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I don't understand this condition: > if using a single backup and index based derived key sets including BIP85 32bit hex keys used as nsec. But if I could get away with ignoring it, I would only quote this part: "I think it is the only option" and then I would say "it's clearly not the only option because bitcoin core doesn't use bip39." Sadly, I can't say that without doing an injustice to what you said
There are two companies building Ark Ark Labs: docs.arkadeos.com Second: docs.second.tech There is trust involved to an extent in Ark, you need to trust that the Ark server won't collude with the sender of the payment to double spend it, but that's only for payments that you receive instantly, if you wait for some time your balance (called VTXOs) can be included as part of an aggregate txn that the Ark server makes after an interval which removes the trust assumption
I don't think they have a recovery tool in place, so you would probably lose your bitcoins forever if the server went down. But in theory, whenever a user sends or receives money, they are supposed to get a set of signed transactions that they can broadcast in order to exit unilaterally with their new balance, without needing further assistance from the server. If the users keep a local cache of these signed transactions (which I do not think is currently being done, but it might be), then the user can broadcast them even if the server disappears, and thus get their balance out. There is an important caveat, though: when you receive money in this implementation of Ark, the default way that happens is through something called an "out of round payment." In that mode, you *do* get signed transactions that you can broadcast at any time, but they do *not* offer a true unilateral exit. (But it's not as bad as it sounds, keep reading.) The transactions are timelocked, and some of their inputs can be doublespent by the server if he colludes with other users. If he did that, your exit transactions would be invalidated, and you would lose money. But that is only the "default" way you receive money in this implementation of Ark. There is a second way to receive money, called an "in round payment," and if you receive money *that* way, the server does not have the ability to doublespend the inputs to your exit transactions, so you have a "true" unilateral exit. Also, I believe this Ark wallet does something called a periodic refresh, and whenever it does this, your coins should automatically change from "out of round" to "in round," thus (eventually) giving you true unilateral exit even for coins received the "default" way. I'm not sure when these periodic refreshes happen. As a further caveat, if you "miss" one of these periodic refreshes (e.g. your wallet was offline), and you don't come online within a grace period after that, I believe your exit transactions become invalid anyway, and the service becomes fully custodial.