rich1.0Tomas Susanka [ARCHIVE] (npub1pz…xnfhv)https://yabu.me/npub1pzhqhlvhvdg3ygr8keypprax6gg0qmsrtypmez67few4uzsvhqaqkxnfhvnjumphttps://yabu.me📅 Original date posted:2018-06-21 📝 Original message:Hi, On 19.6.2018 19:16, Pieter Wuille via bitcoin-dev wrote: > Yes, the reason is address reuse. It may be discouraged, but it still > happens in practice (and unfortunately it's very hard to prevent > people from sending to the same address twice). > > It's certainly possible to make them per-input (and even per-output as > suggested below), but I don't think it gains you much. At least when a > signer supports any kind of multisig, it needs to match up public keys > with derivation paths. If several can be provided, looking them up > from a global table or a per-input table shouldn't fundamentally > change anything. > > However, perhaps it makes sense to get rid of the global section > entirely, and make the whole format a transaction plus per-input and > per-output extra fields. This would result in duplication in case of > key reuse, but perhaps that's worth the complexity reduction. I think having a global section with just one record (the transaction) is just fine, in case we come up with some other fields later on which would fit the global section. Otherwise I totally agree. >> 2) The global items 0x01 (redeem script) and 0x02 (witness script) are >> somewhat confusing. Let's consider only the redeem script (0x01) to make >> it simple. The value description says: "A redeem script that will be >> needed to sign a Pay-To-Script-Hash input or is spent to by an output.". >> Does this mean that the record includes both input's redeem script >> (because we need to sign it), but also a redeem script for the output >> (to verify we are sending to a correct P2SH)? To mix those two seems >> really confusing. >> >> Yet again, adding a new output section would make this more readable. We >> would include the input’s redeem script in the input section and the >> output’s redeem script again in the output section, because they’ll most >> likely differ anyway. > I think here it makes sense because there can actually only be (up to) > one redeemscript and (up to) one witnessscript. So if we made those > per-input and per-output, it may simplify signers as they don't need a > table lookup to find the correct one. That would also mean we can drop > their hashes, even if we keep a key-value model. Yes, indeed. Just to clarify: in the first sentence you mean "per output", right? There can actually only be (up to) one redeemscript and (up to) one witnessscript *per output*. >> 4) Is it a good idea to skip records which types we are unaware of? We >> can't come up with a reasonable example, but intuitively this seems as a >> potential security issue. We think we should consider introducing a >> flag, which would define if the record is "optional". In case the signer >> encounters a record it doesn't recognize and such flag is not set, it >> aborts the procedure. If we assume the set model we could change the >> structure to <type><optional flag><length>{data}. We are not keen on >> this, but we wanted to include this idea to see what you think. > Originally there was at least this intuition for why it shouldn't be > necessary: the resulting signature for an input is either valid or > invalid. Adding information to a PSBT (which is what signers do) > either helps with that or not. The worst case is that they simply > don't have enough information to produce a signature together. But an > ignored unknown field being present should never result in signing the > wrong thing (they can always see the transaction being signed), or > failing to sign if signing was possible in the first place. Another > way of looking at it, the operation of a signer is driven by queries: > it looks at the scriptPubKey of the output being spent, sees it is > P2SH, looks for the redeemscript, sees it is P2WSH, looks for the > witnessscript, sees it is multisig, looks for other signers' > signatures, finds enough for the threshold, and proceeds to sign and > create a full transaction. If at any point one of those things is > missing or not comprehensible to the signer, he simply fails and > doesn't modify the PSBT. The rationale behind this was, what if at some point we come up with a PSBT record, which forbids some kind of operation or alters some behaviour. In another words, by omitting such record the signer would create a signature, which is valid, but actually signed something different than the Creator intended. > However, if the sighash request type becomes mandatory, perhaps this > is not the case anymore, as misinterpreting something like this could > indeed result in an incorrect signature. I believe this use case illustrates it quite well. Let’s suppose the sighash record is binding and the Signer does not know it. The Creator creates a PSBT with sighash set SIGHASH_SINGLE. The Signer sings the transaction with SIGHASH_ALL, because they are not aware of such field. This results in a valid signature, however not what the Creator intended it to be. >> We’d also like to note that the “number of inputs” field should be >> mandatory - and as such, possibly also a candidate for outside-record field. > If we go with the "not put signatures/witnesses inside the transaction > until all of them are finalized" suggestion, perhaps the number of > inputs field can be dropped. There would be always one exactly for > each input (but some may have the "final script/witness" field and > others won't). Agree. I'm be fine with dropping the field completely in that case. Thanks, Tomas