That would be an actual crime. If you guessed my key, perhaps by knowing I generated it insecurely or watched me type it (when not under an NDA or some duty to

That would be an actual crime. If you guessed my key, perhaps by knowing I generated it insecurely or watched me type it (when not under an NDA or some duty to me), you could do this legally though. But yes, once you've stolen the thing from me I have less control over it... Also, this would never happen as written. If you tried to claim my asset without having changed the key, then I would. We'd never be in court with shared custody, and thus they'd never get a chance to rule on this. The only time a court would come in would be criminally if you actually hacked my system. Otherwise, owning the key is the entirely of ownership. Blockchain is intentionally a knowledge-based version of a bearer bond. (It could be based on hash-preimages which would count as a password and probably be legally protected, but intentionally it's based on math because factors in an equation aren't protected.) Also, you aren't really purchasing art because it's untenable. You're purchasing something like the right to show that the artist considers you to have purchased art. To actually purchase art would be to actually codify a contract (from what jurisdictions?) in an NFT, and that's not what anyone is doing. It's all just digital signatures of a phrase like "Thanks for being the first sponsor of my work 'Autumn Leaves', with shasum XYZ".