Date: Wed, 01 May 2019 16:57:38 +0000
To: bitcoin-dev@lists.linuxfoundation.org
From: Andrew Chow <achow101-lists@achow101.com>
Reply-To: Andrew Chow <achow101-lists@achow101.com>
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Subject: Re: [bitcoin-dev] Adding xpub field to PSBT to make multisig more
	secure
Precedence: list

Hi Stepan,

I think that this would be a good extension.

Just for clairty, by xpub, do you mean the extended serialization format=20
defined in BIP 32 or the Base58 check encoded string of that serialization?

Andrew

On 4/26/19 11:21 AM, Stepan Snigirev via bitcoin-dev wrote:
> Hi list,
>
> I was looking at the bip174 PSBT specs, in particular for=20
> multisignature setup, and I think with current spec there is a way to=20
> steal user funds in M of N setup with M =E2=89=A4 N/2.
>
> I made a small write-up on this:=20
> https://github.com/stepansnigirev/random_notes/blob/master/psbt_multisig.=
md
>
> To compress:
>
> Currently in PSBT there is no way to reliably say if the output uses=20
> the keys derived from the same root keys as the inputs aside from the=20
> key owned by the signer =3D> there is no way to verify that the output=20
> is a change output in multisig setup.
>
> Therefore an attacker can replace half of the keys in the change=20
> address by his own keys and still get the transaction signed.
>
> I suggest to add an xpub field to the inputs and outputs metadata,=20
> then signers can verify that the same xpubs are used for public keys=20
> in inputs and outputs =3D> output is indeed a change.
>
> Normally change and receiving addresses are derived from the same xpub=20
> with non-hardened derivation pathes, so providing xpub after the last=20
> hardened index should be enough to see that public keys of inputs and=20
> change output are derived from the same xpub.
>
> I suggest to add the following key-value pairs to PSBT:
>
> Type: BIP 32 public key `PSBT_IN_BIP32_XPUB =3D 0x10`
> - Key: derivation path for xpub
> =C2=A0 `{0x10}|{master key fingerprint}|{32-bit int}|...|{32-bit int}`
> - Value: 78-byte xpub value
> =C2=A0 `{xpub}`
>
> Type: BIP 32 public key `PSBT_OUT_BIP32_XPUB =3D 0x03`
> - Key: derivation path for xpub
> =C2=A0 `{0x03}|{master key fingerprint}|{32-bit int}|...|{32-bit int}`
> - Value: 78-byte xpub value
> =C2=A0 `{xpub}`
>
> Derivation paths are in the key of the key-value pair as they are used=20
> for lookup, and xpub itself is the actual value being looked up.
>
> I also want to mention that Trezor for example doesn't suffer from=20
> this problem as they use xpubs to verify change outputs. So it may=20
> make sense to go through the communication protocols of existing=20
> hardware / multisignature wallets and see if there is something else=20
> we are missing.
>
> If everyone is happy about the proposal I would prepare a pull request=20
> to the bip.
>
> Best regards,
> Stepan Snigirev.
>