Subject: Re: [bitcoin-dev] Blockchain verification flag (BIP draft)

From: Jannes Faber <[email-masked]>
Date: Fri, 4 Dec 2015 13:44:52 +0100

1) (I would assume this is already current default behaviour, but just in
case.) Would it not make sense to *never* send a blockheader to an SPV
client unless the node itself fully validated that block? Regardless of who
mined the block and whether this verification flag has been set or not.

2) Besides having your verification flag in the block, would it not also
make sense to have such a flag in the P2P protocol when blocks (or headers)
are communicated? That way a node could simply do some quick sanity checks
(difficulty as anti-DOS) on an incoming block and then immediately
propagate it to the next (non-SPV) node, but with a flag "Looks good, but I
haven't fully validated it myself, so please don't blame me". And if the
block does turn out to be invalid, the node does not get banned if it was
honest about it.

3) With the above implemented, I can imagine miners running 2 (or more)
nodes side by side, one of them doesn't validate in order to reduce latency
and orphan rates, but the other one does validate and quickly signals the
first one if there's a problem. Both nodes don't necessarily need to be in
the same network or even on the same side of the Great Firewall. Of course
they would be whitelisting each other for trust, or the signal would need
to include some sort of proof.

This probably has been suggested many times already, sorry if this is a
dumb idea.

--
Jannes

On 4 December 2015 at 09:26, Gregory Maxwell via bitcoin-dev <
[email-masked]> wrote:

> For discussion,
>
> A significant fraction of hashrate currently mines blocks without
> verifying them for a span of time after a new block shows up on the
> network for economically rational reasons. This otherwise harmful
> behavior can be made a beneficial to the whole network; but only if it
> is communicated.
>
> This BIP proposal suggests a communication channel and describes its
> use and the motivations for it.  I wrote it in response to suggestions
> that Bitcoin Core add explicit support for this kind of mining, which
> could also implement best in class risk mitigations. I believe
> signaling the behavior is a necessary component for risk mitigation
> here.
>
> -----------------------------------------------------------------
>
> <pre>
>   BIP: draft-maxwell-flagverify
>   Title: Blockchain verification flag
>   Author: Greg Maxwell <[email-masked]>
>   Status: Draft
>   Type: Standards Track
>   Created: 2015-12-02
> </pre>
>
> ==Abstract==
>
> This BIP describes a flag that the authors of blocks can use to voluntarily
> signal that they have completely validated the content of their
> block and the blocks before it.
>
> Correct use of this signaling is not enforced internally to the network
> but if used it can act as a hint allowing more intelligent risk analysis.
>
> If deployed and adhered to, this mechanism turns otherwise harmful
> validation skipping by miners into a behavior which benefits the public.
>
> ==Summary==
>
> The version field in a Bitcoin block header is a 32-bit signed integer.
>
> The most significant bit (30) of the block version is defined to signal
> that
> the author of the block has validated the whole chain up to and including
> the
> content of the block.
>
> Conforming miners MUST NOT set this flag when they have not completely
> validated the prior block(s) or the content of their own block.  Miners
> should continue to try to minimize the amount of time spent mining
> on a non-validated chain.  Blocks which extend an invalid chain will
> continue to be rejected and ultimately orphaned as validation catches up.
>
> It is recommended, but not required, that miners also not set the flag on
> blocks
> created by the same device which created the block immediately prior.  This
> will reduce the incorrect implication of independent validation when the
> two
> most recent blocks are both the product of the same, single, faulty system.
>
> The set state for the bit is defined as verified so that that
> un(der)maintained systems do not falsely signal validation.
>
> Non-verifying clients of the network may check this bit (e.g. checking
> that the version is >= 1073741824) and use it as an input to their risk
> modeling.  It is recommended that once this BIP is widely accepted by the
> network that non-full-node wallets refrain from counting confirmations on
> blocks where the bit is not set.
>
> The authors of non-verifying clients should keep in mind that this flag
> is only correct with the cooperation of the block author, and even then
> a validating miner may still accidentally accept or produce an invalid
> block due to faulty hardware or software.  Additionally, any miner which
> correctly uses this flag could stop doing so at any time, and might
> do so intentionally in order to increase the effectiveness of an attack.
> As a result of misunderstanding, misconfiguration, laziness, or other
> human factors some miners may falsely set the flag.  Because invalid
> blocks are rare it may take a long time to detect misuse of the flag.
>
> As such, the accuracy of this field MUST NOT be strongly relied upon.
>
> Especially due to the non-enforceability of the flag, the user community
> should keep in mind that both setting the flag correctly and mining
> without verification (for brief periods of time) are healthy for the
> network.  If participants are punished for following this specification
> they will simply lie, and its utility will be diminished.
>
> ==Motivation==
>
> Some applications of the Bitcoin system such as thin-client wallets make
> a strong assumption that all the authors of the blocks have faithfully
> verified the blockchain.  Because many of these applications also take
> irreversible actions based on only one or two confirmations and the time
> between blocks is often very short, these clients are vulnerable to
> even small and short-duration violations of this assumption.
>
> Processing and propagation delays resulting from increased transaction
> load contribute to block orphaning when multiple blocks are found at
> close to the same time. This has caused some miners to work on extending
> the chain with the most proof-of-work prior to validating the latest
> block(s).
>
> Although this validation skipping undermines the security assumptions
> of thin clients, it also has a beneficial effect: these delays also
> make the mining process unfair and cause increased rewards for the
> largest miners relative to other miners, resulting in a centralization
> pressure.  Deferring validation can reduce this pressure and improve
> the security of the Bitcoin system long term.
>
> This BIP seeks to mitigate the harm of breaking the thin client
> assumption by allowing miners to efficiently provide additional
> information on their level of validation.  By doing so the
> network can take advantage of the benefits of bypassed
> validation with minimal collateral damage.
>
> ==Deployment==
>
> Because there is no consensus enforced behavior there is no special
> deployment strategy required.  [BIP 9 will need to be updated.]
>
> ==Credits==
>
> Thanks goes to Jeremy Rubin for his two-phase mining suggestion
> which inspired this simplified proposal.
>
> ==Copyright==
>
> This document is placed in the public domain.
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