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BIP-341 describes how to construct the message to be signed for taproot (subsection Common signature message). It mostly makes sense but I have a couple of quick questions:

  • Is the ext_flag 0 when spending default key path and 1 if spending with a script?

  • In the 'Data about this input' it says that scriptPubKey is always 35 bytes. How so? I would assume it is (a) "OP_1 OP_PUSHDATA(1) <32 bytes>" so 34 bytes. Does "serialized as script inside CTxOut" mean that we need another byte for the length of (a), thus making it 35 bytes?

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Yes, ext_flag is (currently) only 0 for key path spends. The only defined script path spend (Tapscript, BIP342) sets ext_flag to 1.

The size of a Taproot ScriptPubKey is indeed always 35 bytes because there is:

  • 1 byte for the size of the script
  • 1 OP_1
  • A 32 bytes push (which is 33 bytes accounting for the length prefix)
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