Multi-owned coins

[fabianhjr]

Hey, I am just thinking that an added feature might be the ability for a coin to be "owned" by multiple addresses. So in order to make a payment with this keys the owners, all of them, must agree. This could also increase the security against fraud in the following scenario:

A wants to buy from B
A adds B's public key to the owner list of the involved coins so now A and B own the coins
B delivers the goods and services and then A removes his key from the owner list
Now B can use the coin freely

Please discuss.

[theymos]

That's already supported by the protocol with transaction scripts.

Satoshi described that escrow use of multi-owner coins here:
http://bitcointalk.org/index.php?topic=750.0

[fabianhjr]

Well, the last message was by august. It is a shame it didn't continue.

Now: What about 3 or four owners? Say for instance that I operate a bank and want to exchange Bitcoins. As a precaution there is the cashier's key, the vault-master's key and my key. So neither the cashier or the vault-master can get the Bitcoins to disappear and only if we all agree the Bitcoins can be release. Of course, neither the cashier or the vault-master can disappear the key because you keep a backups.

[theymos]

Up to 20 keys can be required.

[fabianhjr]

Then what this:
A buys from B
A uses the multi-own feature to escrow the Bitcoins to B
B then buys from C and uses the multi-own feature to add him
B never gives the goods to A
C delivers goods to C who keeps them

The coins never really get released and are still owned by A , who got scammed.

[theymos]

You can't add owners without making a new transaction.

[Mike Hearn]

Look at OP_CHECKMULTISIG for how to do this.

[Nefario]

Posting here just to make it easier for me to find.