On Tue, Sep 4, 2018, 04:32 Alex Bosworth via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> I've been experimenting with a format tag for BIP 174 to help support
> HTLC scripts I've been working with.
>

I've been thinking about this as well.

A useful way to look at it IMHO is to see a hash as the analogue of a
public key, and the preimage as the analogue of a signature.

That would suggest adding two fields to PSBT:
* A request for the preimage of a given hash (similar to the pubkey/path
field currently)
* A revealed preimage for a given hash (similar to the partial signature
field currently).

The workflow would in this case would be:
* An updater recognizes an output/script as being one that requires a
preimage, and adds a preimage request field to the input (along with pubkey
fields for all involved keys).
* A "signer" who knows the preimage sees the request field, verifies he's
willing to divulge the secret, and adds a preimage field (along with any
signatures he may be able to create).
* A finalizer who is compatible with the type of hashlock script combines
all signatures and preimages into a final scriptSig/witness stack.

An obvious difficulty is having updaters and finalizers which are
compatible with all possible variations of hashlocks and other scripts.

Not sure on the best format for this, but what I have been thinking
> about is a new input type that defines elements that should be
> inserted in the final p2sh/p2wsh stack such as a preimage or a refund
> path flag.
>

That's one approach to reducing the complexity of the finalizer: adding
information about the composition of the scriptSig to the PSBT itself.
However, I don't think that approach scales very well (you'd need new
fields for all kinds of new script constructions). In particular, dealing
with multiple possible satisfactions may complicate things, especially when
the number of combinations is intractable.

I've been working on another approach that doesn't involve changes to PSBT,
but instead uses an easily-parsable subset of script (which includes
and/or/threshold/pubkey/locktimes/hashlocks). I hope to publish something
soon about it.

Cheers,

-- 
Pieter
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