# AggLayer and Pessimistic Proofs
With the recent update to AggLayer v0.2, a **pessimistic transaction verification mechanism** (Pessimistic Proofs) has been introduced, offering a different approach to solving trust issues between blockchains.
Most cross-chain solutions rely on **Optimistic Proofs**, assuming that participants act honestly — until proven otherwise. This is the foundation of **Optimistic Rollups**.
Instead, AggLayer adopts a **pessimistic model**, where every blockchain is considered potentially unreliable by default. Before confirming a cross-chain operation, the system verifies all input data. If a valid proof cannot be formed, the transaction fails.
**The primary goals of Pessimistic Proofs are to ensure:**
➡️ Every cross-chain transaction is backed by a real deposit.
➡️ No chain can withdraw more than it has deposited.
➡️ Even if one chain is compromised, others remain unaffected.
**The model is built around a Unified Bridge and state trees that track each transaction:**
1️⃣ **Local Exit Tree** — Each blockchain generates an exit tree recording all outgoing transactions.
2️⃣ **Global Exit Root** — A globally aggregated state stored on Ethereum, consolidating all local exit trees.
3️⃣ **Nullifier Tree** — A control system preventing duplicate withdrawal requests.
Before executing a cross-chain transfer, the **pessimistic proof** verifies:
✅ Does the network state match the declared one?
✅ Are there sufficient funds for the withdrawal?
✅ Is the balance integrity maintained within the global bridge?
If any condition fails, the atomic transaction does not proceed — effectively stopping bridge attacks before they can be executed.
**In practice, this update has led to several key improvements:**
🔹 **Expanded support for any blockchain.**
Previously, only **Polygon CDK-based** blockchains could interact with AggLayer. Now, even chains **without ZK-proofs** can connect, with security ensured via **Pessimistic Proofs**.
🔹 **Unified liquidity without wrapped tokens.**
Unlike most bridges that rely on wrapped versions of assets, **AggLayer v0.2** enables native token transfers across chains **without wrapping**, reducing risk and simplifying cross-chain operations.
🔹 **Flexible choice for blockchains.**
Blockchains using **Polygon CDK** can continue operating under the **legacy settlement** mechanism, while new networks gain an extra security layer with **Pessimistic Proofs**.
An important point: if a blockchain initially adopts **Pessimistic Proofs**, it can later transition to **ZK verification** if it meets the criteria. However, the reverse transition is **not possible**.
**Final Thoughts**
Overall, we view this update **positively**. There is an ongoing race to develop the **most effective cross-chain solution** capable of onboarding **the maximum number of blockchains**. Previously, the requirement to use only **ZK rollups** significantly limited **AggLayer**. This pivot might also be motivated by the goal of **integrating native Ethereum**. We believe **AggLayer is well-positioned** to compete in this space. Haust Network will be leveraging Pessimistic Proofs within the AggLayer framework to enhance the security and reliability of its cross-chain operations
*written by Haust Network CTO Andrew Nalichaev*
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