--- Log opened Sun Aug 06 00:00:21 2023
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01:50 < Ademan> What is the purpose of PSBT_GLOBAL_XPUB ? Is the signer supposed to look for xpubs it has the xpriv for and then use the derivation path and search for inputs it can sign for? That seems like strictly more work for the signer than just using PSBT_IN_{,TAP_}BIP32_DERIVATION (have to search through inputs, vs them being explicitly marked). Am I missing something obvious?
01:53 < Ademan> providing the xpub obviously lets the transaction constructor derive other keys, is this more for the output side? (but there's similar BIP32 keys on the output side)
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02:24 < salvatoshi> yes, I'd say being able to derive the co-signers' pubkeys is the advantage. Without that, you can't verify what inputs/outputs are "internal" (e.g. going back to the sender), which is critical for change outputs. Of course, you also need to know the exact descriptor (and there is currently no defined field for that purpose in PSBT), so there could be some redundancy in that case.
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10:48 < Ademan> isn't PSBT_IN_*BIP32_DERIVATION enough for the *sender* to verify that a given output is going back to them though? I suppose other signers can't, but in terms of incentives surely they'd be the one who cares?
10:49 < Ademan> there's probably plenty of cases though where signers have a strong interest in verifying the outputs, I wasn't thinking about that aspect
10:50 < Ademan> strong interest, beyond the obvious change address case which I *do* think is covered already?
10:52 < Ademan> I'm working on a little rust library to facilitate musig signing with some proprietary keys. I'm kind of trying to understand the design motivation of some keys to make sure my design lines up with that, since I have one very specific use case in mind and probably a million blind spots
11:24 < salvatoshi> No, in a multisig, an output is change only if it goes back to the same exact script that you expect. If your pubkey is there, but your cosigner's pubkeys are replaced, money is gone! This would also hold for wallets based on musig/frost. For single-sig, yes, I agree you could do without the GLOBAL_XPUBs
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11:54 < Ademan> for a p2wsh we know the pubkeys of the other participants in the quorum via PSBT_OUT_WITNESS_SCRIPT right? then you have the derivation path potentially via PSBT_OUT_BIP32_DERIVATION for verification ? I guess you may not have the xpub for your (future) cosigners so you'd want PSBT_GLOBAL_XPUB for that?
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13:19 < salvatoshi> Any information in the PSBT is not trusted, at least from the point of view of hardware signers, as it could be have been modified by malware. Say the wallet is wsh(multi(2,xpubA/<0;1>/*,xpubB/<0;1>/*)). In a hardware signer, you typically register the policy/descriptor with xpubA and xpubB (the same that you find in the GLOBAL_XPUBs). The hardware signer can derive the cosigner's pubkeys from the xpubs 
13:19 < salvatoshi> independently (using the BIP32_DERIVATIONs to identify the paths) and verify if the script it obtains is the expected one. This prevents any malware from tampering with cosigners' keys. Even in hardware signers that don't allow persisting the policy on the device, you can at least check that the change is indeed going to the same exact policy as the inputs. You can only do that if you have all the xpubs and all 
13:19 < salvatoshi> the derivations.
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13:26 < salvatoshi> note that the pubkeys in the witness script are _derived_ from the GLOBAL_XPUBs (that's why you need the BIP32_DERIVATIONs). E.g. a concrete address for the script above would be wsh(multi(2,xpubA/0/4,xpubB/0/4)). The global xpubs would have xpubA and xpubB (and their key origin info); BIP32_DERIVATIONs would have the full pubkeys (not xpubs) at m/path'/of'/A'/0/4, etc. 
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13:47 < Ademan> thanks, I think I was mostly in the same place on most of that. I didn't consider the advantage to signers with no persistence, I was thinking you'd be forced to trust the PSBT regardless in that case, but GLOBAL_XPUB does give you a bit more capacity for verification
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14:12 < Ademan_> I really need to set up a bouncer if libera is going to keep their matrix bridge down for a while
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16:55 < stevenroose> instagibbs: Also not in Script? :)
16:56 < stevenroose> F.e. to prove that you're committing to a certain prevout, i.e. that youre spending a certain UTXO, you need to take all other prevouts on the stack, hash them all together in a streaming hash to not reach the max stack size limit.
16:57 < stevenroose> Is there a maximum number of inputs on a tx? Is there a de-facto limit that is somehow known and documented?
16:58 < stevenroose> instagibbs: in this blob of prevouts, you also need to somehow know the index of the one you're picking out as the prevout you're signing for, because then later you commit to the index. It's pretty annoying to do without something like a simple for loop.
16:59 < stevenroose> > The maximum number of inputs that can fit in a valid transaction is 24386.
16:59 < stevenroose> Thanks, achow101 (https://bitcoin.stackexchange.com/questions/85752/maximum-number-of-inputs-per-transaction), that's insanely high. I'll do similar math using taproot.
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--- Log closed Mon Aug 07 00:00:21 2023