[bitcoindev] Mapping the design space of sibling-aware covenants

[Josh Doman <joshsdoman@gmail.com>]

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Like many others, I'm interested in the OP_CTV and OP_TEMPLATEHASH 
proposals. One subtle difference is that OP_TEMPLATEHASH does not enable 
sibling-aware covenants, like those used in BitVM, Ark, and (potentially) 
LN-Symmetry.[1] This requires a connector output and a sibling prevout 
commitment. This way, additional conditions can be added to the covenant, 
or it can be invalidated altogether.

Personally, I find unsatisfactory the method by which OP_CTV achieves this 
(by committing to the scriptSigs), and I appreciate the simplicity of 
OP_TEMPLATEHASH (TH) and the fact that it requires no additional 
pre-computation.

The purpose of this post isn't to suggest a modification to TH but to 
attempt to list all the ways sibling-aware covenants could be achieved. I 
found this helpful to understand the tradeoffs that surround this 
capability.

The first five options entail an additional commitment to TH:
1.* scriptSigs*: precompute a hash of all scriptSigs.
2. *Dynamic MuHash*: precompute a MuHash commitment to all indexed prevouts 
and then remove the prevout at the current index.
3. *All-but-first*: restrict execution to the first input and commit to all 
other prevouts.
4. *First-only*: commit to the prevout at the first index, if the current 
index is non-zero.
5. *Next-only*: commit to the prevout at the next index, if present.

An alternative approach is to create a new witness program, which 
indirectly enforces a sibling prevout commitment:
6. * Pay-to-Prevout-Anchor (P2PA):* an anyone-can-spend output that *cannot 
be created *unless the output's program is the hash of the prevout at the 
same index in that transaction.

The final options involve new introspection opcodes (not exhaustive):
7. *OP_TXHASH*: a general introspection opcode that can push a hash that 
commits to one or more prevouts at specific indices.
8. *OP_CAT*: a simple opcode that enables introspection via complex script.

I won't go into the pros and cons of each option, except to say that I 
think Option 5 strikes a nice balance between minimalism, pragmatism, and 
flexibility.[2]

Mostly, I wanted to list what options I think exist. If I missed any, 
please let me know!

Best,
Josh

*[1] Eliminating the 2x-delay problem in LN-Symmetry requires a 
contract-level relative timelock (CLRT) and a sibling-aware covenant. CLRT 
capabilities remain an active area of research and may not exist in the 
first iteration of LN-Symmetry.*

*See: https://delvingbitcoin.org/t/contract-level-relative-timelocks-or-lets-talk-about-ancestry-proofs-and-singletons/1353 
<https://delvingbitcoin.org/t/contract-level-relative-timelocks-or-lets-talk-about-ancestry-proofs-and-singletons/1353>*

*[2] Option 5 (Next-Only) strikes me as a balanced option for several 
reasons:*
*- Identical to TH in the common single-input use case.*
*- Solves for the two-input sibling-aware covenant common to L2 protocols.*
*- Leaves the door open to multi-input covenants with no sibling 
commitments.*
*- Requires no additional pre-computation.*

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