From mboxrd@z Thu Jan 1 00:00:00 1970 Delivery-date: Thu, 19 Feb 2026 15:36:15 -0800 Received: from mail-oa1-f58.google.com ([209.85.160.58]) by mail.fairlystable.org with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (Exim 4.94.2) (envelope-from ) id 1vtDZ8-0003yp-Pu for bitcoindev@gnusha.org; Thu, 19 Feb 2026 15:36:15 -0800 Received: by mail-oa1-f58.google.com with SMTP id 586e51a60fabf-4094b3db1dcsf2172643fac.1 for ; Thu, 19 Feb 2026 15:36:14 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=googlegroups.com; s=20230601; t=1771544168; x=1772148968; darn=gnusha.org; h=list-unsubscribe:list-subscribe:list-archive:list-help:list-post :list-id:mailing-list:precedence:x-original-sender:mime-version :subject:references:in-reply-to:message-id:to:from:date:sender:from :to:cc:subject:date:message-id:reply-to; bh=LSHo+j+f1vq6aLRhOUv0kypX8ZwGU5MuLUCtr9+z+aE=; b=DahARb2trNcEdAaAWhlmW7vnwXHsmhneumSk1ckwn7uoSXMAycmctnxnUQi8a0aqFE djXIceiHkOlQMAcPRVGd0zmsV7fd7txkNKDmXrELfp63sX0Bp/X3qVLDuLkJffxWccwR 4rxJltDzLlSGdaZwouEcY2HecMLcoLeVmDd1ZzgeVmVpQCeroPbr/2DYnpqMh1iDyGiA rHnSIzzP6rZHoRFUCVdG0zYJKepz+Eln1xB9z4YVIPY194p+oAQ9/pMEwuKmHyBhk9mb tN8aUd34MdzHKBEBMYggHlv4/CqWlv2gWb+5ju9dneBMeLi8/uWdiUmx3145yuAhrA6v m3sw== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20230601; t=1771544168; x=1772148968; darn=gnusha.org; h=list-unsubscribe:list-subscribe:list-archive:list-help:list-post :list-id:mailing-list:precedence:x-original-sender:mime-version :subject:references:in-reply-to:message-id:to:from:date:from:to:cc :subject:date:message-id:reply-to; bh=LSHo+j+f1vq6aLRhOUv0kypX8ZwGU5MuLUCtr9+z+aE=; b=fsApOcMI855GZZx4ABxFzTJ8zsDI1Ag9vhTRxnp3xcsU9HzQLYlS3RRSOw+Dowp2sT JucZWaQ2HiHWu/NveDH77AKnhLwxubxISf1vddr8xxNXPsLQK97maGk4RPQ/vlJQeSVZ 1fj+V5oyqLo7aPyLLy9dFs5Sp8/D0ecaEWOEX1Nyz0RLvAvNZ6TQ6CLs2nSMaDddnl/Q itqh9UCUr9/xyA02DVJ3omQWD9A8craKVo4VYeY4mmiQPdrokn35ewsv/QdY3FhGhd02 1IXx4kyROeZz5k78dwX9jl0IbQjX4DXF+S1Km1IVnweIlyrh3RA0sLel3qZT1/jIyzOQ 8m8A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1771544168; x=1772148968; h=list-unsubscribe:list-subscribe:list-archive:list-help:list-post :list-id:mailing-list:precedence:x-original-sender:mime-version :subject:references:in-reply-to:message-id:to:from:date:x-beenthere :x-gm-message-state:sender:from:to:cc:subject:date:message-id :reply-to; bh=LSHo+j+f1vq6aLRhOUv0kypX8ZwGU5MuLUCtr9+z+aE=; b=VXqnabgqD8TgzCmHf8AFb3qYrekprrnnCCU9ZqEvWEFQ3bAhgwhddPsNGptrj72zzW HGxxNqSuEjpsvbv+P1Z+QgQ/n2w+pqXGAiLRKyhK9wx+YxFct9RDm9wavr/YTDTZd1kH sB09sXxJJ1IV5i4HmJ2xbrhHC99qQB4EDpJExbVJvml7jB3QrYdJBe89OfUAoH/DiRND mun3SibdGb/Dcdsvf4BXVmvPyzZltnXiMDbZBwISgHCz8rnUVBbwRQrUfemIgsMNe94y az0/mOeM0xu2/I/kLzgE5P0yUYsY7otVPycTHXV8hZJXhNQzxmeahrOBN7gVJKIET7m+ H0Lg== Sender: bitcoindev@googlegroups.com X-Forwarded-Encrypted: i=1; AJvYcCUB5+h5Nl7Pzf98jOZDDVJ+zusy9w9KGuYsx1PQ84nrhlH6TdPq73jr190F+t3ckLFjTFuH/v2VKP55@gnusha.org X-Gm-Message-State: AOJu0YztM6u+gk2qbewIq8UttSKyVIatc2/7PE9NcGao/2/MxZ7RBDG6 CZaSJF2r0OKZSSh28fqOkK1rg3pk+bOz6yLsTbmOoJ9b44IXZOsl2k+h X-Received: by 2002:a05:6871:2b13:b0:40e:a6e7:5319 with SMTP id 586e51a60fabf-40eeec9f09bmr10708208fac.8.1771544168065; Thu, 19 Feb 2026 15:36:08 -0800 (PST) X-BeenThere: bitcoindev@googlegroups.com; h="AV1CL+GU6UAZe4nOGo9Cb0goCuo8gm4T2HgE+tKikL62DmAbxw==" Received: by 2002:a05:6870:2f0e:b0:3e8:9f07:3b9 with SMTP id 586e51a60fabf-40ef2809067ls5078403fac.0.