Subject: Re: [Bitcoin-development] New P2P commands for diagnostics, SPV clients

From: Mike Hearn <[email-masked]>
Date: Tue, 24 Jul 2012 10:16:12 +0200

> Really lightweight clients (like Bitcoincard), clients with shared
> private keys (electrum-style), or brainwallets - will ask the following
> question quite often to "supernodes": Given my public keys/addresses,
> what is the list of unspent outputs. i think it would make sense to
> include such a command, instead or in addition to the filterload/filterinit.

Ultra-lightweight clients like Electrum or smart cards have a
fundamentally different security model to SPV clients, which mean they
cannot connect directly to the P2P network no matter what commands or
db indexes are added.

This seems to be a common point of confusion. Andreas brought up
something similar in a chat yesterday.

To connect to the P2P network, you MUST understand how to walk the
block chain and handle re-orgs. This is not optional. The reason is
that you are connected to random arbitrary nodes who can and maybe
will lie to you. The block chain is a self-proving data structure, a
node cannot lie about it or make you believe garbage unless they can
outrun the rest of the miners combined.

If all you're doing is asking a remote node to tell you about what
coins are available, that node can simply say "guess what, you're a
millionaire!" and you have no way to discover it's wrong. This can be
dangerous in the case where you think you've received a payment but
actually did not, eg, because your internet connection got tampered
with in some way. SPV clients have the same issue for zero-confirmed
transactions, but once you see confirmations at high speeds you can be
pretty sure the network accepted the transaction. For clients that
don't understand the block chain confirmations don't have any meaning.

That's why Electrum requires a trusted server and connects to it via SSL.

> And perhaps more severe: as far as i understand classic bloom filters,
> the server has no method of indexing his data for the expected requests.

It doesn't matter. CPU wise Bloom filtering of blocks is very cheap
and can be trivially parallelised in the unlikely event it's
necessary. The expensive part of serving a Bloom filtered chain to an
SPV client is simply moving the disk head into the right position and
waiting for the platter to rotate. Blocks are stored sequentially and
modern hard disks transfer data once positioned at gigabit speeds so
requesting 1 or 2000 blocks is not significantly different.