Subject: Re: [bitcoindev] OP_CHECKUTXOSETHASH idea

From: "'moonsettler' via Bitcoin Development Mailing List" <[email-masked]>
Date: Thu, 02 Oct 2025 22:40:00 +0000

Hi Erik,

Since it is costly for nodes to compute, this is a bit of a DOS vector. I would suggest to limit UTXO set commitments to every 2016 blocks (either the first or the last block of a difficult adjustment epoch).

If it checks the UTXO set commitment of a previous block, it will not interfere with mining, for example always commit to the initial state of the difficult adjustment epoch at the end of the epoch. The hash can be calculated well in advance. It also would be the same in every check, so it's not possible to use it for denial of service.

It's a bit interesting that the script can not be fully validated before it hits an actual block. It also allows for submitting a transaction into the mempool that might be invalid to mine, that needs additional steps for eviction. Wonder if this allows for weird new pinning attacks for free?

Overall I have low confidence that this belongs in script instead of the coinbase transaction structure via a more specific soft fork.

BR,
moonsettler

On Tuesday, September 30th, 2025 at 2:11 AM, Erik Aronesty <[email-masked]> wrote:

> A soft fork could introduce a new opcode, `OP_CHECKUTXOSETHASH`, allowing miners to optionally commit a deterministic hash of the current UTXO set into a block. If present, all nodes must verify its correctness or reject the block; if absent, the block is still valid. Old nodes treat the opcode as unspendable, so backward compatibility is preserved.
> Because computing the full UTXO root is costly, this makes each checkpoint intentionally expensive to produce, ensuring that miners will only include them when compensated with sufficient fees. Additionally, it could be limited to one per block.
> 
> The result is a voluntary, self-limiting, incentive-aligned, fee-driven system where checkpoints are cheaply consensus-enforced when included but never mandatory.
> 
> Most nodes could operate on a rolling history validated by occasional, high-value commitments, while archival nodes remain free to preserve the full chain. This reduces the burden of initial sync and resource use without sacrificing Bitcoin’s security model, since any invalid checkpoint would invalidate its block.
> 
> In practice, the chain becomes more efficient for everyday use while the historical record remains intact for those willing to bear the expense of maintaining it.
> 
> 
> 
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