Date: Thu, 9 May 2019 22:08:47 +0500
From: Dmitry Petukhov <dp@simplexum.com>
To: jan matejek via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
Message-ID: <20190509220847.11ff9c33@simplexum.com>
In-Reply-To: <9e85b47c-6ba9-ab85-03f1-eb0ddf3022de@satoshilabs.com>
References: <CACL8y1v9fpZ+gWLVHMx-bGUCaSd0=0ecHU-u4FF=LnhT7s1zTg@mail.gmail.com>
	<20190503132945.GR810@coinkite.com>
	<CACL8y1tesev2OLrkfYfvmkgbR2xuk-0JPqdmYGtrUcser9GPfg@mail.gmail.com>
	<20190507184034.0a72a9c7@simplexum.com>
	<9e85b47c-6ba9-ab85-03f1-eb0ddf3022de@satoshilabs.com>
Organization: simplexum.com
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Subject: Re: [bitcoin-dev] Adding xpub field to PSBT to make multisig more
 secure
Precedence: list


> Therefore, the input==output check is sufficient: if I use the same
> set of signers for an input and an output, I can be sure that the
> change goes to the same multisig wallet.

This is sufficient, in a simple case.

I consider cases where spending from different wallets ('wallet
compartments') can be aggregated into one transaction, for efficiency
and convenience in certain circumstances. (ex: legacy addresses that
cannot be abandoned due to users still sending to them, but managing
them separately is inconvenient; wallet 'compartments' that each have
different multisig policies and spending priorities, and change would
go to most secure compartment used, etc.)

This is most likely a 'borader problem', as you said, but this is just
what my code already does, although with stateful signers that
store trusted xpubs. I had an idea how to apply this to stateless hw
wallets, and shared it.

> > 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.
> 
> This seems to be an attempt at a different, much broader problem. And
> it won't help if the attacker can replay a different trusted-xpub
> package (e.g., one that contains a revoked previously compromised
> key).

The auxiliary text can be constructed to include some code word that
would mark 'epoch' of the package, and will be displayed prominently.
Upon compromise, new trusted-xpub packages would use different 'epoch'
code word. This is one method to make it stateless (stateful way would
be to just have a counter inside hw wallet and check package version
against it).