From 9159b40629dd1b8a02b8983783c5c8e766c45133 Mon Sep 17 00:00:00 2001 From: nymius <155548262+nymius@users.noreply.github.com> Date: Mon, 19 Jan 2026 19:00:00 -0300 Subject: [PATCH 1/7] bip-draft: spending silent payment outputs with psbts --- ...lent-payments-outputs-with-psbts.mediawiki | 110 ++++++++++++++++++ 1 file changed, 110 insertions(+) create mode 100644 bip-spending-silent-payments-outputs-with-psbts.mediawiki diff --git a/bip-spending-silent-payments-outputs-with-psbts.mediawiki b/bip-spending-silent-payments-outputs-with-psbts.mediawiki new file mode 100644 index 0000000000..512d552dbe --- /dev/null +++ b/bip-spending-silent-payments-outputs-with-psbts.mediawiki @@ -0,0 +1,110 @@ +
+  BIP: ?
+  Layer: Applications
+  Title: Spending Silent Payment outputs with PSBTs
+  Authors: nymius 
+  Status: Draft
+  Type: Specification
+  Assigned: ?
+  License: BSD-2-Clause
+  Discussion: 2024-05-17: https://delvingbitcoin.org/t/bip352-psbt-support/877/30 [delving bitcoin post] Original discussion
+              2025-12-05: https://gist.github.com/nymius/b3dd0b8a08c6735d617e6216b73c4260 [gist] First draft
+              2025-12-15: https://gnusha.org/pi/bitcoindev/R53cG3TeXgXDUUS4kH_q226GlaFCjI0DZVT6mdTQzSQdj3RnNqWA-bFT7uGgGQFJG6938kDGvDJVoFQj8ItEMsJ6NyOjCTvpVEarYiyW6-8=@proton.me/ [bitcoin-dev] [BIP Proposal] Add PSBT_IN_SP_TWEAK field
+  Version: 0.1.0
+  Requires: 352, 375, 371
+
+ +== Introduction == + +=== Abstract === + +This document proposes an additional per input field for BIP 370 PSBTv2 that allows BIP 352 silent payment tweaks to be included in a PSBT of version 2. This field will be relevant to silent payment outputs spending. + +=== Motivation === + +BIPs 352 specify silent payments protocol, which provides a new way to create P2TR outputs and spend them. + +The existing PSBT fields are unable to support silent payments without changes, due to the new method by which outputs are created. + +BIP 375 and complementary BIP 374 specify how to create outputs locked with silent payment keys using PSBTs. But they don't specify how to unlock these outputs in a transaction.''' Why not including this new field in BIP 375?''' Historically, Silent Payments have been categorized by the perspective of the user of the protocol: receiver or sender. BIP 375 has followed this convention, and its stated on its title: Sending Silent Payments with PSBTs. Given that spending belongs to the sphere of the receiver, and considering this convention, this specification should be a different BIP. + +Therefore a new field must be defined to allow PSBTs to carry the information necessary for tweaking taproot keys without following the BIP 340 tagging scheme. + +== Specification == + +We use the following functions and conventions: + +* ser32(i): serializes a 32-bit unsigned integer ''i'' as a 4-byte sequence, most significant byte first. +* ser256(p): serializes the integer p as a 32-byte sequence, most significant byte first. +* serP(P): serializes the coordinate pair P = (x,y) as a byte sequence using SEC1's compressed form: (0x02 or 0x03) || ser256(x), where the header byte depends on the parity of the omitted Y coordinate. +* ''hashtag(x)'': refers to ''SHA256(SHA256(tag) || SHA256(tag) || x)''. + +=== Fields === + +{| class="wikitable" +! Name +! +! +! Description +! +! Description +! Versions Requiring Inclusion +! Versions Requiring Exclusion +! Versions Allowing Inclusion +|- +| Silent Payment Tweak +| PSBT_IN_SP_TWEAK = 0x1f +| None +| No key data +| <32-byte hash> +| A 32 byte raw tweak. Finalizers should remove this field after PSBT_IN_FINAL_SCRIPTWITNESS is constructed. +| +| 0 +| 2 +|} + +Per [https://github.com/bitcoin/bips/blob/master/bip-0352.mediawiki#spending BIP 352 spending] the <32-byte-hash> is ''hashBIP0352/SharedSecret(serP(ecdh_shared_secret) || ser32(k))'' + +or ''hashBIP0352/SharedSecret(serP(ecdh_shared_secret) || ser32(k)) + hashBIP0352/Label(ser256(bscan) || ser32(m)'', + +where ''hashBIP0352/Label(ser256(bscan) || ser32(m))'' is the optional label derived by some integer ''m''. + +== Rationale == + +On PSBTs, when spending non silent payment outputs, one can rely on the PSBT_IN_BIP32_DERIVATION or any of the allowed PSBT_IN_TAP_* combinations available to get the right private keys to sign for each input. + +To spend silent payment outputs you have to combine the private key with the tweak obtained from the transaction corpus. + +Passing the prevouts together with the PSBT to allow the computation of the tweaks, forces more computation on the signer side. + +Once a silent payment UTXO is scanned, is easier to store the output together with the tweak that generated it. + +To avoid the burden