Eric Lombrozo [ARCHIVE] on Nostr: 📅 Original date posted:2015-12-26 📝 Original message:Note: my stupid email ...

📅 Original date posted:2015-12-26
📝 Original message:Note: my stupid email client didn't indent Peter Todd's quote correctly.
The first paragraph is his, the second is my response.

------ Original Message ------
From: "Eric Lombrozo" <elombrozo at gmail.com>
To: "Peter Todd" <pete at petertodd.org>; "Emin Gün Sirer"
<el33th4x0r at gmail.com>
Cc: nbvfour at gmail.com; "Bitcoin Dev"
<bitcoin-dev at lists.linuxfoundation.org>
Sent: 12/26/2015 12:23:38 AM
Subject: Re[2]: [bitcoin-dev] We need to fix the block withholding
attack

>Peter Todd wrote:
> Fixing block withholding is relatively simple, but (so far) requires a
>SPV-visible hardfork. (Luke-Jr's two-stage target mechanism) We should
>do this hard-fork in conjunction with any blocksize increase, which
>will
>have the desirable side effect of clearly show consent by the entire
>ecosystem, SPV clients included.
>
>I think we can generalize this and argue that it is impossible fix this
>without reducing the visible difficulty and blinding the hasher to an
>invisible difficulty. Unfortunately, changing the retargeting algo to
>compute lower visible difficulty (leaving all else the same) or
>interpreting the bits field in a way that yields a lower visible
>difficulty is a hard fork by definition - blocks that didn't meet the
>visible difficulty requirement before will now meet it.
>
>jl2012 wrote:
>>After the meeting I find a softfork solution. It is very inefficient
>>and I am leaving it here just for record.
>>
>>1. In the first output of the second transaction of a block, mining
>>pool will commit a random nonce with an OP_RETURN.
>>
>>2. Mine as normal. When a block is found, the hash is concatenated
>>with the committed random nonce and hashed.
>>
>>3. The resulting hash must be smaller than 2 ^ (256 - 1/64) or the
>>block is invalid. That means about 1% of blocks are discarded.
>>
>>4. For each difficulty retarget, the secondary target is decreased by
>>2 ^ 1/64.
>>
>>5. After 546096 blocks or 10 years, the secondary target becomes 2 ^
>>252. Therefore only 1 in 16 hash returned by hasher is really valid.
>>This should make the detection of block withholding attack much
>>easier.
>>
>>All miners have to sacrifice 1% reward for 10 years. Confirmation will
>>also be 1% slower than it should be.
>>
>>If a node (full or SPV) is not updated, it becomes more vulnerable as
>>an attacker could mine a chain much faster without following the new
>>rules. But this is still a softfork, by definition.
>jl2012's key discovery here is that if we add an invisible difficulty
>while keeping the retarget algo and bits semantics the same, the
>visible difficulty will decrease automatically to compensate. In other
>words, we can artificially increase the block time interval, allowing
>us to force a lower visible difficulty at the next retarget without
>changing the retarget algo nor the bits semantics. There are no other
>free parameters we can tweak, so it seems this is really the best we
>can do.
>
>Unfortunately, this also means longer confirmation times, lower
>throughput, and lower miner revenue. Note, however, that confirmations
>would (on average) represent more PoW, so fewer confirmations would be
>required to achieve the same level of security.
>
>We can compensate for lower throughput and lower miner revenue by
>increasing block size and increasing block rewards. Interestingly, it
>turns out we *can* do these things with soft forks by embedding new
>structures into blocks and nesting their hash trees into existing
>structures. Ideas such as extension blocks
>[https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2015-May/008356.html]
>have been proposed before...but they add significant complications to
>the protocol and require nontrivial app migration efforts. Old nodes
>would not get forked off the network but backwards compatibility would
>still be a problem as they would not be able to see at least some of
>the transactions and some of the bitcoins in blocks. But if we're
>willing to accept this, even the "sacred" 21 million asymptotic limit
>can be raised via soft fork!
>
>So in conclusion, it *is* possible to fix this attack with a soft fork
>and without altering the basic economics...but it's almost surely a lot
>more trouble than it's worth. Luke-Jr's solution is far simpler and
>more elegant and is perhaps one of the few examples of a new feature
>(as opposed to merely a structure cleanup) that would be better to
>deploy as a hard fork since it's simple to implement and seems to stand
>a reasonable chance of near universal support...and soft fork
>alternatives are very, very ugly and significantly impact system
>usability...and I think theory tells us we can't do any better.
>
>- Eric
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