Committee Node Liveness Proof via Lightweight Proof of Work

nep-33.md

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+# NEP: Committee Node Liveness Proof via Lightweight Proof of Work
+- NEP: TBA
+- Title: Mechanism for Removing Inactive Committee Nodes
+- Author: Fernando Diaz Toledano <shargon@gmail.com>
+- Discussed in: https://github.com/neo-project/neo/pull/4304
+- Type: Standard (Core)
+- Status: Draft
+- Created: 2025-11-11
+
+## Summary
+
+This NEP proposes a new mechanism to verify the liveness of committee nodes through a **lightweight proof of work (PoW)** based on the hash of previous blocks.  
+The goal is to automatically detect and expel nodes that fail to demonstrate activity, ensuring a functional and reliable committee.
+
+## Motivation
+
+In the current protocol, committee nodes may stop participating without immediate detection, causing delays or loss of fault tolerance.  
+This NEP introduces a **cryptographic liveness proof**, verifiable by other committee members, allowing inactive nodes to be identified and replaced automatically.
+
+## Specification
+
+Each committee node must periodically prove it is **alive** by performing a **PoW** over the hash of a block `X` blocks in the past.
+
+### 1. Proof Base
+
+- The hash of block `BlockHeight - X` is used as the base.
+- Each node must find a nonce such that:
+
+  ```
+  SHA256(blockHash || nonce) < Target
+  ```
+
+  where `Target` defines the minimum difficulty threshold.
+
+- The result is included in a `LivenessProof` message broadcast to other committee members.
+
+### 2. Automatic Update for Primary
+
+- If the node is **primary**, the height used for computing the proof (`BlockHeight - X`) is automatically updated with every confirmed block.
+- Secondary nodes must update it upon receiving a new block or `LivenessRequest` message.
+
+### 3. Verification and Expulsion
+
+Each node validates received proofs. If a member does not submit a valid proof within the `LivenessWindow`, it is marked as **inactive** and automatically expelled from the committee.
+
+Expulsion occurs by consensus during the next committee round, without external intervention.
+
+## Rationale
+
+This approach replaces network-based heartbeat detection with verifiable cryptographic evidence.  
+The lightweight PoW ensures minimal computation cost while making falsification or replay infeasible.
+
+## Compatibility
+
+Backward compatible. The proposal introduces no new message types or block structures beyond internal committee validation.
+
+## Reference Implementation
+
+Based on the logic introduced in [neo-project/neo#4304](https://github.com/neo-project/neo/pull/4304), extended with PoW-based liveness checks.
+
+## Security Considerations
+
+The mechanism prevents false reporting and requires no external trust. Only nodes that actively participate and compute valid proofs remain in the committee.
+
+## Conclusion
+
+This NEP enhances NEO network reliability by enforcing continuous liveness verification through a lightweight proof of work, ensuring the committee remains composed solely of active nodes.