on the signer it would be better to pass this data into the PSBT together with the input spending the silent payment. Currently, there is no field prescribed for this. + +The PSBT_IN_BIP32_DERIVATION field cannot be used because its different nature, neither the PSBT_IN_TAP_MERKLE_ROOT field because of the tagged hash used for tweaking. + +A change of the hash tag used for silent payments to TapTweak or something compatible with taproot tweaking wouldn't make sense: although the raw tweak can be disguised as the tap tree merkle root for spending, at the moment of verifying change outputs, you need the full tap tree, and there would be none backing this fake merkle root. + +The use of proprietary fields is possible but brittle, as one may end up having to perform extra lookups for keys that are not unified across implementations. + +Assuming different tweaking schemes available, PSBT_IN_TAP_RAW_TWEAK would be a more general solution. However is unclear how a hardware wallet will determine what the content of the field were in the first more general case. In addition, PSBT fields are usually specified as to the nature of the contents. + +== Backward compatibility == + +This is a new field added to the existing PSBT format. Because PSBT is designed to be extensible, old software will ignore the new fields. + +== Test vectors == + +'''''TODO''''' + +== Reference implementation == + +'''''TODO''''' + +== Acknowledgements == + +Thanks to Craig Raw, macgyver, josibake and all others who participated in discussions on this topic. + +== References == + + From fd66836e43bfb1feebc2a26aa8887866b64d5cd8 Mon Sep 17 00:00:00 2001 From: nymius <155548262+nymius@users.noreply.github.com> Date: Thu, 29 Jan 2026 16:41:02 -0300 Subject: [PATCH 2/7] bip-draft(bip352/spending/psbt): reference BIP 370 as dependency for this proposal --- bip-spending-silent-payments-outputs-with-psbts.mediawiki | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/bip-spending-silent-payments-outputs-with-psbts.mediawiki b/bip-spending-silent-payments-outputs-with-psbts.mediawiki index 512d552dbe..e1486580aa 100644 --- a/bip-spending-silent-payments-outputs-with-psbts.mediawiki +++ b/bip-spending-silent-payments-outputs-with-psbts.mediawiki @@ -11,7 +11,7 @@ 2025-12-05: https://gist.github.com/nymius/b3dd0b8a08c6735d617e6216b73c4260 [gist] First draft 2025-12-15: https://gnusha.org/pi/bitcoindev/R53cG3TeXgXDUUS4kH_q226GlaFCjI0DZVT6mdTQzSQdj3RnNqWA-bFT7uGgGQFJG6938kDGvDJVoFQj8ItEMsJ6NyOjCTvpVEarYiyW6-8=@proton.me/ [bitcoin-dev] [BIP Proposal] Add PSBT_IN_SP_TWEAK field Version: 0.1.0 - Requires: 352, 375, 371 + Requires: 352, 370, 371, 375 == Introduction == From 4ac8948f87664e9bd9d26bae75501763d22f9032 Mon Sep 17 00:00:00 2001 From: nymius <155548262+nymius@users.noreply.github.com> Date: Thu, 29 Jan 2026 16:42:03 -0300 Subject: [PATCH 3/7] bip-draft(bip352/spending/psbt): fix typo `BIP 340` -> `BIP 341` --- bip-spending-silent-payments-outputs-with-psbts.mediawiki | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/bip-spending-silent-payments-outputs-with-psbts.mediawiki b/bip-spending-silent-payments-outputs-with-psbts.mediawiki index e1486580aa..2c7e940ae4 100644 --- a/bip-spending-silent-payments-outputs-with-psbts.mediawiki +++ b/bip-spending-silent-payments-outputs-with-psbts.mediawiki @@ -28,7 +28,7 @@ The existing PSBT fields are unable to support silent payments without changes, BIP 375 and complementary BIP 374 specify how to create outputs locked with silent payment keys using PSBTs. But they don't specify how to unlock these outputs in a transaction.''' Why not including this new field in BIP 375?''' Historically, Silent Payments have been categorized by the perspective of the user of the protocol: receiver or sender. BIP 375 has followed this convention, and its stated on its title: Sending Silent Payments with PSBTs. Given that spending belongs to the sphere of the receiver, and considering this convention, this specification should be a different BIP. -Therefore a new field must be defined to allow PSBTs to carry the information necessary for tweaking taproot keys without following the BIP 340 tagging scheme. +Therefore a new field must be defined to allow PSBTs to carry the information necessary for tweaking taproot keys without following the BIP 341 tagging scheme. == Specification == From 9b4b2004e0c9cdd592f446913400cd86a349563d Mon Sep 17 00:00:00 2001 From: nymius <155548262+nymius@users.noreply.github.com> Date: Thu, 29 Jan 2026 16:42:36 -0300 Subject: [PATCH 4/7] bip-draft(bip352/spending/psbt): fix grammar --- bip-spending-silent-payments-outputs-with-psbts.mediawiki | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/bip-spending-silent-payments-outputs-with-psbts.mediawiki b/bip-spending-silent-payments-outputs-with-psbts.mediawiki index 2c7e940ae4..fa2a84911a 100644 --- a/bip-spending-silent-payments-outputs-with-psbts.mediawiki +++ b/bip-spending-silent-payments-outputs-with-psbts.mediawiki @@ -81,9 +81,9 @@ Once a silent payment UTXO is scanned, is easier to store the output together wi To avoid the burden on the signer it would be better to pass this data into the PSBT together with the input spending the silent payment. Currently, there is no field prescribed for this. -The PSBT_IN_BIP32_DERIVATION field cannot be used because its different nature, neither the PSBT_IN_TAP_MERKLE_ROOT field because of the tagged hash used for tweaking. +The PSBT_IN_BIP32_DERIVATION field cannot be used because of its different nature, neither can the PSBT_IN_TAP_MERKLE_ROOT field because of the tagged hash used for tweaking. -A change of the hash tag used for silent payments to TapTweak or something compatible with taproot tweaking wouldn't make sense: although the raw tweak can be disguised as the tap tree merkle root for spending, at the moment of verifying change outputs, you need the full tap tree, and there would be none backing this fake merkle root. +A change of the hash tag used for silent payments to TapTweak or something compatible with taproot tweaking wouldn't make sense: although the raw tweak can be disguised as the script tree merkle root for spending, at the moment of verifying change outputs, you need the full script tree, and there would be none backing this fake merkle root. The use of proprietary fields is possible but brittle, as one may end up having to perform extra lookups for keys that are not unified across implementations. From 26c6685c064dd195e9ff620fccc9f591769fbc60 Mon Sep 17 00:00:00 2001 From: nymius <155548262+nymius@users.noreply.github.com> Date: Thu, 29 Jan 2026 16:43:24 -0300 Subject: [PATCH 5/7] bip-draft(bip352/spending/psbt): add missing Copyright section --- bip-spending-silent-payments-outputs-with-psbts.mediawiki | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/bip-spending-silent-payments-outputs-with-psbts.mediawiki b/bip-spending-silent-payments-outputs-with-psbts.mediawiki index fa2a84911a..2342cb7ab4 100644 --- a/bip-spending-silent-payments-outputs-with-psbts.mediawiki +++ b/bip-spending-silent-payments-outputs-with-psbts.mediawiki @@ -101,6 +101,10 @@ This is a new field added to the existing PSBT format. Because PSBT is designed '''''TODO''''' +== Copyright == + +This document is licensed under the 2-clause BSD license. + == Acknowledgements == Thanks to Craig Raw, macgyver, josibake and all others who participated in discussions on this topic. From 858275ea6c3c3d21ea3f8f633a5b2edcf675198f Mon Sep 17 00:00:00 2001 From: nymius <155548262+nymius@users.noreply.github.com> Date: Thu, 29 Jan 2026 16:44:11 -0300 Subject: [PATCH 6/7] bip-draft(bip352/spending/psbt): add Changelog section --- bip-spending-silent-payments-outputs-with-psbts.mediawiki | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/bip-spending-silent-payments-outputs-with-psbts.mediawiki b/bip-spending-silent-payments-outputs-with-psbts.mediawiki index 2342cb7ab4..8d10ff07e7 100644 --- a/bip-spending-silent-payments-outputs-with-psbts.mediawiki +++ b/bip-spending-silent-payments-outputs-with-psbts.mediawiki @@ -101,6 +101,11 @@ This is a new field added to the existing PSBT format. Because PSBT is designed '''''TODO''''' +== Changelog == + +* 0.1.0 (YYYY-MM-DD '''''TODO'''''): +** Initial version + == Copyright == This document is licensed under the 2-clause BSD license. From d3b7256d5240f3f3d3125872f8943cbae22fce74 Mon Sep 17 00:00:00 2001 From: nymius <155548262+nymius@users.noreply.github.com> Date: Thu, 29 Jan 2026 16:44:58 -0300 Subject: [PATCH 7/7] bip-draft(bip352/spending/psbt): add 'Test vectors' section as subsection of 'Reference implementation' --- bip-spending-silent-payments-outputs-with-psbts.mediawiki | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/bip-spending-silent-payments-outputs-with-psbts.mediawiki b/bip-spending-silent-payments-outputs-with-psbts.mediawiki index 8d10ff07e7..ed0e700905 100644 --- a/bip-spending-silent-payments-outputs-with-psbts.mediawiki +++ b/bip-spending-silent-payments-outputs-with-psbts.mediawiki @@ -93,11 +93,11 @@ Assuming different tweaking schemes available, PSBT_IN_TAP_RAW_TWEAK wo This is a new field added to the existing PSBT format. Because PSBT is designed to be extensible, old software will ignore the new fields. -== Test vectors == +== Reference implementation == '''''TODO''''' -== Reference implementation == +=== Test vectors === '''''TODO'''''