Bitcoin and stock clearing

07 Feb 2014

Quoth Felix:

A large part of the technology underpinning stock exchanges is the way in which they have to ensure that once you sell a share, you no longer own it, and you can’t sell it a second time. That’s also the technology which underpins bitcoin, except bitcoin transactions take many orders of magnitude more time to clear than stock transactions do.

There are two claims here. One is that stock exchanges enforce rules against “double selling” of stock. Others will be more qualified to comment on this, but my impression is that they don’t actually do this. Brokers do. That’s why brokers can some times lose track and let customers do things that aren’t really allowed. And that’s part of the reason why we have failures to deliver in the stock market.

The second claim is that Bitcoin transactions take much longer to clear than stock transactions. How long Bitcoin transactions take depends on how many confirmations (blocks in which the transaction appears) you wait for, and how much time passes between blocks. Bitcoin blocks come every 10 minutes. The convention used to be 6 confirmations for most transactions, but these days I think it’s closer to 3. With 3 confirmations, Bitcoin transactions will an average of 25 minutes to clear, after which the transaction is irrevocable.

In the United States, stock transactions take 3 business days to settle, which is when you hand over your cash and receive stock (or vice versa), and you can withdraw that cash or buy other stock with it. I know that “clearing” can technically mean something else—the netting of multilateral obligations between brokers—but I don’t think that happens much sooner than 3 business days after the trade.