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Aegis: Tethering a Blockchain with Primary-Chain Stake

Yogev Bar-On, Roi Bar-Zur, Omer Ben-Porat, Nimrod Cohen, Ittay Eyal, Matan Sitbon

TL;DR

Aegis proposes a tethered blockchain that derives security from primary-chain stake by using primary-block–defined committees, checkpoints, and resets to manage epoch transitions. The paper proves an impossibility result: with partial synchrony and only delayed, bounded access to the primary ledger the tethered chain cannot be secure; synchronous primary-chain access is necessary for safety and progress. It then details Aegis, combining a primary-ledger smart contract for checkpoints and resets with a P2P BFT-style consensus among active committees that reference primary blocks to extend the tethered chain; forensics and slashing deter misbehavior. The approach enables fast, secure progress for tethered chains (e.g., rollups oracles, randomness sources) while maintaining independence from the primary chain beyond stake-based security, offering a practical path to decentralized scalability and interoperability in blockchain ecosystems.

Abstract

Blockchains implement decentralized monetary systems and applications. Recent advancements enable what we call tethering a blockchain to a primary blockchain, securing the tethered chain by nodes that post primary-chain tokens as collateral. The collateral ensures nodes behave as intended, until they withdraw it. Unlike a Proof of Stake blockchain which uses its own token as collateral, using primary-chain tokens shields the tethered chain from the volatility of its own token. State-of-the-art tethered blockchains either rely on centralization, or make extreme assumptions: that all communication is synchronous, that operators remain correct even post-withdrawal, or that withdrawals can be indefinitely delayed by tethered-chain failures. We prove that with partial synchrony, there is no solution to the problem. However, under the standard assumptions that communication with the primary chain is synchronous and communication among the tethered chain nodes is partially synchronous, there is a solution. We present a tethered-chain protocol called Aegis. Aegis uses references from its blocks to primary blocks to define committees, checkpoints on the primary chain to perpetuate decisions, and resets to establish new committees when previous ones become obsolete. It ensures safety at all times and rapid progress when latency among Aegis nodes is low.

Aegis: Tethering a Blockchain with Primary-Chain Stake

TL;DR

Aegis proposes a tethered blockchain that derives security from primary-chain stake by using primary-block–defined committees, checkpoints, and resets to manage epoch transitions. The paper proves an impossibility result: with partial synchrony and only delayed, bounded access to the primary ledger the tethered chain cannot be secure; synchronous primary-chain access is necessary for safety and progress. It then details Aegis, combining a primary-ledger smart contract for checkpoints and resets with a P2P BFT-style consensus among active committees that reference primary blocks to extend the tethered chain; forensics and slashing deter misbehavior. The approach enables fast, secure progress for tethered chains (e.g., rollups oracles, randomness sources) while maintaining independence from the primary chain beyond stake-based security, offering a practical path to decentralized scalability and interoperability in blockchain ecosystems.

Abstract

Blockchains implement decentralized monetary systems and applications. Recent advancements enable what we call tethering a blockchain to a primary blockchain, securing the tethered chain by nodes that post primary-chain tokens as collateral. The collateral ensures nodes behave as intended, until they withdraw it. Unlike a Proof of Stake blockchain which uses its own token as collateral, using primary-chain tokens shields the tethered chain from the volatility of its own token. State-of-the-art tethered blockchains either rely on centralization, or make extreme assumptions: that all communication is synchronous, that operators remain correct even post-withdrawal, or that withdrawals can be indefinitely delayed by tethered-chain failures. We prove that with partial synchrony, there is no solution to the problem. However, under the standard assumptions that communication with the primary chain is synchronous and communication among the tethered chain nodes is partially synchronous, there is a solution. We present a tethered-chain protocol called Aegis. Aegis uses references from its blocks to primary blocks to define committees, checkpoints on the primary chain to perpetuate decisions, and resets to establish new committees when previous ones become obsolete. It ensures safety at all times and rapid progress when latency among Aegis nodes is low.
Paper Structure (22 sections, 2 theorems, 3 figures, 1 algorithm)

This paper contains 22 sections, 2 theorems, 3 figures, 1 algorithm.

Table of Contents

  1. Introduction
  2. Related Work
  3. Classical Reconfiguration.
  4. Tethered Chains.
  5. Long-Range Attacks and Checkpoints.
  6. Layer-2 Solutions.
  7. Chain Combining.
  8. Model
  9. Principals and Network
  10. Consensus Algorithm
  11. Goal
  12. Bounded Primary-Chain Access is Necessary
  13. Naive Attempts
  14. Impossibility Proof
  15. Aegis
  16. ...and 7 more sections

Key Result

Lemma 1

Let $\mathcal{A} \xspace$ be a protocol, and let $\sigma$ be an execution of $\mathcal{A} \xspace$ where all nodes are correct. Let $t_k$ be the time when the first node decides on height $k$, and let $t_{k+1}$ be the time when the first node decides on height $k+1$. If $\mathcal{A} \xspace$ satisfi

Figures (3)

  • Figure 1: Aegis and primary-chain blocks.
  • Figure 2: Impossibility proof.
  • Figure 3: Aegis's toplogy and key mechanisms.

Theorems & Definitions (2)

  • Lemma 1
  • Theorem 2