Transcript for #bitcoin-dev 2018/01/29

18:42 ProfMac when I sign a message, is there a sha256 digest that can be recovered from the signature?
18:45 arubi only the signature?
18:46 ProfMac Yes, only the signature. I am deciding whether to explicitly include the sha256 in a report.
18:46 arubi ProfMac, if you have the signature, the k value and the private key, then you can know what digest that was signedwas. but not with just the sig
18:47 ProfMac No, signed with someone else's public key, then reported publicly.
18:48 arubi there's nothing you can recover from just a signature. basically almost any two numbers are a valid signature
18:48 ProfMac It is a matter of how succinct I can make my report.
18:48 ProfMac Thanks.
18:48 arubi np
18:50 ProfMac *both the other person's signing address as well as the signature are reported.
18:54 arubi even if it were the pubkey, I don't think you could recover z
18:54 arubi you'd get some point that is z*G, now how do you go back to z? :)
18:55 arubi s/were/had/ ..
18:56 arubi err, no. "even if it were" is right
20:26 ghost43 how should a txn's witness be parsed if the parent txn's scriptPubKey is not available? do I have no way of determining witness program version for example?
20:27 gmaxwell ghost43: a witness program is all just data pushes. Generally you should treat it as totally opaque.
20:28 ghost43 gmaxwell: so I do need the scriptPubKey it spends?
20:28 gmaxwell also this channel is basically dead, I wouldn't recommend asking any questions here and expecting answers.
20:29 gmaxwell ghost43: Well, need for what? FWIW, this isn't unique to segwit-- any script pubkey could contain more or less arbritary stuff, what looks like a pubkey and signature might be something else completely.
20:29 ghost43 where should I ask? #bitcoin is usually full of casual talk; so then #bitcoin-core-dev?? seems off-topic there
20:29 gmaxwell I'd recommend stackoverflow for this sort of question.
20:29 gmaxwell And tech questions are welcome in #bitcoin fwiw, most of the ops there will tell the casual talk to quiet down when someone has a serious question.