash power, an adversary can “take control” of a Chainweb chain, as if the chain were a voting node in a deterministic (e.g. PBFT) or non-POW (e.g. POS) Byzantine Fault Tolerant system.

These scenarios attempt to view Chainweb through the lens of a staked sharding system, in which there is some game theoretic or information theoretic model under which you trust ledger states that you didn’t generate. However, these issues do not apply to Chainweb, and arise from a fundamental misunderstanding in how Chainweb functions, and I’ll address and provide additional clarity on these attacks below.

Proof-of-Work in a culture of Proof-of-Stake

It’s important to not forget that when Bitcoin was first proposed, PoW was still unproven. Once economics are injected into a consensus algorithm setting, it’s very difficult to show that a formal proof of the theoretical behavior of an algorithm will apply when it is set loose upon the real world. Game theory is our best tool for modeling human behavior at the required granularity and yet it makes assumptions about the rationality of people’s behavior. Only after Bitcoin was released into the wild and behaved as expected for years did many begin to believe that PoW-based trustless consensus actually works. It has proven to truly be a master stroke of design.

The work in Proof of Work, i.e. exchanging energy for hashes, has been proven by the test of time to be a valid primitive for securing the consensus protocol of a network. We have no evidence so far that staking, or securing a network using its own internal currency, will be a valid alternative to underpin a consensus protocol, and the field of trustless consensus is still too new to know how to test the strengths of its foundations at this time. This isn’t to say that staking cannot function as a consensus primitive, just that we don’t yet know if it is a viable alternative to work.

Chainweb is an evolution of Nakamoto Consensus

Much of discussion around scaling has been shaped by Jae Kwon, Dominic Williams, and Vlad Zamfir’s work to apply traditional academic BFT research to blockchains. Chainweb is explicitly not part of that paradigm; Chainweb is unique in its status as a purely-POW-based scaling solution.

People quite commonly misinterpret the way our consensus mechanism functions, generally when they are using a perspective based on PoS, but these systems use fundamentally different models for replication of a ledger. To properly consider replication in Chainweb, we must emphasize a core principle of the network:

Chainweb is analogous to a braided Bitcoin. Miners mine the network, which in Chainweb means mining multiple chains simultaneously. When we say nodes, we mean parallel chains, NOT validator nodes in a voting system. We use the word “chain” for a reason: each node in the Chainweb base graph is its own complete blockchain. Under this architecture, miners have an opportunity to mine blocks on all chains all at the same time.

Miners are free to select which chains they mine and are not sorted via allotment (e.g. sortition in PoS) to specific chains. Moreover, they have an incentive to efficiently allocate their hash power to different chains on the braid in ad hoc manner so that they maximize their chances of finding as many blocks as possible. The biggest difference between Chainweb and sortation-based solutions is that in a sortation-based solution only the members of the committee have an incentive to monitor and validate a shard. In Chainweb, there might be a profitable mining opportunity on any chain at any time, so all miners have incentives to watch and validate all chains.

Sharding in PoS is fundamentally different from the partitioning of ledger state seen in Chainweb (e.g. peer/parallel chains). In PoS, sharding consensus participants are not watching (replicating and validating) every shard looking for opportunities to create/validate the next block. In contrast, Chainweb has an incentive for every consensus participant to watch for opportunities everywhere across the network. This structure is how Chainweb allows for local (single chain) transactions that fall under a global consensus umbrella.

We repeat in many materials, talks, and posts that we expect most miners for Chainweb will mine the entire Chainweb network in large pools because of the economies of scale. Large mining pools will validate headers and peer blocks internally (i.e. cross-check and execute previous/new blocks) and mine (i.e. create new “latest” blocks) for every chain in Chainweb. The benefits of scale for electricity, hardware, and direct trusted message propagation in pools are too critical to ignore in an attempt to design a world in which all mining is performed by single-GPU miners. Mining a network should be a profitable business in which a miner will invest significant resources–this participation aligns the interests of miners, users, and token holders. Invested miners will want a correctly-aligned network to succeed as designed.