-pod-prod-03-us; Thu, 19 Feb 2026 15:36:03 -0800 (PST) X-Received: by 2002:a05:6808:2518:b0:45e:6697:f983 with SMTP id 5614622812f47-4639ab5f455mr11201334b6e.5.1771544163231; Thu, 19 Feb 2026 15:36:03 -0800 (PST) Received: by 2002:a05:690c:2891:b0:794:2788:2ae4 with SMTP id 00721157ae682-7979c669d0cms7b3; Thu, 19 Feb 2026 12:39:17 -0800 (PST) X-Received: by 2002:a05:690c:b:b0:794:ff4d:9201 with SMTP id 00721157ae682-7979e899694mr204797967b3.35.1771533556402; Thu, 19 Feb 2026 12:39:16 -0800 (PST) Date: Thu, 19 Feb 2026 12:39:15 -0800 (PST) From: Sebastian Falbesoner To: Bitcoin Development Mailing List Message-Id: In-Reply-To: References: Subject: [bitcoindev] Re: BIP-352: Limiting the number of per-group recipients (K_max) MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_44786_332659962.1771533555925" X-Original-Sender: sebastian.falbesoner@gmail.com Precedence: list Mailing-list: list bitcoindev@googlegroups.com; contact bitcoindev+owners@googlegroups.com List-ID: X-Google-Group-Id: 786775582512 List-Post: , List-Help: , List-Archive: , List-Unsubscribe: , X-Spam-Score: 2.6 (++) ------=_Part_44786_332659962.1771533555925 Content-Type: multipart/alternative; boundary="----=_Part_44787_1751165140.1771533555925" ------=_Part_44787_1751165140.1771533555925 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable 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=20 opened a corresponding PR in the BIPs repository, where the discussion can= =20 be continued: https://github.com/bitcoin/bips/pull/2106 Best, Sebastian [1] https://github.com/bitcoin-core/secp256k1/issues/1799#issuecomment-3842= 046237=20 ff. On Wednesday, February 4, 2026 at 6:21:09=E2=80=AFPM UTC+1 Sebastian Falbes= oner=20 wrote: > Hi list,=20 > > In the course of working on a Silent Payments module for libsecp256k1 [1]= ,=20 > we=20 > discovered that the scanning approach suggested in BIP-352 [2] suffers fr= om=20 > very poor performance for adversarial transactions [3].=20 > > One more recent proposal to mitigate this issue is by introducing a "K_ma= x"=20 > protocol limit. This effectively limits the number of per-group recipient= s=20 > within a single transaction, i.e. the number of recipients sharing the sa= me=20 > scan public key. In theory this is a backwards incompatible protocol=20 > change,=20 > in practice we believe that none of the existing SP wallets would be=20 > affected,=20 > for a reasonably high K_max (the example value used is K_max=3D1000, but = this=20 > can be seen as a placeholder).=20 > > See the following BIP change draft for more details and motivation:=20 > > https://github.com/theStack/bips/commit/961d1442139ceecd6c0cc5775ef911d69= aabed4c=20 > > The discussion is on-going at the following issue:=20 > https://github.com/bitcoin-core/secp256k1/issues/1799 [4]=20 > > If you have any concerns or feedback for this change, either for currentl= y=20 > existing wallets or potential future use-cases that you could think of,= =20 > please=20 > comment there. Most SP wallet developers that we are aware of have alread= y=20 > been=20 > pinged on the issue. We are posting this here to reach a wider audience= =20 > and to=20 > provide an alternative opportunity to comment, in case anyone doesn't wan= t=20 > to=20 > use GitHub.=20 > > Best, > Sebastian > > [1] https://github.com/bitcoin-core/secp256k1/pull/1765=20 > [2]=20 > https://github.com/bitcoin/bips/blob/5d0f70a5cf4cfc429267cd6cc246ba3bcb94= 9cb3/bip-0352.mediawiki?plain=3D1#L330=20 > [3]=20 > https://github.com/bitcoin-core/secp256k1/pull/1698#pullrequestreview-334= 1766084=20 > [4]=20 > https://github.com/bitcoin-core/secp256k1/issues/1799#issuecomment-384204= 6237=20 > ff. in particular --=20 You received this message because you are subscribed to the Google Groups "= Bitcoin Development Mailing List" group. To unsubscribe from this group and stop receiving emails from it, send an e= mail to bitcoindev+unsubscribe@googlegroups.com. To view this discussion visit https://groups.google.com/d/msgid/bitcoindev/= ea67d9d0-3e3e-4906-9c47-495f8011468en%40googlegroups.com. ------=_Part_44787_1751165140.1771533555925 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Thanks for all the feedback left on the secp256k1 issue [1].

