Extending the Alert messages in the protocol

[martin]

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[Gavin Andresen]

Here's a scheme that would work:

1. Exchange creates a special bitcoin keypair for each exchange rate it wants to publish.
  E.g. maybe there is a   1mtgxbtcusd9873919fp876...   address for mtgox btc/usd

2. Exchange funds that address with a bunch of bitcoins.

3. Every 10 minutes the exchange performs a send-to-self transaction FROM that address TO that address with the number of bitcoins that correspond to the current exchange rate.

Voila, exchange rate is broadcast to anybody who cares to listen. You know it is the exchange, because the exchange is the only entity that can spend bitcoins from the special exchange address.

You have to trust the exchange not to broadcast a bogus price...

[jackjack]

It depends on the amount
No problem for MtGox $/BTC, but for bitparking BTC/NMC it can be costly indeed

[Pieter Wuille]

There is no need to put these in the block chain.

As the exchange is inherently centralized, you lose nothing by publishing the exchange rate data through http or whatever out-of-p2p protocol, possibly mirrored by those who find it interesting enough, if you care about availability.

The data itself can still be signed by a private key held by MtGox, whose address is publically known.

[Mike Hearn]

The Bitcoin protocol already has a meet-in-the-middle broadcast channel system.  AFAIK it was never used and I have no idea if it works, but it seemed complete enough when I looked over it.

That said, the network is already pretty busy. I'm not sure blasting exchange rates over it, even with a pubsub system, is the right way to go. There's no real advantage.

If you want to build a distributed exchange, having a separate chain that contains USDcoins or EURcoins is one way to do it. The chain would do merged mining with the Bitcoin chain, but coins would be created by transactions signed by pre-agreed keys owned by members of a consortium. They would issue USDcoins and keep the dollars backing them in full reserve. You can then trade Bitcoin for USDcoin cryptographically across chains using some of the contracts/cross-chain protocols described on the wiki.

Wanted trades can be broadcast on the alternative network using flood fill. Software could potentially auto-complete trades according to pre-set policies. When you want to cash out, a consortium member destroys the coins and wires you the dollars.