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
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