Subject: [bitcoindev] BIP: Sending Silent Payments in PSBTs

From: Andrew Toth <[email-masked]>
Date: Thu, 17 Oct 2024 06:40:03 -0700 (PDT)

This BIP adds support for sending silent payments using PSBTs.

If there are multiple entities handling the PSBT that do not have access to 
some input private keys, a DLEQ proof by the signer may be added for other 
entities to verify the corresponding ECDH shares used to derive the output 
scripts were generated correctly. This will be specified in a following 
BIP. For the common case of a single entity that has access to all private 
keys, the DLEQ proof generation is unnecessary.

Spending support is trivial and can be done with a modification to BIP370 
to add a new input field for the tweak data.

Any feedback appreciated, thank you.

<pre>
  BIP: ?
  Layer: Applications
  Title: Sending Silent Payments with PSBTs
  Author: Andrew Toth <[email-masked]>
          Ava Chow <[email-masked]>
          josibake <[email-masked]>
  Comments-Summary: No comments yet.
  Comments-URI: TBD
  Status: Draft
  Type: Standards Track
  Created: 2024-05-14
  License: BSD-2-Clause
</pre>

==Introduction==

===Abstract===

This document proposes additional fields and updated role responsibilities 
for BIP 370 PSBTv2
which adds support for sending to silent payments as described in BIP352.

===Copyright===

This BIP is licensed under the 2-clause BSD license.

===Motivation===

Partially Signed Bitcoin Transaction Version 2 as described in BIP 370 is 
not compatible with sending to silent payments as described in BIP352. In 
particular, the output script of a silent payment cannot be computed until 
after all transaction inputs have been added.
Also, any inputs that the Signer has the private keys for must be signed 
with SIGHASH_ALL and all inputs must not have any scriptPubKeys with Segwit 
version > 1.
Additionally, the silent payment outputs computed by a signer must be 
verifiable to other entities.
Therefore, new fields and role responsibilities must be added to carry, 
compute, and verify the silent payment data.

==Specification==

This document specifies new fields and new field inclusion/exclusion 
requirements.

<tt>PSBT_OUT_SCRIPT</tt> is modified to be optional for outputs in silent 
payments capable PSBTs. If this field is not included in the output, then 
the field PSBT_OUT_SP_V0_INFO must be included.

The new global types are defined as follows:

{|
! Name
! <tt><keytype></tt>
! <tt><keydata></tt>
! <tt><keydata></tt> Description
! <tt><valuedata></tt>
! <tt><valuedata></tt> Description
! Versions Requiring Inclusion
! Versions Requiring Exclusion
! Versions Allowing Inclusion
|-
| Silent Payment Global ECDH Share
| <tt>PSBT_GLOBAL_SP_ECDH_SHARE = 0x07</tt>
| <tt><33 byte scan key> <34 byte outpoint>*</tt>
| The scan key and a list of outpoints corresponding to the prevouts of the 
inputs that this ECDH share is for. The outpoints are composed of a 32 byte 
txid followed by a 32-bit little endian uint.
| <tt><32 byte share></tt>
| An ECDH share for a scan key, followed by a list of outpoints. The ECDH 
shared is computed with ''a * B_scan'', where ''a'' is the sum of all 
private keys of the inputs matching the list of outpoints, and ''B_scan'' 
is the scan key of a recipient.
|
| 0
| 2
|-
| Silent Payment Global DLEQ Proof
| <tt>PSBT_GLOBAL_SP_DLEQ = 0x08</tt>
| <tt><33 byte scan key> <34 byte outpoint>*</tt>
| The scan key and a list of outpoints corresponding to the prevouts of the 
inputs that this proof covers. The outpoints are composed of a 32 byte txid 
followed by a 32-bit little endian uint.
| <tt><64-byte proof></tt>
| A DLEQ proof computed for the matching ECDH share.
|
| 0
| 2
|}

One new per-output type is defined as follows:

{|
! Name
! <tt><keytype></tt>
! <tt><keydata></tt>
! <tt><keydata></tt> Description
! <tt><valuedata></tt>
! <tt><valuedata></tt> Description
! Versions Requiring Inclusion
! Versions Requiring Exclusion
! Versions Allowing Inclusion
|-
| Silent Payment Data
| <tt>PSBT_OUT_SP_V0_INFO = 0x08</tt>
| None
| No key data
| <tt><33 byte scan key> <33 byte spend key></tt>
| The scan and spend public keys from the silent payments address.
|
| 0
| 2
|}

===Unique Identification===

Silent payment capable PSBTs can be uniquely identified the same way as 
PSBTv2s, except when including silent payment outputs. For silent payment 
capable PSBTs, all silent payment outputs must use the PSBT_OUT_SP_V0_INFO 
instead of PSBT_OUT_SCRIPT as the output script when creating the unsigned 
transaction used for unique identification.

