> So the sha256 of the span of the group doesn't commit to start and end > -- it just serializes a vector, so commits to the number of elements, > the order, and the elements themselves.

Gotcha wasn't clear to me that the new state pair isn't committed as part of the annex. Have been confused by "Introduce a new SIGHASH_GROUP flag, as an alternative to ALL/SINGLE/NONE, that commits to each output i, start <= i < end." > Does the above resolve that? I think so. It shouldn't be susceptible to any spend replay attack, as the state pair prevents output group overlapping though you might still have to be careful about siphoning ? Something you should already care about if you use SIGHASH_SINGLE and your x's amount > y's value. Le ven. 9 juil. 2021 à 21:47, Anthony Towns <a...@erisian.com.au> a écrit : > On Fri, Jul 09, 2021 at 09:19:45AM -0400, Antoine Riard via bitcoin-dev > wrote: > > > The easy way to avoid O(n^2) behaviour in (3) is to disallow partial > > > overlaps. So let's treat the tx as being distinct bundles of x-inputs > > > and y-outputs, and we'll use the annex for grouping, since that is > > > committed to by singatures. Call the annex field "sig_group_count". > > > When processing inputs, setup a new state pair, (start, end), initially > > > (0,0). > > > When evaluating an input, lookup sig_group_count. If it's not present, > > > then set start := end. If it's present and 0, leave start and end > > > unchanged. Otherwise, if it's present and greather than 0, set > > > start := end, and then set end := start + sig_group_count. > > IIUC the design rationale, the "sig_group_count" lockdowns the hashing of > > outputs for a given input, thus allowing midstate reuse across signatures > > input. > > No midstates, the message being signed would just replace > SIGHASH_SINGLE's: > > sha_single_output: the SHA256 of the corresponding output in CTxOut > format > > with > > sha_group_outputs: the SHA256 of the serialization of the group > outputs in CTxOut format. > > ie, you'd take span<CTxOut>{start,end}, serialize it (same as if it were > a vector of just those CTxOuts), and sha256 it. > > > Let's say you want to combine {x_1, y_1} and {x_2, y_2} where {x, y} > denotes > > bundles of Lightning commitment transactions. > > x_1 is dual-signed by Alice and Bob under the SIGHASH_GROUP flag with > > `sig_group_count`=3. > > x_2 is dual-signed by Alice and Caroll under the SIGHASH_GROUP flag, with > > `sig_group_count`=2. > > y_1 and y_2 are disjunctive. > > At broadcast, Alice is not able to combine {x_1,y_1} and {x_2, y_2} for > the > > reason that x_1, x_2 are colliding on the absolute output position. > > So the sha256 of the span of the group doesn't commit to start and end > -- it just serializes a vector, so commits to the number of elements, > the order, and the elements themselves. So you're taking serialize(y_1) > and serialize(y_2), and each of x_1 signs against the former, and each > of x_2 signs against the latter. > > (Note that the annex for x_1_0 specifies sig_group_count=len(y_1) > and the annex for x_1_{1..} specifies sig_group_count=0, for "reuse > previous input's group", and the signatures for each input commit to > the annex anyway) > > > One fix could be to skim the "end > num_ouputs" semantic, > > That's only there to ensure the span doesn't go out of range, so I don't > think it makes any sense to skip it? > > > I think this SIGHASH_GROUP proposal might solve other use-cases, but if I > > understand the semantics correctly, it doesn't seem to achieve the batch > > fee-bumping of multiple Lightning commitment with O(1) onchain footprint > I was > > thinking of for IOMAP... > > Does the above resolve that? > > Cheers, > aj > >

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