Draft post of tr() partial descriptors for delvingbitcoin.org

tr_partial_descriptors.md

tr() Partial Descriptors

Previous discussions on Issue 24114 and Partial Descriptors Gist suggests two new descriptors for tr():

• rawnode(HEXHASH) which allows the specification of a branch using the merkle hash
• rawleaf(HEXSCRIPT,[HEXLEAFVER]) which allows the addition of raw scripts with different leaf versions

Arguments for rawnode() include:

• Adding support for Need-to-Know-Branches (N2KB) see Jeremy's comment on 21365
• Allowing users to specify TR trees with omitted information
• Allowing users to simplify TR trees
  As an example, this descriptor:

tr(
  5a1e61f898173040e20616d43e9f496fba90338a39faa1ed98fcbaeee4dd9be5,{
    multi_a(
      2,
      tprv8btqJtFL5n1drr8jP95PBgTitQiCJyxDb27gGhbXM2vacGt3gV2Bfs5PHSBt6LbpPs5fUb4hGhD5TXudLrisjd14qtM8EZ3nVSv31jhLuop/*,
      415f59b2ce1ad8f714fd40b71865937dd911ad4742724cead5db936f73029d1e,1a3569fb509ababa09cf896c0a66619d0a01fd235d26b479a42503670beb9754
    ),
   {
     {
       and_v(
         v:older(4320),
         multi_a(
           2,
           ae3f20aee8d8798ceafb1671bf5607a45794676498e2edfac042958f7407ee1c,
           tpubD8asTJHaE9hJkKAXGnjyb67qTSE8UK98AKiTZDdpmJiySm8pJsqmrMhFTZV9cQyCdisA71G9bjGCZXcjXgFWCfjK5rfxVYmbnA5XikwzEHt/*
         )
       ),
       and_v(
         v:older(4320),
         multi_a(
           2,
           ae3f20aee8d8798ceafb1671bf5607a45794676498e2edfac042958f7407ee1c,
           415f59b2ce1ad8f714fd40b71865937dd911ad4742724cead5db936f73029d1e
         )
       )
    },
    {
      and_v(
        v:older(4320),
        multi_a(
          2,
          ae3f20aee8d8798ceafb1671bf5607a45794676498e2edfac042958f7407ee1c,
          1a3569fb509ababa09cf896c0a66619d0a01fd235d26b479a42503670beb9754
        )
      ),
      and_v(
        v:older(12960),
        pk(ae3f20aee8d8798ceafb1671bf5607a45794676498e2edfac042958f7407ee1c)
      )
    }
  }
})#vz6zss2j

has been reduced to:

tr(
  5a1e61f898173040e20616d43e9f496fba90338a39faa1ed98fcbaeee4dd9be5,
    {
        multi_a(
          2,
          tprv8btqJtFL5n1drr8jP95PBgTitQiCJyxDb27gGhbXM2vacGt3gV2Bfs5PHSBt6LbpPs5fUb4hGhD5TXudLrisjd14qtM8EZ3nVSv31jhLuop/*,
          415f59b2ce1ad8f714fd40b71865937dd911ad4742724cead5db936f73029d1e,
          1a3569fb509ababa09cf896c0a66619d0a01fd235d26b479a42503670beb9754
        ),
        rawnode(8a62dc0a100c4156cc2a4b7c2a97747ce0dfe90562673fd662678aaae93121fb)
    }
)

See test case verifying this

While it would seem that rawleaf offers no advantage since we have rawnode, sipa makes the argument that since the script and leaf version can be communicated via PSBTs, then descriptors should also be able to pass this information See Sipa's comment on 24114

There's an unanswered question about whether rawleaf(HEXSCRIPT,[HEXLEAFVER]) should just be raw() that takes an optional HEXLEAFVER under tr() but I prefer rawleaf to avoid overloading the raw() descriptor.

Implementation

I have created a PoC here wip-tr-raw-nodes, see diff. I created two new DescriptorImpl subclasses, RawNodeDescriptor and RawLeafDescriptor. Both are only valid under P2TR context and can be inferred by InferDescriptor.

Descriptor Inference

• If no taptree is inferred, and the Merkle root is not null, then tr(internal_key,rawnode(merkle_root)) is returned instead of the previous rawtr(tr_public_key). I am not sure if this should be the case. What advantage does inferring tr(internal_key,rawnode(merkle_root)) have over inferring rawtr(tr_public_key)?
• rawleaf is inferred after no standard scripts could be inferred under a P2TR context.