so lets say for the first commitment (after a mined transaction)
a = miner_reward
this is a known number.
to generate the first commitment, one does;
input = x.G + a.H; output = x.G + a.H
Then using diffie-hellman the sender lets the receiver know the values of both x an a. The problem is, if the receiver now wants to spend the outputted commitment, he must add a further layer of masking/encryption. Because if he doesn't and some malicious actor down the road receives a payment, all the bad actor has to do is reveal x, and everyone in the chain that used x as a blinding key, will have their amounts compromised.
So my question is, how exactly does the receiver add the further encryption to the commitment? Proofs of why its valid would be nice as well.