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To break EC you need to get EC public key. You can't get it unless bitcoin address was reused.

If you found pre-image for ripemd160(sha256(x)), you still need to find a private key for it.

Say public key x receives 1 BTC in block A. I'm guessing it's encoded as ripemd160(sha256(o)) -> 1 BTC -> ripemd160(sha256((x)) where o is some other public key with sufficient funds. I create a new key pair with public key y, such that ripemd160(sha256((x)) = ripemd160(sha256((y)). From now on let's call this address hash h.

In block B, I make a transaction h -> 1 BTC -> s. Where s is a securely generated public key that I own. I then sign this transaction with my forged public key, which hashes to h.

How does this not give me x's money?

If you can create a pair of (private key, public key with collision), then yes bitcoin will be broken.

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