From: Matt Corallo <lf-lists@mattcorallo.com>
To: Sjors Provoost <sjors@sprovoost.nl>,
Antoine Riard <antoine.riard@gmail.com>
Cc: Bitcoin Development Mailing List <bitcoindev@googlegroups.com>,
Antoine Poinsot <darosior@protonmail.com>
Subject: Re: [bitcoindev] Addressing remaining points on BIP 54
Date: Thu, 8 Jan 2026 11:36:50 -0500 [thread overview]
Message-ID: <33ffd6c4-6395-4f6c-a6e8-8b43220cdb00@mattcorallo.com> (raw)
In-Reply-To: <9C946151-D6DD-4CB7-B524-15FD9F625D9D@sprovoost.nl>
On 1/8/26 3:30 AM, Sjors Provoost wrote:
> Hello Riard,
>
>> Thanks for the update. If I'm understanding correctly Luke's concern,
>> currently the coinbase's scriptSig is used to store an extranonce. One
>> has to observe first there is no consensus limit on the size of a
>> transaction, which holds for the coinbase tx too, a fortiori there is
>> no limit on the extranonce size a miner could fit in the scriptSig.
>
>
> The coinbase scriptSig is limited to 100 bytes [0]. Some speculation as to
> why [1].
>
> The main issue I see is complexity of implementation. The nLockTime is always
> the last 4 bytes of a transaction, so an ASIC can roll it without having to
> understand anything about serialisation.
Assuming some future change to stratum v1/v2 to allow for this (which I think is basically a "never
going to happen"), its worth noting that you can't just roll it for free. Its already the case that
nLockTime has consensus meaning on the coinbase transaction - its enforced like any other block. So
there's relatively little rolling you can do until you get to the current block height and have to
go do something else (I imagine this is why its not been used for this purpose in the past, at least
in part). So the ASIC actually has to understand quite a bit to roll this.
Instead, in practice, ASICs (or their controllers) roll nTime, which is even better cause its in the
header and you know you can ~always roll it once a second. Then rolling a nonce in the coinbase is
easy cause you can just do it in the controller and get plenty of headroom on the ASIC itself with
nTime and a few midstates.
> The scriptSig OTOH is variable length, so it needs to read the length byte in
> order to figure out which 4 bytes are at the end. The pool or proxy then also
> needs to ensure those 4 bytes are pre-initialised*.
>
> The approach suggested by Towns [4] of appending a 0-sat OP_RETURN output with
> padding so a 4-byte nonce lands in the final 64-byte SHA256 chunk is probably
> better, but not because like nLockTime it has a small hashing midstate
> benefit. It's easier to implement.
>
> Compared to varying the end of the scriptSig, this can be easier for an ASIC
> because it can update a fixed 4-byte field at a known offset from the end,
> rather than having to parse variable-length fields (notably the scriptSig
> length) to locate the bytes to roll.
>
> I think that extra complexity is doable and justifiable, but I've never built an ASIC.
>
> Note that today Stratum v1 simply splits the scriptSig [5] into two parts, as does
> Stratum v2 [3], but presumably that's all done by the control board and it makes
> sense to want to push rolling functionally into the ASIC silicon, where even
> simple concatenation might be too involved - but updating bytes at known
> positions is easy.
>
>> The point being made is that the nLocktime field of the coinbase
>> transaction could be used as a more efficient extra nonce due to
>> the positional location of nLocktime in a serialized coinbase being
>> one of the latest message block to be processed [0].
>>
>> Nothing prevent a miner in already doing this and draw a speed advantage
>> from the diminished computational work. I have not looked into CGminer code
>> or one of its derivative forks, if there is an implemented option to do that,
>> but yes there could be non-published existing mining firmware doing it. IIUC,
>> BIP54 would nullify this theoretical "speed advantage" for all miners.
>
> I don't think there's currently a speed advantage, so I wouldn't expect to observe
> this behaviour in the wild just yet. The combination of rolling nVersion
> (BIP310) [2] and updating nTime every second, works fine up to 280 TH/s.
>
> Beyond that an ASIC will need to touch the coinbase.
>
> - Sjors
>
> [0] https://github.com/bitcoin/bitcoin/blob/v30.1/src/consensus/tx_check.cpp#L47-L51
> [1] https://bitcoin.stackexchange.com/questions/35455/why-bother-having-limitations-on-bitcoin-coinbase-transaction-scriptsigs
> [2] https://github.com/bitcoin/bips/blob/master/bip-0310.mediawiki
> [3] https://github.com/stratum-mining/sv2-spec/blob/main/05-Mining-Protocol.md#511-standard-job
> [4] https://delvingbitcoin.org/t/great-consensus-cleanup-revival/710/88?u=sjors
> [5] https://en.bitcoin.it/wiki/Stratum_mining_protocol#mining.notify
>
> * = otherwise the ASIC needs to know how to extend it, know that it can't be
> more than 100 bytes, and that it can't touch the BIP34 part, or really any
> subsequent bytes that a future soft fork might constrain
>
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next prev parent reply other threads:[~2026-01-08 18:19 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-12-30 15:59 'Antoine Poinsot' via Bitcoin Development Mailing List
2026-01-08 4:29 ` [bitcoindev] " Antoine Riard
2026-01-08 8:30 ` [bitcoindev] " Sjors Provoost
2026-01-08 16:36 ` Matt Corallo [this message]
2026-01-13 2:16 ` Antoine Riard
2026-01-13 16:59 ` Mubarek Juhar
2026-01-08 16:40 ` Matt Corallo
2026-01-13 1:49 ` Antoine Riard
2026-01-14 0:23 ` Murch
2026-01-14 10:15 ` Sjors Provoost
2026-01-14 15:33 ` 'Antoine Poinsot' via Bitcoin Development Mailing List
2026-01-14 18:58 ` Murch
2026-01-30 4:08 ` Antoine Riard
2026-02-05 22:48 ` 'Antoine Poinsot' via Bitcoin Development Mailing List
2026-02-12 3:57 ` Antoine Riard
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