==Roles==

This document modifies some existing roles.

===Constructor===

All rules must be followed from PSBTv2 for this role, with the following 
exception:
When an output is added, it must have either PSBT_OUT_SCRIPT or 
PSBT_OUT_SP_V0_INFO, or both, set.

Additionally to PSBTv2, the Constructor must also follow additional rules:

Inputs must not spend an output with Segwit version > 1 may only be added 
if no silent payment outputs are added.
Outputs with PSBT_OUT_SP_V0_INFO set may only be added if there are no 
inputs spending an output with Segwit version > 1.

===Signer===

All rules must be followed from PSBTv2 for this role, in addition to the 
following:

If there are any silent payment outputs present and any input is spending 
from a Segwit version > 1, the Signer must fail.

For all outputs with PSBT_OUT_SP_V0_INFO set, the Signer should:
 - Compute and set an ECDH share and DLEQ proof using all inputs it has the 
private key for.
 - Verify the DLEQ proofs for all inputs it does not have the private keys 
for.
 - If all eligible inputs have an ECDH share, compute and set the 
PSBT_OUT_SCRIPT.

If the Signer sets any missing PSBT_OUT_SCRIPTs, it must set the Inputs 
Modifiable flag to False.

If any output does not have PSBT_OUT_SCRIPT set, the Signer must not yet 
add a signature.

The Signer should additionally compute the silent payment addresses, 
optionally showing this data to the user instead of the computed segwit v1 
addresses.

If a sighash type is provided and there are silent payment outputs present, 
the signer must fail if the sighash type is not SIGHASH_ALL.
If a sighash type is not provided and there are silent payment outputs 
present, the signer must sign using SIGHASH_ALL.

====Computing the DLEQ Proof====

For each output, the Signer may generate a proof for other entities to 
generate the output scripts and verify that the output scripts were 
generated correctly.

Generate a global ECDH share for each scan key ''B<sub>scan</sub>'' and all 
eligible inputs the Signer has private keys for as follows:

Using the notation from 
[https://github.com/bitcoin/bips/blob/master/bip-0352.mediawiki#specification 
BIP352]

* Let ''A<sub>n</sub>'' be the sum of the public keys ''A'' of all eligible 
inputs
* Let ''a<sub>n</sub>'' be the sum of the private keys ''a'' of all 
eligible inputs
* Let ''C =  a<sub>n</sub>·B<sub>scan</sub>''

Use a key ''B<sub>scan</sub>'' followed by a list of the outpoints of all 
eligible inputs.

Set the value for the key of PSBT_GLOBAL_SP_ECDH_SHARE to ''C''.

Compute the DLEQ proof for ''C'' using ''a<sub>n</sub>'' and 
''B<sub>scan</sub>''.
Set the value for the key of PSBT_GLOBAL_SP_DLEQ to the proof.

====Verifying the DLEQ Proof====

For each output, the Signer should verify the ECDH shares for all eligible 
inputs it does not have the private key for using the proofs provided by 
other Signers.

====Computing the Output Scripts====

Compute the PSBT_OUT_SCRIPT using the procedure in 
[https://github.com/bitcoin/bips/blob/master/bip-0352.mediawiki#user-content-Creating_outputs 
BIP352] but substituting ''a·B<sub>scan</sub>'' with the sum of all 
PSBT_GLOBAL_SP_ECDH_SHAREs for that scan key.
If there are multiple silent payment codes with the same scan key, sort the 
codes lexicographically in descending order to determine the ordering of 
the ''k'' value.
If there are multiple silent payment codes with both the same scan and 
spend keys, sort the subgroup by output index in descending order.

====Change Detection====

Updaters may add two PSBT_OUT_BIP32_DERIVATION key-value-pairs with the 
corresponding derivation path of both the scan and spend keys. The Signer 
can then use these fields to verify that the silent payment code is change.

===Transaction Extractor===

For silent payment capable PSBTs, the transaction extractor should compute 
all output scripts for silent payment codes and verify they are correct 
using the ECDH shares and DLEQ proofs, otherwise fail.

==Backwards Compatibility==

Silent payment capable PSBTs are backwards compatible with PSBTv2 once all 
outputs have PSBT_OUT_SCRIPT set. Otherwise they are not backwards 
compatible.

==Test Vectors==

Todo

==Rationale==

<references/>

==Reference implementation==

Todo

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