Subject: Re: [bitcoin-dev] Proposal: Bitcoin Secure Multisig Setup

From: Hugo Nguyen <[email-masked]>
Date: Mon, 12 Apr 2021 10:55:36 -0700

Hello Salvatore,

On Mon, Apr 12, 2021 at 8:03 AM Salvatore Ingala <[email-masked]>
wrote:

> Hi Hugo,
>
> First of all, thank you for the impressive work on leading the
> standardization efforts!
>
> I believe one ought to more clearly distinguish the "Signer" (as in: one
> of the parties in the multisig setup), from the "*Signing device*" (which
> is likely a hardware wallet).
>

Actually, in the current spec, a "Signer" is *any software/hardware that
possesses the private keys and can sign using those keys* -- it doesn't
have to be hardware. "Signer" does not mean the human user. I will clarify
the definition and clear up any ambiguous language in the spec. Thanks for
bringing this to my attention!

> BSMS defines a "Signer" as "a participating member in the multisig",
> therefore a person/entity who is likely using both a hardware wallet and
> some BSMS-friendly software wallet (e.g. the next version of Specter
> Desktop).
>

As mentioned above, "Signer" does not refer to the user or any entity that
does not have the private keys / signing capability.

> It is therefore relevant to discuss which parts of the BSMS mechanism are
> implemented in the Signer's software wallet, and which should be in the
> Signer's hardware wallet.
> From the discussion, it appears to me that different people might have
> different expectations on what the signing device/HWW should do, so I would
> like to comment on this point specifically (while I reckon that it mostly
> falls within the realm of concerns #4 and #5 of the motivation paragraph,
> which are explicitly left out of scope).
>
> I fully agree that a *Signer* must persist the full wallet's description,
> and should also create physical backups which include the full descriptor
> and the cosigner's information. I would disagree, however, if any standards
> were to force *hardware wallets* to persist any substantial amount of
> state other than the seed, as I believe that it gives no substantial
> advantage over externally stored signed data for many use cases.
>
> The following is the *wallet registration flow* I am currently working on
> (in the context of adding support to multisig wallets at Ledger). The goal
> is to allow a *Signer* (the person) to persist a multisig setup in its
> storage, while achieving a similar level of security you would have if you
> were storing it on the hardware wallet itself (note that the following flow
> would happen as part of Round 2):
>
> 1) The desktop wallet of the requests the HWW to register a new multisig
> wallet. The request includes the full multisig wallet description, and some
> extra metadata (e.g.: a name to be associated to this multisig wallet).
> 2) The HWW validates the wallet and verifies it with the user with the
> trusted screen (as per BSMS Round 2); on confirmation, it returns a wallet
> id (which is a vendor-specific hash of all the wallet description +
> metadata) and signature
> 3) The desktop wallet stores the full wallet description/id/signature.
> (Optionally, a backup could be stored elsewhere).
>

> Whenever an operation related to the multisig wallet is required
> (verifying a receiving address, or signing a spending transaction), the HWW
> first receives and verifies all the data stored at step 3 above (without
> any user interaction). Then it proceeds exactly the same way as if it had
> always stored the multisig wallet in their own storage.
>

Now that we're clear on definitions, then it should become obvious that
redefining the "Coordinator-Signer" pair as "a Signer" does not address the
underlying problem. (What you call "the desktop wallet" here is a
Coordinator, not a Signer).

As long as the Signer does not own up the task of storing the wallet
configuration, it must rely indefinitely on others for critical data when
working in a multisig wallet, as I have explained in my last email.

Best,
Hugo