As no objections were raised to the proposed K_max protoc= ol change, I've opened a corresponding PR in the BIPs repository, where the= discussion can be continued:
https://github.com/bitcoin/bips/pul= l/2106

Best, Sebastian

[1]=C2=A0https://github.com/bitcoin-core/secp256k1/issues/1799#issuecomme= nt-3842046237 ff.
On Wednesday, February 4, 2026 at 6:21:09=E2=80=AFPM UTC+1 = Sebastian Falbesoner wrote:
Hi list, =20
=20
In the course of working on a Silent Payments module fo= r libsecp256k1 [1], we =20
discovered that the scanning approach suggested in BIP-= 352 [2] suffers from =20
very poor performance for adversarial transactions [3].= =20
=20
One more recent proposal to mitigate this issue is by i= ntroducing a "K_max" =20
protocol limit. This effectively limits the number of p= er-group recipients =20
within a single transaction, i.e. the number of recipie= nts sharing the same =20
scan public key. In theory this is a backwards incompat= ible protocol change, =20
in practice we believe that none of the existing SP wal= lets would be affected, =20
for a reasonably high K_max (the example value used is = K_max=3D1000, but this =20
can be seen as a placeholder). =20
=20
See the following BIP change draft for more details and= motivation: =20
https://github.com/theStack/bips/commit/961d1442139ceecd6c0cc5775ef9= 11d69aabed4c =20
=20
The discussion is on-going at the following issue: =20
https://github.com/bitcoin-core/secp256k1/issue= s/1799 [4] =20
=20
If you have any concerns or feedback for this change, e= ither for currently =20
existing wallets or potential future use-cases that you= could think of, please =20
comment there. Most SP wallet developers that we are aw= are of have already been =20
pinged on the issue. We are posting this here to reach = a wider audience and to =20
provide an alternative opportunity to comment, in case = anyone doesn't want to =20
use GitHub. =20
=20
Best,
Sebas= tian

[1] https://github.com/bitcoin-core/secp256k1/pull/1765 =20
[2] https://github.com/bitcoin/bips/blob/5d0f70a5cf4cfc429267cd6cc246ba3bcb949= cb3/bip-0352.mediawiki?plain=3D1#L330 =20
[3] https://github.com/bitcoin-core/secp256k1/pull/1698#pullreques= treview-3341766084 =20
[4] https://github.com/bitcoin-core/secp256k1/issues/1799#issuecomment-3= 842046237 ff. in particular

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