From: Sebastian Falbesoner <sebastian.falbesoner@gmail.com>
To: Bitcoin Development Mailing List <bitcoindev@googlegroups.com>
Subject: [bitcoindev] Re: BIP-352: Limiting the number of per-group recipients (K_max)
Date: Wed, 4 Mar 2026 01:45:01 -0800 (PST) [thread overview]
Message-ID: <4bd9b8e5-1233-4959-b34f-281b5e91dcc1n@googlegroups.com> (raw)
In-Reply-To: <ea67d9d0-3e3e-4906-9c47-495f8011468en@googlegroups.com>
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Update: the proposed BIP-352 protocol change [1] was merged, setting K_max
= 2323.
This limit was chosen intentionally, as it represents the maximum number of
taproot
outputs that can fit within a <= 100 kvB transaction with the smallest
Silent Payments
eligible input (a taproot key-path spend) [2].
Consequently, any transaction that adheres to the current transaction size
policy rule [3] is guaranteed to also comply with the K_max protocol limit.
It
is therefore extremely unlikely that any wallet would ever encounter this
limit
in practice, as such a transaction would not propagate on the network.
Thanks to everyone involved in proposing, discussing and reviewing this
change.
The secp256k1 silentpayments module PR [4] will be updated and taken out of
draft state shortly.
Best,
Sebastian
[1] https://github.com/bitcoin/bips/pull/2106
[2] a 1-in-2323-out P2TR transaction has a vsize of 99959 vbytes, see e.g.
https://jlopp.github.io/bitcoin-transaction-size-calculator/ or
https://bitcoinops.org/en/tools/calc-size/ (note that the latter calculator
doesn't account the two extra bytes needed for the output-count compactSize
encoding); adding one more taproot output (taking 43) vbytes would result
in 100002 vbytes
[3] `MAX_STANDARD_TX_WEIGHT` in Bitcoin Core (expressed in WUs):
https://github.com/bitcoin/bitcoin/blob/2702711c3a54b2ba9ae3781b61c90d28ee951de6/src/policy/policy.cpp#L110-L114
[4] https://github.com/bitcoin-core/secp256k1/pull/1765
On Friday, February 20, 2026 at 12:36:08 AM UTC+1 Sebastian Falbesoner
wrote:
> Thanks for all the feedback left on the secp256k1 issue [1].
>
> As no objections were raised to the proposed K_max protocol change, I've
> opened a corresponding PR in the BIPs repository, where the discussion can
> be continued:
> https://github.com/bitcoin/bips/pull/2106
>
> Best, Sebastian
>
> [1]
> https://github.com/bitcoin-core/secp256k1/issues/1799#issuecomment-3842046237
> ff.
> On Wednesday, February 4, 2026 at 6:21:09 PM UTC+1 Sebastian Falbesoner
> wrote:
>
>> Hi list,
>>
>> In the course of working on a Silent Payments module for libsecp256k1
>> [1], we
>> discovered that the scanning approach suggested in BIP-352 [2] suffers
>> from
>> very poor performance for adversarial transactions [3].
>>
>> One more recent proposal to mitigate this issue is by introducing a
>> "K_max"
>> protocol limit. This effectively limits the number of per-group recipients
>> within a single transaction, i.e. the number of recipients sharing the
>> same
>> scan public key. In theory this is a backwards incompatible protocol
>> change,
>> in practice we believe that none of the existing SP wallets would be
>> affected,
>> for a reasonably high K_max (the example value used is K_max=1000, but
>> this
>> can be seen as a placeholder).
>>
>> See the following BIP change draft for more details and motivation:
>>
>> https://github.com/theStack/bips/commit/961d1442139ceecd6c0cc5775ef911d69aabed4c
>>
>> The discussion is on-going at the following issue:
>> https://github.com/bitcoin-core/secp256k1/issues/1799 [4]
>>
>> If you have any concerns or feedback for this change, either for currently
>> existing wallets or potential future use-cases that you could think of,
>> please
>> comment there. Most SP wallet developers that we are aware of have
>> already been
>> pinged on the issue. We are posting this here to reach a wider audience
>> and to
>> provide an alternative opportunity to comment, in case anyone doesn't
>> want to
>> use GitHub.
>>
>> Best,
>> Sebastian
>>
>> [1] https://github.com/bitcoin-core/secp256k1/pull/1765
>> [2]
>> https://github.com/bitcoin/bips/blob/5d0f70a5cf4cfc429267cd6cc246ba3bcb949cb3/bip-0352.mediawiki?plain=1#L330
>> [3]
>> https://github.com/bitcoin-core/secp256k1/pull/1698#pullrequestreview-3341766084
>> [4]
>> https://github.com/bitcoin-core/secp256k1/issues/1799#issuecomment-3842046237
>> ff. in particular
>
>
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next prev parent reply other threads:[~2026-03-04 10:38 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-02-04 17:20 [bitcoindev] " Sebastian Falbesoner
2026-02-19 20:39 ` [bitcoindev] " Sebastian Falbesoner
2026-03-04 9:45 ` Sebastian Falbesoner [this message]
2026-04-01 0:56 ` Sebastian Falbesoner
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