[bitcoin-dev] Fraud proofs for block size/weight

Jorge Timón jtimon at jtimon.cc
Thu Mar 23 18:27:39 UTC 2017

Great stuff, although the ordering of the sections seems a little bit confusing.

I think it would be clearer to put the "Creation of proofs" section
before "Proof verification", maybe even before "Proof format" if a
high level defintion of "full tx size proof" is provided before.

Also, in "For the full-size proof, each transaction should be assumed
to be at a minimum the stripped-size rather than the fixed 60 bytes."
it seems you are referring to a "full-size block proof" as opposed to
a "full size tx proof", perhaps a better term could be "full-weight
block proof" if what you are referring to is the proof of the weight
instead of only the pre-segwit size.

Perhaps some short definitions for "stripped-size proof", "full tx
size proof", "full-size proof" and maybe also "size component" at the
beginning would be enough.

In "Network protocol", "It should not recheck blocks known to be
valid, " does "known to be valid" include the blocks that the peer
told us where valid (with their hash and 0 in the enumerated varint)?
Those could be invalid too if the peer was lying, no?
Do you mean "It should not recheck blocks known to be invalid,"?

Why do you need to have at least one full tx size?

In Rationale you have:
Why must a full tx size proof be included?

This is necessary to establish that the claimed block transaction
count is correct.

Why do you need to establish that? If you can establish that the
number of transactions is at least N and that N * 60 bytes is greater
than the size/weight limit, isn't it that enough?

On Wed, Mar 22, 2017 at 10:51 PM, Matt Corallo via bitcoin-dev
<bitcoin-dev at lists.linuxfoundation.org> wrote:
> It works today and can be used to prove exact size: the key observation is
> that all you need to show the length and hash of a transaction is the final
> SHA256 midstate and chunk (max 64 bytes). It also uses the observation that
> a valid transaction must be at least 60 bytes long for compression (though
> much of that compression possibility goes away if you're proving something
> other than "too large").
> On March 22, 2017 1:49:08 PM PDT, Bram Cohen via bitcoin-dev
> <bitcoin-dev at lists.linuxfoundation.org> wrote:
>> Some questions:
>> Does this require information to be added to blocks, or can it work today
>> on the existing format?
>> Does this count number of transactions or their total length? The block
>> limit is in bytes rather than number of transactions, but transaction number
>> can be a reasonable proxy if you allow for some false negatives but want a
>> basic sanity check.
>> Does this allow for proofs of length in the positive direction,
>> demonstrating that a block is good, or does it only serve to show that
>> blocks are bad? Ideally we'd like an extension to SPV protocol so light
>> clients require proofs of blocks not being too big, given the credible
>> threat of there being an extremely large-scale attack on the network of that
>> form.
>> On Wed, Mar 22, 2017 at 1:47 AM, Luke Dashjr via bitcoin-dev
>> <bitcoin-dev at lists.linuxfoundation.org> wrote:
>>> Despite the generalised case of fraud proofs being likely impossible,
>>> there
>>> have recently been regular active proposals of miners attacking with
>>> simply
>>> oversized blocks in an attempt to force a hardfork. This specific attack
>>> can
>>> be proven, and reliably so, since the proof cannot be broken without also
>>> breaking their attempted hardfork at the same time.
>>> While ideally all users ought to use their own full node for validation
>>> (even
>>> when using a light client for their wallet), many bitcoin holders still
>>> do
>>> not. As such, they are likely to need protection from these attacks, to
>>> ensure
>>> they remain on the Bitcoin blockchain.
>>> I've written up a draft BIP for fraud proofs and how light clients can
>>> detect
>>> blockchains that are simply invalid due to excess size and/or weight:
>>>     https://github.com/luke-jr/bips/blob/bip-sizefp/bip-sizefp.mediawiki
>>> I believe this draft is probably ready for implementation already, but if
>>> anyone has any idea on how it might first be improved, please feel free
>>> to
>>> make suggestions.
>>> Luke
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>>> bitcoin-dev at lists.linuxfoundation.org
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