Let's say you find an address which has funds on it. More precisely - of course - you find one of the private keys to that address.
case A) - You found that key by "brute force" aka lot's of effort and lots of luck. You seize the funds. Is it stealing?
Yes, this would be cracking and then stealing. It is like breaking into a bank vault and then taking the money.
case B) - Your wallet generates that address aka no effort and lots of lots of luck. The funds appear in your wallet balance. Is it stealing?
No, this would be chance and "finders keepers". It is like finding money in the street and you have choice on what to do.
Take or turn in. (If you take from a system that is governed by laws or regulations, "take" as above, will be theft.)
I'm asking, because a collision attack is considered infeasible. It seems not forbidden (because you can't forbid or effectively hinder this attack on a public ledger) - so is it illegal?
A provably purposeful collision attack, if successful, IMO is cracking and thus leads to theft, which is illegal.
Let's assume two parties were known who made claims on the funds of a given address. One party X was the one who "really" put the funds in there, the other Y was the one who found the private key. If Y took the funds - would that legally be considered stealing?
Depends on how Y managed to get the privatekey. Every way, other than collision accident, is likely theft.
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I found myself being legally quite bare in this matter. Especially regarding the question:
"Does one OWN a privkey/pubkey combination?" (I think no)
"Does one OWN the blockchain?" (obviously no)
You can not OWN anything in Bitcoin, you can only CONTROL certain aspects, such as privatekeys.
OWNing something is applying certain rights to the thing. You have no rights within Bitcoin.
Bitcoin allows you to control your keys to secure your portion in the bitcoin blockchain, but
the bitcoin blockchain does not grant you OWNERSHIP over those bitcoins.
For example, paper money is OWNED by the governments and their treasuries/etc,
but they allow the banks and you to CONTROL it temporarily for use.
Bitcoin is the government, blockchain(bitcoin) is the treasury, and your privatekey is your use(control).
But if one does own neither, how can he own the balance the blockchain defines as on that given address?
IMHO, the one OWNS the balance who owns the privkey/pubkey combination and that may very well be several persons.
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As stated above, you do not own anything in Bitcoin, including the balance in an address. You only control it.
In Bitcoin, there is no way to determine who rightfully controlled the keys first (unless ID message
signed upon first tx) so it is not actually possible within the protocol to prove who "owns" what, and thus,
issues of ownership can not exist within the system.
As a side note, if you transferred your bitcoins to a company or site, and that site gets hacked or etc
and your coins are stolen from the company, you only have a right for refund/return by that company
since they took on the liability and must conform to certain regulations and laws. Not only does Bitcoin
not accept any liability, but also does not conform to any regulations and laws, and thus, is the reasoning
that by using bitcoin and controlling your own privatekeys, they say: "Be your own bank". You are
singularly responsible and (in theory of low probability of collision) control your keys.