BIP47 - Reusable Payment Codes - Notification Tx

[redPanda]

From BIP47 on mediawiki:
https://github.com/bitcoin/bips/blob/master/bip-0047.mediawiki
step 3 of the notification tx:

3. Alice serializes her payment code in binary form.

According the Binary Serialization on the same page,
the Payment Code is essentially Alice's pubkey (A = a * G) and Alice's Chain Code(c)
in the next step:

4. Alice renders her payment code (P) unreadable to anyone except Bob:
  i. Replace the x value with x':

The problem is x is the x coordinate of the secret point S = a * B = (x, y)
where a is Alice's privatekey and B is Bob's publickey point (B = b * G)
Question: even if I replace x by x', it won't change the value of the Payment Code
since x is not involved in this value !
What I'm missing ?

[1713561078]

1713561078

[1713561078]

1713561078

[justusranvier]

4. Alice renders her payment code (P) unreadable to anyone except Bob:
  i. Replace the x value with x':

The problem is x is the x coordinate of the secret point S = a * B = (x, y)
where a is Alice's privatekey and B is Bob's publickey point (B = b * G)
Question: even if I replace x by x', it won't change the value of the Payment Code
since x is not involved in this value !
What I'm missing ?

In step 2, you generate a 64 byte blinding factor.

In step 4, you xor the x value and chain code portions of the payment code with the blinding factor.

This means the data you put in the OP_RETURN output is only readable by the recipient.

[thejaytiesto]

Im looking forward to this BIP to increase the overall privacy of Bitcoin. Do you think this will get added soon? I think this idea of mixing all transactions by default is an excellent idea.. the optional should be to not mix it, but by default the transaction should be mixed so default state of transaction is "cash mode" ..

I wonder what core devs like gmaxwell think of this BIP and when can we expect a Bitcoin where all transactions are mixed natively so it feels more like cash like it should be.

[justusranvier]

Do you think this will get added soon?

Wallets can implement this whenever they like. Several have already started.