Draft BIP for OP_TXHASH and OP_CHECKTXHASHVERIFY

Protocol Design
8

This is super cool @stevenroose! It’s nice to see a concrete full TXHASH proposal as something to think about.

A few comments (some of which we’ve already discussed offline, but I wanted to put into a public forum as well):

  1. Given that BIP119 has working code and is already ready already, why not make OP_CHECKTXHASHVERIFY an upgrade path for non-32-byte hashes on OP_CHECKTEMPLATEVERIFY?
  2. I wonder whether this OP_TXHASH + OP_CHECKSIGFROMSTACKVERIFY is more likely to get merged (and/or more useful) than adding these same OP_TXHASH modes as the sighash for a new Tapscript key type/version (a la APO).
  3. With the above in mind, it seems like these modes are mostly useful with at least 2 bytes of flags, so perhaps it would be worthwhile to reserve 1-byte flags for some pre-determined modes (possibly similar to Template Key). Then we would have an OP_TXHASH with 1-byte modes or multi-byte flags, OP_CHECKTEMPLATEVERIFY with 32, 33, or > 33-byte hashes, and Tapscript 33-byte keys with 64, 65 or > 66-byte signatures all sharing the same basic set of hash modes and flags.

Thanks!

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