Andy Parkins [ARCHIVE] on Nostr: 📅 Original date posted:2011-08-04 🗒️ Summary of this message: A proposal to ...

📅 Original date posted:2011-08-04
🗒️ Summary of this message: A proposal to solve the issue of double-spending in Bitcoin transactions by broadcasting a "MSG_DOUBLESPEND" message when a transaction is dropped.
📝 Original message:Hello,

Here's a scenario (it's contrived to make the players easy to identify, more
likely this would be low value automated vendors):

Two scammers get together to buy two Ferraris using only one set of BTC. They
travel to opposite ends of the world to two car dealerships that accept
bitcoins without waiting for confirmations. They are in contact by mobile.
They each buy the car and come to pay. At exactly the same moment, they both
spend the same coins. They both walk away with a car.

The current solution is the recommendation that vendors wait for six
confirmations before releasing goods. That's a long time though; more than
most would be willing to wait.

Some points:
- The bitcoin network is essentially honest
- If a block chain fork happens, the transactions that are orphaned get added
to the pending transaction list again, meaning ...
- A valid transaction will _eventually_ make it into the (longest) block
chain.
- Actual distribution time for a transaction through the network is in the
order of seconds not minutes
- A double spend attempt has to enter the network near simulateously at
different places, otherwise the second spend will be rejected instantly by
the whole network.

New transactions propagate through the network if they are found to be valid.
If they aren't valid, they are silently dropped. In the event of a double
spend attempt one of those transactions goes to (say) half the network, the
other goes to the other half. Whichever one reaches a node first is seen as
the real one, the second being seen as invalid. One or other of these will
therefore end up in the "longest" chain; but there is no way to know which.

Here's my proposal then: when a node drops a transaction, it should not be
silent. It should be broadcast just as it always was going to be had it been
valid. Only it is broadcast with a new "inv" type, let's say
"MSG_DOUBLESPEND" instead of "MSG_TX".

Now run the Ferrari test again. The vendor sees the transaction that pays for
the car appear near instantly (within the propagation time of the network). A
short while later they also see a MSG_DOUBLESPEND of the same coins that they
have just accepted. They can then operate whatever policy they want: wait for
six, ten, twenty confirmations. Call the police. Whatever. Miners can also
significantly lower the priority of any transactions that get flagged in this
way.

When there isn't a double spend attempt message within the network propagation
time, they can be sure that their transaction is the one that miners are
working on, and they'll eventually get their money. In other words, they can
accept the payment on zero confirmations.

At first I was concerned that this would make it possible to DOS a
transaction, but of course it doesn't -- the transaction has to be internally-
valid to result in a MSG_DOUBLESPEND, meaning it can only be DOSed by someone
with the appropriate private keys.



Andy
--
Dr Andy Parkins
andyparkins at gmail.com
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