Date: Mon, 01 Aug 2022 11:38:29 +0000
To: Ruben Somsen <rsomsen@gmail.com>
From: Alfred Hodler <alfred_hodler@protonmail.com>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] New BIP: Private Payments
Precedence: list

Hi Ruben,

I have incorporated your feedback. Using only the first four bytes of the n=
otification code is a very valuable suggestion, so thank you for that. I ha=
ve added you as a co-author.

In regards to hiding the recipient in the notification, the purpose is not =
only to allow Alice to send a notification herself, but also to break the l=
ink between the notifier (be that Alice or a third-party service) and Bob. =
Not doing so would reintroduce the same problem we have with BIP47 and uniq=
ue per-recipient notification addresses -- namely that of social graph buil=
ding. The tradeoff, as you noticed, is that light clients have to rely on s=
ome kind of OP_RETURN indexing service. I personally consider any inconveni=
ence (to developers, as end users never see this) stemming from that to be =
acceptable because:

1) it reduces the amount of social metadata on the blockchain
2) notification services might otherwise be pressured into censoring certai=
n recipients
3) it allows wallets to decide the level of outsourcing they are comfortabl=
e with
4) adversaries monitoring notifications might see a lot of notifications to=
 someone and use that information to mount an attack

Thanks for all the feedback.

Alfred