Subject: [bitcoin-dev] Silent Payment v4 (coinjoin support added)

From: woltx <[email-masked]>
Date: Tue, 11 Oct 2022 07:02:47 +0000

Silent Payment v4 (coinjoin support added)

Changes:

. Silent payments now use all inputs to create transactions. Previously, they only used the first input. This change increases privacy and makes silent payments compatible with coinjoin.

. `getspaddress` RPC renamed to `getsilentaddress` for clarity

. Added support for silent payment in PSBT via `walletcreatefundedpsbt` RPC.

. Added a new index scheme (which stores the sum of input public keys for each transaction). The previous index `bitcoin/signet/indexes/silentpaymentindex` should be removed as it is no longer compatible with this new version.

For reviewers:

Now, silent payments use the scheme `hash(i1*X + i2*X + i3*X + ...)*G + X == hash(x*(I1+I2+I3+...))*G + X`, as described here: https://gist.github.com/RubenSomsen/c43b79517e7cb701ebf77eec6dbb46b8#variant-using-all-inputs

As inputs can be Taproot, this introduced a new issue as `bitcoin-core/secp256k1` does not support x-only public key sum (perhaps due to missing prefix byte).

I opened a new PR (#1143) to add a function to convert from x-only to compressed public key with even y. This is the solution being used by the current silent payment implementation.
Tutorial updated: https://gist.github.com/w0xlt/c81277ae8677b6c0d3dd073893210875