Re: [bitcoin-dev] Adding xpub field to PSBT to make multisig more secure

[Dmitry Petukhov <dp@simplexum.com>]

> > Even with this additions to the PSBT format, I think PSBT-signing
> > devices still need to store the xpubs of their co-signers. It's not
> > possible to safely show an incoming address to the user without a
> > full understanding of the other keys in a "multisig wallet". Also,
> > it represents data that should not change between PSBT instances
> > (ie. tomorrow's co-signers should match today's).
> 
> I would like to keep hardware wallets state-less, otherwise wiping and
> recovering them would be problematic.
> At the setup phase the user can verify a multisignature address (or
> xpub) on the screens of all devices,
> after that we just need to verify that xpubs in the inputs and in the
> change output are the same.

At the setup phase, hardware wallet can sign a message that consists of
xpubs of participants, and some auxiliary text. It can use the key
derived from the master key, with path chosen specifically for this
purpose.

Hardware wallet then gives out this signature to the software.

The software will store the message and the signature (or maybe it can
take xpubs from PSBT), and will send this 'trusted-xpub-package' to
hardware wallet along with the transaction.

Hardware wallet can then verify that the message is indeed signed by
the key for that purpose, and then can mark the ouputs that use the
pubkeys derived from 'verified' xpubs as 'trusted' outputs. It can also
display the auxiliary text along with the information about the
'trusted' output.

This way, hardware wallet does not need to store anything extra besides
the master key.

This would allow to distinguish the trusted output even if the inputs
are not all derived from the same set of xpubs, that could happen in
more complex scenarios (batching, key rotation, etc.), and can possibly
be used to have several different types of 'trusted' outputs.

If the user loses the signature for trusted-xpub-package, the signature
can be re-created again - but maybe the procedure should be more
involved than ordinary signing, because creating creating such
'trusted-xpub-package' with malicious keys can enable attackers to
bypass these checks.