Hi Yuval,

This is an interesting attack. Usually I think of spending with a big
weight witness in the context of slowing down a confirmation of a
transaction, especially a DLC creation tx. There you can delay its
confirmation past some time (i.e. see if your team won the game, and then
either trying to confirm it by providing the slimmed down witness or double
cancelling it by double spending). In this case you are not trying to delay
it but to dilute your portion of the fee.

Another mitigation is to aggresively RBF double spend your input any time a
counterparty doesn't use the spending path they said they would and don't
deal with them again. Of course, various pinning attacks may prevent this
depending on how your joint tx is structured.


On Tue, 7 Feb 2023 at 13:59, Yuval Kogman via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> ## Summary
> Since Taproot (more generally any kind of MAST) spends have variable size
> which
> depends on the path being used, the last such input to be signed in a
> multiparty
> transaction can always use a larger than estimated signature to unfairly
> extract
> a fee contribution from the other parties to the transaction (keeping the
> absolute fees the same and reducing the feerate for the transaction).
> ## Attack Scenario
> Alice et al wish to perform a multiparty transaction, such as a CoinJoin or
> lightning dual funding at a relatively high feerate.
> Mallory has a P2TR output with a large script spend path, e.g. an ordinal
> inscription commitment transaction output.
> Mallory registers this coin as an input into the multiparty transaction
> with a
> fee obligation calculated on the basis of a key spend. When all other
> participants have provided signatures, the script spend path can be used.
> Since the absolute fee amount is already committed to by the provided
> (`SIGHASH_ALL`) signatures but the total transaction weight is not,
> Mallory can
> broadcast any valid signatures up to the maximum standard weight and
> minimum
> relay fees, or in collusion with a miner, up to consensus limits.
> This effectively steals a fee from Alice et al, as their signatures do not
> commit to a feerate directly or indirectly.
> ## Mitigations
> ### RBF
> All parties could negotiate a (series of) transaction(s) ahead of time at a
> lower feerate, giving a lower bound minimum feerate that Mallory can force.
> ### Minimum Weight Before Signing
> Enforcing a minimal weight for all non-witness data in the transaction
> before
> the transaction is considered fully constructed can limit the
> effectiveness of
> this attack, since the difference between the predicted weight and the
> maximum
> weight decreases.
> ### Trusted Coordinator
> In the centralized setting if BIP-322 ownership proofs are required for
> participation and assuming the server can be trusted not to collude with
> Mallory, the server can reject signatures that do not exercise the same
> spend
> path as the ownership proof, which makes the ownership proof a commitment
> to the
> spend weight of the input.
> ### Reputation
> Multiparty protocols with publicly verifiable protocol transcripts can be
> provided as weak evidence of a history of honest participation in
> multiparty
> transactions.
> A ring signature from keys used in the transaction or its transcript
> committing
> to the new proposed transaction can provide weak evidence for the honesty
> of the
> peer.
> Such proofs are more compelling to an entity which has participated in
> (one of)
> the transcripts, or proximal transactions. Incentives are theoretically
> aligned
> if public coordinators publish these transcripts as a kind of server
> reputation.
> ### Increasing Costliness
> A minimum feerate for the previous transaction or a minimum confirmation
> age
> (coindays destroyed implies time value, analogous to fidelity bonds) can be
> required for inputs to be added, in order to make such attacks less
> lucrative
> (but there is still a positive payoff for the attacker).
> ### Signature Ordering
> Signatures from potentially exploitative inputs can be required ahead of
> legacy
> or SegWit v0 ones. The prescribed order can be determined based on
> reputation or
> costliness as described in the previous paragraphs.
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