Hi list,

I was looking at the bip174 PSBT specs, in particular for multisignature setup, and I think with current spec there is a way to steal user funds in M of N setup with M ≤ N/2.

I made a small write-up on this: 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 the keys derived from the same root keys as the inputs aside from the key owned by the signer => there is no way to verify that the output is a change output in multisig setup.

Therefore an attacker can replace half of the keys in the change address by his own keys and still get the transaction signed.

I suggest to add an xpub field to the inputs and outputs metadata, then signers can verify that the same xpubs are used for public keys in inputs and outputs => output is indeed a change.

Normally change and receiving addresses are derived from the same xpub with non-hardened derivation pathes, so providing xpub after the last hardened index should be enough to see that public keys of inputs and 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 = 0x10`

- Key: derivation path for xpub

  `{0x10}|{master key fingerprint}|{32-bit int}|...|{32-bit int}`

- Value: 78-byte xpub value

  `{xpub}`

Type: BIP 32 public key `PSBT_OUT_BIP32_XPUB = 0x03`

- Key: derivation path for xpub

  `{0x03}|{master key fingerprint}|{32-bit int}|...|{32-bit int}`

- Value: 78-byte xpub value

  `{xpub}`

Derivation paths are in the key of the key-value pair as they are used 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 this problem as they use xpubs to verify change outputs. So it may make sense to go through the communication protocols of existing hardware / multisignature wallets and see if there is something else we are missing. 

If everyone is happy about the proposal I would prepare a pull request to the bip.

Best regards,

Stepan Snigirev.