Blinded PSET incompatibility with elementsd

[pavel-kokolemin]

When I create and blind PSET with two or more outputs the result can't be loaded in elementsd:

elements-cli analyzepsbt $(cat pset.txt)
error code: -22
error message:
TX decode failed Size of value was not the stated size: iostream error

I'm attaching small rust program to reproduce the issue and resulted PSET:
pset.txt
pset_test.zip
Same program generates valid PSET when there is only one output.

I tested with latest release (0.21.0) and latest dev version of elements (commit 04cedf3f6be9e8eeb76af6206656ad2516d16f5a) and
latest rust-elements (commit 53d5729f8cdc3623f9f110edba86aeaa9059af5b).
I don't know if the issue on elements or rust-elements side.

Thank you for the very good library!

[sanket1729]

Hello Pavel. First off, thanks for the minimal stand alone example. This really helped me find out what the problem is. There are a couple of issues here.

First bug: there is a bug in elementsd (c++ codebase) global scalar decoding that is causing the issue here.

Second bug: In rust-elements, the scalars must be cleared after the blinding is done. This is not being done currently and is updated in #100.

I just tested your code by replacing dependencies with

elements = { package = "elements", git = "https://github.com/sanket1729/rust-elements", branch = "update_pset" }

and I am able to analyzepsbt with it.

This means that currently:

• Semi-blinded psets (ones after blind(), but not blind_last()) are correctly implemented in rust-elements, but not in elementsd
• Fully blinded psets(ones after calling blind_last()) are incorrectly implemented rust-elements, but will be fixed as of Update pset spec #100. There are a few other updates to the spec, so I suggest prototyping with Update pset spec #100 till we have another release.

So, if you are completely blinding using rust-elements, I think #100 should solve all your issues.

[pavel-kokolemin]

Works nicely now. Thank you, Sanket!