[bitcoin-dev] Signature bundles

[Rusty Russell]

Anthony Towns via bitcoin-dev <bitcoin-dev at lists.linuxfoundation.org> writes:
> If you've got one bundle that overpays fees and another that underpays,
> you can safely combine the two only if you can put a SIGHASH_ALL sig in
> the one that overpays (otherwise miners could just make their own tx of
> just the overpaying bundle).

This is a potential problem, yes :( And I'm not sure how to solve it,
unless you do some crazy thing like commit to a set of keys which are
allowed to bundle, which kind of defeats the generality of outsourcing.

> This could replace SINGLE|ANYONECANPAY at a cost of an extra couple of
> witness bytes.
>
> I think BUNDLESTART is arguably redundant -- you could just infer
> BUNDLESTART if you see an INBUNDLE flag when you're not already in
> a bundle. Probably better to have the flag to make parsing easier,
> so just have the rule be BUNDLESTART is set for precisely the first
> INBUNDLE signature since the last bundle finished.

Indeed.

>> One of the issues we've struck with lightning is trying to guess future
>> fees for commitment transactions: we can't rely on getting another
>> signature from our counterparty to increase fees.  Nor can we use
>> parent-pays-for-child since the outputs we can spend are timelocked.
>
> That doesn't quite work with the HTLC-Success/HTLC-Timeout transactions
> though, does it? They spend outputs from the commitment transaction
> and need to be pre-signed by your channel partner in order to ensure
> the output address is correct -- but if the commitment transaction gets
> bundled, its txid will change, so it can't be pre-signed.

Not without SIGHASH_NOINPUT, no.

> FWIW, a dumb idea I had for this problem was to add a zero-value
> anyone-can-spend output to commitment transactions, that can then be
> used with CPFP to bump the fees. Not very nice for UTXO bloat if fee
> bumping isn't needed though, and I presume it would fail to pass the
> dust threshold...

Yeah, let's not do that.

> I wonder if it would be plausible to have after-the-fact fee-bumping
> via special sighash flags at the block level anyway though. Concretely:
> say you have two transactions, X and Y, that don't pay enough in fees,
> you then provide a third transaction whose witness is [txid-for-X,
> txid-for-Y, signature committing to (txid-for-X, txid-for-Y)], and can
> only be included in a block if X and Y are also in the same block. You
> could make that fairly concise if you allowed miners to replace txid-for-X
> with X's offset within the block (or the delta between X's txnum and the
> third transaction's txnum), though coding that probably isn't terribly
> straightforward.

What would it spend though?  Can't use an existing output, so this
really needs to be stashed in an *output script*, say a zero-amount
output which is literally a push of txids, and is itself unspendable.

        <txid1>... <txidN>

That's pretty large though, and it's non-witness data (though
discardable).  How about 'OP_NOP4 <N> <ripemd160-of-last-N-txids>'?
Then the miner just bundles those tx all together?

Cheers,
Rusty.