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Kind of an odd question, and I'm pretty sure the answer is, "extremely unlikely," but wanted to see what the community thought.
Secondary question: How sure can one be that the addresses walletgenerator.net and similar paper wallet generators generate are not all related to an already-in-use xPub?
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If you assume that an address is generated from a xpub/xpriv keypair, then yes, it would be safe to assume that the owner of that address also owns the xpub/xpriv keypair that that address was derived from. The probability that someone was able to generate the same address randomly or that someone was able to generate another keypair which also has that address as a child is so low that it is effectively 0.
There is a chance that the owner may not be aware of that address especially if it's at some odd derivation path not supported by the wallet he is using. – Abdussamad – 2018-05-03T08:27:15.660
Do you mean a secret master private key that the website controls that the private keys are actually generated from or do you mean two people generating the same private key, one person doing so randomly and the other doing so via a master private key? – Andrew Chow – 2018-02-11T07:13:26.347
Hmmm... when you put it that way, the second scenario is obviously extremely improbable, but maybe another way of saying it: Can I assume that for any given address the same person aka "account holder" owns all other possible addresses that could be generated from the parent xpub/xpriv pair? – EvilJordan – 2018-02-11T07:24:06.603