Hi woltx,

Excellent work.

>Implements the new scheme suggested by Ruben Somsen that allows multiple silent addresses per wallet with minimal overhead

To expand on this, the scheme basically allows the resulting address to be recognizably marked (only recognizable by the recipient of course), which enables you to distinguish between different payment purposes (e.g. some people donate to you for purpose A, others for purpose B). Here's my original comment describing it:

"Naively, the issue is that two keys means twice the scanning, but an interesting alternative would be to simply use the same key (assuming you're OK with using the same identity) but add a public identifier f to it when tweaking. So instead of hash(i*X)*G + X you get hash(i*X)*G + X + f*G . This means every additional "address" only costs one additional ECC addition when scanning (relatively cheap compared to doing ECC multiplications).

The main downside with this is that f becomes crucial for recovering from backup. If we set f as an index (0, 1, 2, 3...) then you'd only have to remember how many "addresses" you issued (and perhaps overshoot when unsure) to ensure recovery of funds, though of course you'd rather also remember which index is associated with what payment reason.

Absolute worst case scenario you could even do something similar to the gap limit where you scan the full history (not just the UTXO set so you don't miss spent outputs) with a default max index of e.g. 100, and then if you find out most of them are in use, you scan the next 100, etc. Costly, but thorough, and only needed as a last resort."

Original comment here:
https://gist.github.com/RubenSomsen/c43b79517e7cb701ebf77eec6dbb46b8#xpub-sharing

Also good to note that f needs to be communicated to the sender somehow, perhaps as part of the address format.

Cheers,
Ruben

On Fri, Sep 30, 2022 at 12:35 AM woltx via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:

This new version addresses most (or all) requests made in PR:

. Implements the new scheme suggested by Ruben Somsen that allows multiple silent addresses per wallet with minimal overhead.
. Implements a new RPC to retrieve silent addresses, which allows users to assign different labels to different addresses. That way, the user knows which silent address the UTXO came from.

Example:

./src/bitcoin-cli -signet -rpcwallet="receiver" getspaddress
tsp001pjgcwd9p6f2rcgf35dlgvj77h2afylg6lp5cdn0cztrk4k54w99kqxn48tq

# This will return the same address as above (both have no label)
./src/bitcoin-cli -signet -rpcwallet="receiver" getspaddress
tsp001pjgcwd9p6f2rcgf35dlgvj77h2afylg6lp5cdn0cztrk4k54w99kqxn48tq

# New label, new address
./src/bitcoin-cli -signet -rpcwallet="receiver" getspaddress 'donation'
tsp011pjgcwd9p6f2rcgf35dlgvj77h2afylg6lp5cdn0cztrk4k54w99kq80t7lt

In this new scheme, the address has a new field called identifier, which tells the receiver and sender how to set the address correctly.

If the receiver, for whatever reason, doesn't know which identifiers have been used, there is no problem. The wallet can scan all identifiers from 0 to 99. Currently, only 100 different identifiers per wallet are allowed. This limit, however, can be increased at any time in the future.

Unlike address formats so far, sp addresses are not script-related and may eventually include any additional information needed, such as an expiration timestamp (or block height). That way, users don't have to track the address indefinitely.

As usual I wrote a step by step tutorial:
https://gist.github.com/w0xlt/c81277ae8677b6c0d3dd073893210875

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