Hi,
Regarding to BIP47 there's a newer version (v3 and v4) proposed here:
https://github.com/OpenBitcoinPrivacyProject/rfc/blob/master/obpp-05.mediawiki
This newer version addresses some issues from v1.
Now the notification from Alice to Bob is a transaction from Alice to Alice as
a bare 1 of 3 multisig. The other 2 pubkeys represent Alice's payment code and
Bob's payment identifier. Eliminating the toxic change issue.
The overhead is a one time 64 byte for the two pubkeys. This overhead would be
amortized over the lifetime of the Alice / Bob relationship.
Additionally the first economic payment from Alice to Bob can be included along
with the notification transaction.
Payment codes are recoverable from the bip32 seed. No extra backups required.
This new version is in production with Samourai wallet.
This BIP47 v3 allows Alice to receive Bob's address without exposing her
IP/identity to Charlie who can watch Alice receive the payment code material
from Bob without knowing if Alice acted by sending a payment to Bob.
An xpub doesn't accomplish this because if you have your xpub in a crowdfunding
platform the platform or anyone who hacks it can identify your payments. With
the payment code you can associate yourself publicly with your payment code and
no one (who is not the sender) will know if you received funds as your payment
code is not visible in the blockchain.
Regards
Peter
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