0.05 BTC bounty: doing "listunspent" without full blockchain

[agorism]

I am on the last step of building an escrow system. The user has a shared 2x2 address, and the user knows both public keys and both private keys.

How can they build a transaction to move money from the shared address to somewhere else?

My current solution involves sending the user to https://coinb.in/multisig/ to try and build the transaction, and that works, but it is not a very stream-lined process.

Electrum is able to find unspent transactions associated with an address without having to download the entire blockchain, so I know that this is possible.

Whoever answers this question best will get the prize after I get it to work.

[RGBKey]

Are you looking to contact an outside server or do it with like half the blockchain? If you don't have the entire blockchain, you can't tell what has already been spent or not. The reason electrum can do this is because it can connect to the electrum servers that already have the whole blockchain downloaded.

[artw1982]

The only way you could do this is to run a server that is running the bitcoind or daemon alternative that provides an RPC/API. Electrum does this. The client simply makes a call to a remote server to check the balances and so on.

If you don't want to run your own server, you can use the blockchain.info API's freely and get the same effect.