Support for sending to silent payments

[benthecarman]

https://github.com/bitcoin/bips/pull/1458

Supporting receiving for silent payments would be a very large undertaking. However, supporting sending should be relatively pretty trivial.

[LLFourn]

Although sending has less moving parts I'm less sure what's the right API inside transaction building system (mostly because we haven't re-designed transaction building yet). With receiving I know precisely how to approach it (as a new tx graph index) so I'd be tempted to do receiving first and leave sending until later. Of course the basic function to produce a silent payment tx would be needed at the same time as the receiving code which would allow you to manually construct a silent payment tx. We could consider upstreaming this to rust-bitcoin also.

This would go all go in bdk_chain with bdk_tx_build proving more user friendly API coming later (when it exists).

[realeinherjar]

This BIP is recent and still with a lot of comments addressing implementation details, e.g. https://github.com/bitcoin/bips/pull/1458#discussion_r1313885038.

[evanlinjin]

This BIP is recent and still with a lot of comments addressing implementation details, e.g. https://github.com/bitcoin/bips/pull/1458#discussion_r1313885038.

I think Steven was mostly addressing HOW the BIP should be structured, not WHAT was specified in the BIP.

[LLFourn]

While implementing it we might be able to contribute to the BIP if we find places it could be improved.

[notmandatory]

I assigned this to the 1.1 alpha release, but we may need to move it to 2.0 depending on priorities and who's available/able to work on this.

[sethforprivacy]

Traction behind Silent Payments is rapidly growing, with two wallets releasing with full Silent Payments support this week. The most important step for their traction to continue is that wallets simply add the ability to send to Silent Payment addresses (as this PR is for) so that there is proper incentive for receiving to be added to wallets, exchanges, etc.

I started a bounty for this PR to be completed on X, and am offering 100k sats to the dev who's PR solves this issue (total bounty of 870k sats thanks to others on X).

Would be amazing for all BDK wallets to be able to trivially enable sending to Silent Payment addresses!

[sethforprivacy]

Note that a crate for Silent Payment support already exists here:

https://github.com/cygnet3/rust-silentpayments

[storopoli]

The parsing of a SP address is being discussed upstream here https://github.com/rust-bitcoin/rust-bitcoin/issues/2778

[junderw]

I think this should wait for

1. https://github.com/bitcoin-core/secp256k1/pull/1519
2. merging 1 into the secp256k1 wrapper crate.

rust-silentpayments crate is a great prototype, but it is weak to timing attacks.

[cygnet3]

rust-silentpayments crate is a great prototype, but it is weak to timing attacks.

This should be mitigated in v0.3.0. If you find other weaknesses or oversights, please let us know!

Having said that, I agree that the purpose of rust-silentpayments is somewhat obsolete once the silentpayments module is added to secp256k1. Perhaps we will transition to become a more high-level silent payments utility library. Either way, we will rewrite rust-silentpayments to work on top of the silentpayments module once that is finalized.

[junderw]

This should be mitigated in v0.3.0.

I see you just released this about an hour ago. Nice!

[notmandatory]

Tracking issues for SP sending support:

• [ ] PSBT SP data fields spec (delving bitcoin post)
• [ ] Discrete Log Equality Proofs spec (github gist)
• [ ] secp256k1 SP modules (SP and DLEQ modules) (bitcoin-core/secp256k1#1519))
• [ ] rust-libsecp256k1 with SP and DLEQ modules
• [ ] rust-bitcoin SP address types (rust-bitcoin/rust-bitcoin#2778)
• [ ] rust-bitcoin PSBT SP data fields
• [ ] rust-bitcoin upgrade to rust-libsecp256k1 with SP and DLEQ
• [ ] rust-miniscript upgrade to rust-bitcoin with SP support
• [ ] bdk workspace and dependencies upgrade to rust-bitcoin and rust-miniscript with SP support
• [ ] bdk_wallet coin select for SP inputs, single key types
• [ ] bdk_wallet coin select for SP inputs, always uses key path spend for TR SP inputs

Related issues

• [ ] rust-libsecp256k1 with MuSig2 and FROST support
• [ ] rust-miniscript with MuSig2 and FROST support
• [ ] bdk_wallet coin select for SP inputs, aggregate key types (MuSig2, FROST)