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.