Add SignedExtension to check Metadata Root Hash

[SBalaguer]

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Motivation

Offline signer experience in the Polkadot ecosystem can be massively improved. Today there is either too much manual, no scalable work (Ledger) or simply too much trust deposit on a third party (Polkadot Vault / Kampela).
The Zondax and Alzymologist team provided a way to solve these issues to the Polkadot Community on Ref #62. This gave way to the 'Metadata for Offline Signers' project that can be tracked here.

Related: #291

Request

The proposed solution is to maximize the usage of the Metadata so as to provide the right information to the offline signer. As this needs to be passed to the signer by some third party software, in order to avoid attacks the proposal is to add a new SignedExtension that would add the check of the Metadata Root Hash. This will make it so that any transaction sent from an offline device will include the root hash and this information will be then checked by the runtime.

As to how calculate this metadata root hash, there's a spec description and a reference implementation

Solution

No response

Are you willing to help with this request?

Yes!

[ggwpez]

Yea sounds good. Maybe could to for an RFC (but not sure if its a must here) https://github.com/polkadot-fellows/RFCs

[SBalaguer]

Yep, not super sure myself either! If need be down the line, happy to do it :)

[bkchr]

there's a spec

That is not really a spec ;)

[bkchr]

Having a RFC for the spec would be a good start IMO.

[SBalaguer]

@bkchr edit from spec to description to make it clearer. We'll move ahead then with the RFC first and then come back here.

[bkchr]

Ty! The extension will actually not be that complicated, more integrating this into the build process. However, even the integration to the build process will be easy against the metadata merkleization.

[bkchr]

Duplicate of: #291