How Does Using Pessimistic Proofs Safeguard Aggregated Blockchain Solutions?

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Key Takeaways:

  • Check out the premises of aggregated blockchains pioneered by Polygon Technology and how they refine decentralized internet experiences.
  • Find out why existing mechanisms could hinder the effectiveness of such aggregated blockchain ecosystems.
  • Explore how pessimistic proofs could bolster security in such a complex blockchain system that involves numerous networks.

Since the concept of blockchain became a reality, advancements have been going in diverse directions over the years, leading to, at best, a fragmented ecosystem that is struggling to attract global attention. In fact, only 6.8% of the world’s population is estimated to hold cryptocurrencies, making us wonder why this piece of technology hasn’t seen widespread reach. However, the concept of aggregated blockchains from Polygon Technology opened the chances of unifying the split ecosystem. While the progress is slow and steady, some issues have become prominent, leading to the creation of pessimistic proofs, which we will explore in detail in this blog.

Rewinding the Concept of Aggregated Blockchains

Introduced in early 2024 by Polygon as a foundational aspect driving its AggLayer, aggregated blockchains aim to provide the best of everything for users—user experience, unified liquidity, scalability, and customizability. This technological solution comes as an alternative to traditional monolithic and modular blockchains, which compromise some of these facets to satisfy others.

  • Notably, Polygon’s AggLayer acts as a unified bridge between various layer-1 and layer-2 blockchains to make this concept a real possibility. Its developers stated that the problem that led to its development was the fact that people needed to traverse between multiple blockchains for simple transactions, which took a toll on user experience in Web3.
  • The current blockchain ecosystem is also fragmented so much that users find it challenging to conduct token transactions that often involve the usage of multiple native bridges. Such situations also open the doors for increased security concerns and reduced scalability, ultimately leading to degraded performance capabilities.
  • These aggregated blockchain solutions utilize zero-knowledge proofs (ZKPs) or optimistic proofs for proving transactions and registering them on the main blockchain. If a blockchain is compatible with the Ethereum Virtual Machine or has a derivative of the EVM, the unified bridge (i.e., AggLayer) can be used to link networks without any real hassle.

Why are Existing Mechanisms a Problem for Aggregated Blockchains?

As we saw in the previous section, aggregated blockchain solutions use zero-knowledge proofs and optimistic proofs to prove the existence of transactions on the main chain. But, as more number of blockchains become part of the unified bridging mechanism, a new problem arises, where each of these mechanisms tends to hinder the solution’s intended functionality.

  • In the case of optimistic proofs, it can take up to seven days for a transaction to fully become part of a blockchain. While this time gap sounds good within a small-scale system, bringing it into an aggregated blockchain solution would hamper the transaction process, weakening user experience as a result.
  • While the existing ZKP solutions seem good for aggregated blockchain solutions currently, the increase in the number of networks could usher in new problems. The possibility of reduced soundness of transactions spikes as the unified bridge has to interact with networks with diverse consensus mechanisms and provenance solutions.
  • Without sufficient backups for securing the unified bridge (or relying on the safety of participant blockchains), the chances of a malicious actor from one chain jeopardizing the functioning of the whole aggregated network increase. Such possibilities are concerning, given that they are not impossible despite taking a lot of technological effort.

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How Can Pessimistic Proofs Resolve the Issue?

As the name suggests, pessimistic proofs assume that all participants (or blockchains in an aggregated solution) are malicious. They use a unique approach to secure the unified bridging solution that can boost the entire network’s safety, enhancing user experience as a result. The main motto here is that a participant blockchain cannot withdraw more than it had deposited in its bridge contract.

  • Typically, the creation of pessimistic proofs involves checking three vital pieces of information that will determine the outcome: 
    • Updates are executed correctly on the participant blockchain
    • The participant network executed internal accounting correctly
    • All participant blockchains conduct all their accounting activities correctly
  • This way, the unified aggregation layer can check each participant network for trustworthiness. This can ensure there have been no attempts to withdraw extra from the bridge other than the deposited amount. If it fails in the check, a network can only be a threat to itself and not the bridge or the others, effectively saving the aggregation from a digital apocalypse.
  • Technically speaking, the unified bridge requires three inputs to generate a pessimistic proof:
    • The participant blockchain’s local exit tree (indicating withdrawals) up to the previous iteration
    • The list of new withdrawals to be included in the current update
    • The blockchain’s projected new local exit root
  • The mechanism calculates the network’s new exit root from the first two inputs and compares it with the third input. If they match, pessimistic proof is generated, guaranteeing the local exit root is updated correctly. 
  • The mechanism also calculates the crypto token balances of participant blockchains before proceeding with a new global exit root. If the withdrawal balance is more than the deposited balance, the update becomes invalid, and its state cannot be verified on the layer-1 network.  


Thus, we have explored the pioneering concept of pessimistic proofs and how they function to enhance the safety of aggregated blockchain solutions. With more layer-2 networks building up on Ethereum, the need for such provenance mechanisms can bolster the security of all participant networks when using a unified bridging solution. While Polygon Technology’s developers have built the mechanism to support its AggLayer, the concept is all set to capture blockchains worldwide, given its revolutionary appeal. While this solution might not be directly applicable to Web3 dApp projects, those working on their native blockchain infrastructure (especially those building an L2 on Ethereum) could tap into the mechanism to become part of a global network. Connect with our experts now to explore the possibilities and implement pessimistic proofs in your L2 solution with Polygon’s solid infrastructure.


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