Support sweeping of private keys

[javgh]

On the topic of importing/sweeping private keys, I was thinking we might go for a compromise, where we support keys that are specified in a format like this: privkey|timestamp . Where timestamp is a unix timestamp of when that key was generated. To sweep such a key, one would launch another instance of bitcoinj, have it scan the blockchain starting from that timestamp, then empty the wallet into a key of the primary wallet.

An alternative would be to use the block height, but that would require access to the block chain when creating the private key, which I think would be inconvenient. In https://github.com/schildbach/bitcoin-wallet/issues/38 Andreas Schildbach talked about the idea of adding the necessary UXTO details to a paper wallet:

It is possible to rescan the blockchain, yes. However it will take about half a day currently because you don't even know the birth date of the key to swipe.

I have proposed adding the UXTOs to the QR code of private keys. This would essentially extend paper wallets in a way so they are redeemable without connectivity at all (except for broadcasting the signed transaction of course). I could not find followers for the idea, however I'm not very good at evangelizing. If you want to make it happen, I'd appreciate.

But if I understand it correctly, you would then not detect any funds that are added later. I think that would be a big disadvantage.

So privkey + timestamp seems like a good middle ground to me, to have reasonable fast sweeping and not require some extra server.

[mackuba]

I've tried rebuilding the wallet from the blockchain in Hive and it took half an hour, not half a day. I'm pretty sure it went through the whole chain from the beginning. (Though I suppose it will be much slower on a phone...)

[ghost]

Closing since this has been implemented in at least one wallet and can still be referenced.