Re: [bitcoin-dev] [Lightning-dev] Continuing the discussion about noinput / anyprevout

[Chris Stewart <chris@suredbits.com>]

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> I don't find too compelling the potential problem of a 'bad wallet
designer', whether lazy or dogmatic, misusing noinput. I think there are
simpler ways to cut corners and there will always be plenty of good wallet
options people can choose.

In my original post, the business that I am talking about don't use "off
the shelf" wallet options. It isn't a "let's switch from wallet A to wallet
B" kind of situation. Usually this involves design from ground up with
security considerations that businesses of scale need to consider (signing
procedures and key handling being the most important!).

>Because scripts signed with no_input signatures are only really exchanged
and used for off-chain negotiations, very few should ever appear on chain.
Those that do should represent non-cooperative situations that involve
signing parties who know not to reuse or share scripts with these public
keys again. No third party has any reason to spend value to a
multisignature script they don't control, whether or not a no_input
signature exists for it.

Just because some one is your friend today, doesn't mean they aren't
necessarily your adversary tomorrow. I don't think a signature being
onchain really matters, as you have to give it to your counterparty
regardless. How do you know your counterparty won't replay that
SIGHASH_NOINPUT signature later? Offchain protocols shouldn't rely on
"good-will" for their counter parties for security.

>As I mentioned before, I don't think the lazy wallet designer advantage is
enough to justify the downsides of chaperone signatures. One additional
downside is the additional code complexity required to flag whether or not
a chaperone output is included. By comparison, the code changes for
creating a no_input digest that skips the prevout and prevscript parts of a
tx is much less intrusive and easier to maintain.

>I want to second this. The most expensive part of wallet design is
engineering time. Writing code that uses a new sighash or a custom
script with a OP_CODE is a very large barrier to use. How many wallets
support multisig or RBF? How much BTC has been stolen over the entire
history of Bitcoin because of sighash SIGHASH_NONE or SIGHASH_SINGLE
vs ECDSA nonce reuse

I actually think lazy wallet designer is a really compelling reason to fix
footguns in the bitcoin protocol. Mt Gox is allegedly a product of lazy
wallet design. Now we have non-malleable transactions in the form of segwit
(yay!) that prevent this exploit. We can wish that the Mt Gox wallet
designers were more aware of bitcoin protocol vulnerabilities, but at the
end of the day the best thing to do was offering an alternative that
circumvents the vulnerability all together.

Ethan made a great point about SIGHASH_NONE or SIGHASH_SINGLE -- which have
virtually no use AFAIK -- vs the ECDSA nonce reuse which is used in nearly
every transaction. The feature -- ECDSA in this case -- was managed to be
done wrong by wallet developers causing fund loss. Unfortunately we can't
protect against this type of bug in the protocol.

If things aren't used -- such as SIGHASH_NONE or SIGHASH_SINGLE -- it
doesn't matter if they are secure or insecure. I'm hopefully that offchain
protocols will achieve wide adoption, and I would hate to see money lost
because of this. Even though they aren't used, in my OP I do advocate for
fixing these.

> understand the desire to be conservative with protocol changes that could
be misused. However, with just taproot and taproot public key types the
anyprevout functionality is already very opt-in and not something that
might accidentally get used. Belt-and-suspender protections like chaperone
signatures and tagged outputs have their own impacts on code complexity,
on-chain transaction sizes and transaction anonymity that also must be
considered.

I'm making the assumption that the business has decided to use this
feature, and in my OP I talk about the engineering decisions that will have
to be made support this. I'm hoping the "lazy wallet designers" -- or
perhaps people that don't follow bitcoin protocol development as rabidly as
us :-) -- can see that nuance.

-Chris



On Tue, Oct 1, 2019 at 8:36 AM Richard Myers <rich@gotenna.com> wrote:

> Thanks Christian for pulling together this concise summary.
>
> 1.  General agreement on the usefulness of noinput / anyprevoutanyscript /
>>     anyprevout.
>>
>
> I certainly support the unification and adoption of the sighash_noinput
> and anyprevoutput* proposals to enable eltoo, but also to make possible
> better off-chain protocol designs generally.
>
> Among the various advantages previously discussed, the particular use case
> benefits from eltoo I want to take advantage of is less interactive payment
> channel negotiation.
>
> In talking with people about eltoo this summer, I found most people
> generally support adding this as an option to Lightning. The only general
> concern I heard, if any,  was the vague idea that rebindable transactions
> could be somehow misused or abused.
>
> I believe when these concerns are made more concrete they can be
> classified and addressed.
>
> I don't find too compelling the potential problem of a 'bad wallet
> designer', whether lazy or dogmatic, misusing noinput. I think there are
> simpler ways to cut corners and there will always be plenty of good wallet
> options people can choose.
>
> Because scripts signed with no_input signatures are only really exchanged
> and used for off-chain negotiations, very few should ever appear on chain.
> Those that do should represent non-cooperative situations that involve
> signing parties who know not to reuse or share scripts with these public
> keys again. No third party has any reason to spend value to a
> multisignature script they don't control, whether or not a no_input
> signature exists for it.
>
> 2.  Is there strong support or opposition to the chaperone signatures
>>     introduced in anyprevout / anyprevoutanyscript? I think it'd be best
>> to
>>     formulate a concrete set of pros and contras, rather than talk about
>>     abstract dangers or advantages.
>>
>
> As I mentioned before, I don't think the lazy wallet designer advantage is
> enough to justify the downsides of chaperone signatures. One additional
> downside is the additional code complexity required to flag whether or not
> a chaperone output is included. By comparison, the code changes for
> creating a no_input digest that skips the prevout and prevscript parts of a
> tx is much less intrusive and easier to maintain.
>
> 3.  The same for output tagging / explicit opt-in. What are the advantages
>> and
>>     disadvantages?
>>
>
> I see the point ZmnSCPxj makes about tagged outputs negatively impacting
> the anonymity set of taproot transactions. The suggested work around would
> impose a cost to unilateral closes of an additional translation transaction
> and not using the work around would cause a hit to anonymity for off-chain
> script users. I feel both costs are too high relative to the benefit gained
> of preventing sloppy reuse of public keys.
>
> 4.  Shall we merge BIP-118 and bip-anyprevout. This would likely reduce the
>>     confusion and make for simpler discussions in the end.
>
>
> I believe they should be merged. I also think both chaperone signatures
> and output tagging should become part of the discussion of security
> alternatives, but not part of the initial specification.
>
> I understand the desire to be conservative with protocol changes that
> could be misused. However, with just taproot and taproot public key types
> the anyprevout functionality is already very opt-in and not something that
> might accidentally get used. Belt-and-suspender protections like chaperone
> signatures and tagged outputs have their own impacts on code complexity,
> on-chain transaction sizes and transaction anonymity that also must be
> considered.
>
> I believe efforts like descriptors will help people follow best practices
> when working with complex scripts without pushing extra complexity for
> safety into the consensus layer of bitcoin. Anywhere we can make core code
> simpler, and handle foot-guns in higher level non-consensus code, the
> better.
>
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