In Chainweb, consensus infrastructure (servers, fiber, memory pools, etc.) is parallelized without sacrificing security or decentralized block production in the pursuit of throughput. Ultimately, this leaves network bandwidth (required for global replication) the resource that becomes the most costly for scale. As such, the maximum size of a Chainweb network looks to be capable of 10K-100K transactions per second (TPS), though the limit of the network is still theoretical. This throughput is still a massive improvement over existing technology. Second layer scaling solutions on Chainweb will provide additional TPS, because any second-layer scaling approach can be applied to any base layer protocol. With Chainweb, however, the base layer will have enough bandwidth to ensure low fees for moving between layer-two channels and the base layer.

A single mining pool will always be able to mine the entire Chainweb network. Even in the most extreme case, e.g. a 100K TPS configuration, anyone would be able to replicate the entire network so long as they are willing to pay for the servers and bandwidth required, and for large mining conglomerates this infrastructure requirement will still be profitable. Chainweb provides a practical roadmap for solving scaling through the use of infrastructure.

Mining a single new block in Bitcoin today is very expensive in terms of hash power. Chainweb provides a mechanism to produce many simultaneous blocks on different peer chains all at the same height, with each block requiring a fraction of the hash power of a single Bitcoin block. This configuration drastically increases the number of transactions per second over the total network.

At its core, Chainweb is a spiritual successor to Bitcoin in its consensus mechanism. Miners behaving in a selfish, opportunistic, and greedy fashion will mine the next block for every chain, all the time, and will thereby safely propagate the network.

Dominate Hash Power on One Chain to Submit Bad Blocks

When we approach Chainweb with the lens that the majority of miners will want to mine all the chains in Chainweb, we can begin to see how the network will react to attacks. One common issue raised about Chainweb is the idea that an adversary could dominate the hash power on one chain and thereby insert bad blocks into the system.

This scenario hinges on the false assumption that any miners who receive a proof of a finished block will include that proof in the header of their peer chain’s next block without checking its validity. On the contrary, most of the hash power comes from miners mining the entirety of the network, and therefore miners will be performing spot validation on proofs from peer chains. Miners will not accept a bad block from a supposedly dominated chain because they will validate the block before including its Merkle root in their peer chains.

In the false scenario, a miner will:

  1. Start mining on Chain “α

  2. Go to the network for α’s peers’ blocks which are 1 layer back

  3. Not validate those blocks

  4. Find a block on α, broadcast it, and thus implicitly add validity to any forged blocks that were in that set the node didn’t validate

But, this doesn’t make sense, because Step 2 in the false scenario doesn’t actually take place. Rather, a miner will:

  1. Start mining on α

  2. Go to the node’s own internal state for α’s peers blocks which are 1 layer back. Because these are blocks that the node has “accepted” for the chain that the block is on, the node has already validated the block. There’s no need to do another re-validation of the neighbors.

  3. Find a block on α and broadcast it.

Halt Communication from One Chain

This scenario is one in which people assume that dominating all the hash power on one chain will allow an adversary to censor that chain, which would eventually halt the network.

Recall that Chainweb is essentially a multiplied Bitcoin. If we think of mining from this perspective, censoring a Chainweb chain would be like trying to censor Bitcoin. You can dedicate all your hash power to one chain and create blocks faster than other miners, but you cannot keep other miners from attempting to mine a Chainweb chain. In fact, a slow chain is attractive to miners, and resources will be reallocated to any temporarily slow chain. If an attacker mines a Chainweb chain in secret but doesn’t tell anyone, then the rest of the network will mine that chain without them. The block production of any one chain in Chainweb cannot be slowed or halted.

Conclusions and Follow Up

It is absolutely accurate to say that the full security analysis of Chainweb has proven to be more complicated than a traditional single-chain approach. The difficulty in describing the interwoven dependence of the probabilities is why we have moved from a closed-form analysis to a simulator-based approach. Thus, we’ve been working with Tarun Chitra’s research consultancy Gauntlet to simulate multiple different attacks.

We definitely have a lot more work to do on formally proving the defense to various attack vectors, which is why we are running simulations of Chainweb and have an updated security paper in progress. Considerations and critiques of the network are always welcome, which is why we’re so transparent and open about how the system will function. Rigorous work arises from open collaboration, and it will continue to guide our progress as our development continues.