Subject: Re: [Bitcoin-development] Protocol extensions

From: Alan Reiner <[email-masked]>
Date: Sun, 18 Dec 2011 13:06:09 -0500

The whole point of having headers built at a constant size and 
generation rate is to minimize the amount of data needed to "understand" 
of the blockchain while simultaneously maximizing integrity/security in 
the presence of untrusted nodes.  Barring the 50%-attack, you only need 
a couple honest nodes out of 50 to stay safe (as long as you're waiting 
for your 6 confirmations).   In fact, I would argue that a full node 
(Satoshi client), has the same level of security as a headers-only 
client... because they both base *all* their verification decisions on 
computations that end with comparing hashes to the longest-chain headers.

In the case that an attacker figures out how to isolate your node 
entirely and start feeing you poisoned blocks, then you are vulnerable 
with any kind of node, full or lightweight.  I don't see where the 
reduced security is.

The only issue I see is that a truly light-weight, headers-only node 
will be having to download an entire block to get one transaction it 
needs.  This would be significantly alleviated if nodes can start 
requesting merkle-trees directly, even without merkle-branch-pruning.   
If a node can ask for a tx and the tx-hash-list of the block that 
incorporated that tx,  he can easily verify his tx against his 
no-need-to-trust-anyone headers, and doesn't have to download MBs for 
every one.

As for blockchain pruning... I think it's absolutely critical to find a 
way to do this, /for all nodes/.  I am swayed by Dan Kaminsky's 
scalability warnings, and my instinct tells me that leaving full 
verification to a select few deep-pockets nodes in the future opens up 
all sorts of centralization/power-corporation issues that is contrary to 
the Bitcoin concept.  It is in everyone's best interest to make it as 
easy as possible for /anyone/ to act as a full node (if possible).  As 
such, I believe that the current system of minimizing TxOut size is the 
right one.  TxIns take up 0 bytes space in the long-run when taking into 
account any blockchain pruning/snapshot idea (except for nLocktime/seq 
transactions where the TxIn might have to be saved).

-Alan

On 12/18/2011 12:09 PM, theymos wrote:
> On Sat, Dec 17, 2011, at 05:27 PM, Jordan Mack wrote:
>> I don't like the idea of a header only client, unless this is just an
>> interim action until the full block chain is downloaded in the
>> background. Development of these types of clients is probably
>> inevitable, but I believe that this breaks the most fundamental
>> aspects of Bitcoin's security model. If a client has only headers, it
>> cannot do full verification, and it is trusting the data from random
>> anonymous peers.
> A headers-only client is much better than trusting anyone, since an
> attacker needs>50% of the network's computational power to trick
> such clients.
>
> For everyone to keep being a full node, hardware costs would need to
> constantly go down enough for all nodes to be able to handle enough
> transactions to meet demand. If hardware doesn't become cheap enough
> quickly enough, either some people would be unable to handle being full
> nodes, or the max block size wouldn't rise enough to meet demand and
> transaction fees would become noncompetitive.
>
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