createmultisig single point of failure

Question

I'm implementing multisig addresses which is a great solution to a problem, but as far as I can see there's no way to create the multisig address without knowing all the private keys at the start. Isn't that a single point of failure? Whatever system creates the multisig address knows all the private keys.

I was expecting to be able to create a partial multisig address (me) pass the hex result to someone else to add their private key and then off to the escrow to add their private key... kinda like signing of multisig transactions.

You can see in Gavin's example he has all 3 keys at the begining: https://gist.github.com/gavinandresen/3966071

Thanks.

Answer 1

You're correct that Gavin has all three private keys available to him at the start, but you're missing that he only uses the public keys to create the P2SH multisig address. Specifically, this line from the example is listing only public keys,

./bitcoind createmultisig 2 '["0491bba2510912a5bd37da1fb5b1673010e43d2c6d812c514e91bfa9f2eb129e1c183329db55bd868e209aac2fbc02cb33d98fe74bf23f0c235d6126b1d8334f86","04865c40293a680cb9c020e7b1e106d8c1916d3cef99aa431a56d253e69256dac09ef122b1a986818a7cb624532f062c1d1f8722084861c5c3291ccffef4ec6874","048d2455d2403e08708fc1f556002f1b6cd83f992d085097f9974ab08a28838f07896fbab08f39495e15fa6fad6edbfb1e754e35fa1c7844c41f322a1863d46213"]'

The createmultisig RPC will let you mix addresses (corresponding to private keys you have in local storage) and public keys (for cases where you don't have the private keys---like when creating a multisig address with your friends).

There's a step-by-step create-and-spend multisig example on the Bitcoin.org Developer Examples page (you probably want to start by reading the immediately previous examples, as they build up to P2SH multisig).