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From the Wiki:

If the attacker has the majority of computer power, "the attacker can't reverse other people's transactions without their cooperation."

I don't quite understand why this must be the case. Let's say attacker wants to undo a transaction k that happens at block i. Why can't an attacker with a majority of hashing power start mining a block j originating with block i-1 that includes everything but k? Then, the attacker will succeed in the long run and the transaction k no longer appears in the longest chain, right? So I'm not quite sure why you cannot reverse other people's transactions in a majority attack.

Diagram for clarity:

     -i(includes k)
-i-1
     -j(does not include k)...(this chain should be longer w/ majority computing power)
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I believe what the wiki is implying, is that the other user's transactions won't be double spent. Because the attacker does not have the other user's private keys, he can't spend his coins to a new transaction during the attack. He could prevent those transactions from going into blocks, but still has no access to the coins. That's what I think is meant by "without their cooperation".

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