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 42021-04-23T01:10:24  <jeremyrubin> does anyone know why we don't sign the branch script/path that we're signing?
 52021-04-23T01:12:01  <jeremyrubin> According to https://github.com/bitcoin/bips/blob/master/bip-0341.mediawiki#common-signature-message we don't sign the actual branch, right?
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 82021-04-23T04:11:24  <jeremyrubin> update: we do sign it
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272021-04-23T14:32:01  <waxwing> From BIP340: "The function hashtag(x) where tag is a UTF-8 encoded tag name and x is a byte array returns the 32-byte hash SHA256(SHA256(tag) || SHA256(tag) || x)."
282021-04-23T14:32:12  <waxwing> is it intention to use 'tag' twice there? in uses below, it seems like there are maybe two distinct tags allowed? (e.g. "BIP340" and "challenge")
292021-04-23T14:34:52  <waxwing> oh, apparently it's literally like that, according to: https://github.com/bitcoin-core/secp256k1/blob/1e5d50fa93d71d751b95eec6a80f6732879a0071/src/modules/schnorrsig/main_impl.h#L96-L98
302021-04-23T14:35:09  <waxwing> is there an easy to explain reason why the two?
312021-04-23T14:40:39  <nickler> waxwing: Because this is a 64-byte long context-specific constant and the SHA256 block size is also 64 bytes, optimized implementations are possible (identical to SHA256 itself, but with a modified initial state).
322021-04-23T14:41:47  <nickler> (it's mentioned in the bip, ctrl-f "tagged hashes")
332021-04-23T14:50:27  <waxwing> i see thanks nickler
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