Cointime

Download App
iOS & Android

Release Blockhash Opcode Restrictions with zk-SNARKs for Optimistic Bridges

Repost from Ethresearch by tomo_tagami : “Release Blockhash Opcode Restrictions with zk-SNARKs for Optimistic Bridges” The full report and all related findings are available on the official website of Ethresearch.

This post is a proposed solution to release the restrictions that exist in Ethereum opcode by using zk-SNARKs. Furthermore, I hope to use this idea to solve the challenges of the optimistic bridges.

TLDR

  • Ethereum has a blockhash opcode restriction commonly known as the “256 block problem”.
  • This restriction means that the dispute period of the optimistic bridge is only about 51 minutes.
  • To increase the dispute period, use a client that proves the blockhash hash-chain by using zk-SNARKs.
  • Create a circuit of hashes leading up to the targeted blockhash by accessing backward from the latest opcode

Review of the optimistic bridge

If you already know about the optimistic bridge, please skip this part.

Optimistic bridges rather than requiring Ethereum to validate every bridge transaction, the protocol inspects only those that might be fraudulent. Suspicious activity can lead to a dispute, triggering a verification process settled by Layer 1.

Optimistic bridge consists of three participants - User, Relayer and Disputer. Relayer’s actions are constantly monitored by disupters and thrashed if fraud is discovered. See this post for more details.

Blockhash opcode

Ethereum has a blockhash opcode restriction commonly known as the “256 block problem”. The maximum number of blocks that can be referenced by Ethereum’s blockhash opcode is 256 blocks, and older blocks cannot be referenced.

This restriction means that at optimistic bridge, the time available for transaction verification by blockhash is only about 51 minutes (= 12 sec x 256 blocks). Consequently, the window for raising disputes is limited to approximately 51 minutes (12-second block time *256 blocks). In practical terms, this means that if disputers wish to challenge a relayer’s action, they have only about 51 minutes to do so, as the dispute process requires them to present the block hash of the contested transaction block as evidence. This tight timeframe could compromise system security and diminish the overall user experience.

Potential solution using zk-SNARKs

To address this restriction, I propose a solution leveraging zk-SNARKs.

Every block hash inherently embeds information from its predecessor. Given this property, it’s possible to create a cryptographic circuit that verifies the chain of block hashes, even beyond the most recent 256. This is done by anchoring the sequence with a block hash currently accessible via the blockhash opcode and tracing back to an older target hash.

In technical terms, if n1 represents the block number with the opcode-accessible block hash and n2 represents the block number of our target hash, the circuit would be designed to take three primary inputs:

  • Block hash from block n1.
  • All block headers between n1 and n2.
  • Target block hash from block n2.

These operations will utilize RLP encoding and the Keccak256 hash function. This circuit proves that there is a hash connection up to the target block hash.

The following image provides a visual representation of this chain of hashes. However, in practice, the circuit computes this in reverse order. While this graphic specifically illustrates a block hash chain on Ethereum, the principle could also extend to Layer 2 networks.

circuit1721×1080 137 KB

Although this approach is in the research phase and awaits full implementation—primarily because of its intricate nature—it seems almost inevitable that such zk circuits will find integration within the bridge dispute mechanism. This is especially true considering some Layer 2 networks impose even tighter restrictions on block hash access than the 256-block restriction.

Comments

All Comments

Recommended for you

  • Spot gold continues to fall

    spot gold continues to decline, with the decline expanding to 2%, at $3315.49 per ounce.

  • BTC breaks through $93,500

    the market shows BTC has broken through $93,500, now trading at $93,506.58, with a 24-hour increase of 6.12%. The market is fluctuating greatly, please manage your risks.

  • U.S. strategic Bitcoin reserves may announce details in the coming weeks

    Trump signed an executive order in early March this year proposing to establish a national strategic reserve of Bitcoin and other tokens, and requested the Treasury Secretary to submit an evaluation report on the legality and feasibility of the plan within 60 days. With less than two weeks remaining until the 60-day deadline set by Trump's executive order, this means that more details about the US Bitcoin reserve will soon be disclosed. Market expectations for this may be one of the important catalysts for the recent rebound in cryptocurrency. In addition, any comments questioning the independence of the Federal Reserve have also had a positive spillover effect on Bitcoin.

  • Bitcoin's market share once rose to 64.67%, but now fell back to 64.30%

    On April 23rd, data, the Bitcoin dominance (BTC.D) briefly rose to 64.67% this morning, reaching a new high since February 2021, and is currently back at 64.30%. The high Bitcoin dominance indicates the quietness of the altcoin market, but it may also suggest that a bottom reversal is imminent. Based on historical data, when Bitcoin dominance surged above 60% in November last year, altcoins started a small bull market. In 2019 and 2021, Bitcoin dominance reached highs above 70%, followed by a broad and spectacular uptrend.

  • Cointime精选 ·

    Are we finally ready for a gas limit increase?

    There has been growing discussion around the possibility of increasing Ethereum’s gas throughput, either by raising the gas limit or reducing slot time. The key argument in favor of this is that the hardware requirements for running a validator have steadily decreased over the past four years.

  • Cointime August 17th News Express

    1.VanEck and 21Shares Solana ETF Form 19b-4 Suspected to be Removed from CBOE Website

  • Ethereum network gas fee falls back below 1 gwei

    According to Etherscan data, the current Ethereum network gas fee has fallen below 1 gwei, currently at 0.937 gwei.

  • Cointime August 10th News Express

    1. The U.S. Internal Revenue Service has released a new draft of the crypto tax form, which no longer requires filling in wallet addresses and transaction IDs

  • Ethereum ACDC #139: Pectra's Devnet 2 upgrade is under debugging, and the release date of Devnet 3 is still to be determined

    Christine Kim, Vice President of Galaxy Research, summarized the main content of the 139th ACDC conference call. The debugging of Pectra's upgraded Devnet 2 is currently underway, and the release date of Devnet 3 is yet to be determined. Developers will hold weekly testing update meetings starting from Monday to better coordinate the release of Pectra's Devnet. The decision to include EIP-7688 in Pectra's upgrade has been postponed again.

  • Ethereum network gas fee drops to 1 gwei

    According to Ether­scan data, the current gas fee on the Ethereum network has dropped to 1 gwei.