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Subject: Re: [bitcoin-dev] BIP 174 thoughts
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Hi,=E2=80=8B
On July 4, 2018 6:19 AM, matejcik <jan.matejek@satoshilabs.com> wrote:
> =E2=80=8B=E2=80=8B
>=20
> hello,
>=20
> we still have some concerns about the BIP as currently proposed - not
>=20
> about the format or data contents, but more about strictness and
>=20
> security properties. I have raised some in the previous e-mails, but
>=20
> they might have been lost in the overall talk about format.
>=20
> - Choosing from duplicate keys when combining.
> =20
> We believe that "choose whichever value it wishes" is not a good
> =20
> resolution strategy. We propose to either change this to "in case of
> =20
> conflicts, software MUST reject the conflicting PSBTs", or explain in
> =20
> more detail why picking at random is a safe choice.
You cannot simply reject PSBTs for having conflicting values for the same k=
ey. Especially
for the Partial Signatures, you can have two signatures for the same pubkey=
that are both
completely valid. This situation could happen, for example, if a signer tha=
t does not use deterministic
k values can sign multiple inputs but one input is missing a UTXO so it doe=
sn't sign it. So it receives
one PSBT and signs the first input but not the second. It receives a PSBT =
for the same transaction
which has the second input's UTXO but does not have its signatures for the =
first input. The signer
would sign both inputs. When the two PSBTs are combined (suppose the first =
PSBT has other=20
signatures too), you will have two keys that have different values. The dif=
ferent values are both
valid signatures, just with different k values since they were randomly gen=
erated instead of
deterministically. If we fail to merge these, then you could potentially ha=
ve a situation where
nothing can be done with the PSBTs now, or now everyone has to resign and i=
n some specific
order to avoid the conflict. That complicates things and is much more annoy=
ing to deal with.
So a simple solution is to allow the combiner to choose any value it wants =
as it is likely that
both values are valid.
Allowing combiners to choose any value also allows for intelligent combiner=
s to choose the
correct values in the case of conflicts. A smart combiner could, when combi=
ning redeem scripts
and witness scripts, check that the redeem scripts and witness scripts matc=
h the hash provided
in the UTXO (or in the redeem script) and choose the correct redeem script =
and witness script
accordingly if there were, for some reason, a conflict there.
Can you explain why it would be unsafe for combiners to arbitrarily choose =
a value?
> =20
> - Signing records with unknown keys.
> =20
> There's been some talk about this at start, but there should be a cle=
ar
> =20
> strategy for Signers when unknown fields are encountered. We intend t=
o
> =20
> implement the rule: "will not sign an input with any unknown fields
> =20
> present".
> =20
> Maybe it is worth codifying this behavior in the standard, or maybe
> =20
> there should be a way to mark a field as "optional" so that strict
> =20
> Signers know they can safely ignore the unknown field.
I think that requiring there to be no unknowns is a safe change.
> =20
> And two minor points:
> =20
> - Fields with empty keys.
> =20
> This might be inferred from the definition, but is probably worth
> =20
> spelling out explicitly: If a field definition states that the key da=
ta
> =20
> is empty, an implementation MUST enforce this and reject PSBTs that
> =20
> contain non-empty data.
> =20
> We suggest adding something to the effect of:
> =20
> "If a key or value data in a field doesn't match the specified format=
,
> =20
> the PSBT is invalid. In particular, if key data is specified as "none=
"
> =20
> but the key contains data beyond the type specifier, implementation M=
UST
> =20
> reject the PSBT."
> =20
> (not sure about the languge, this should of course allow processing
> =20
> unknown fields)
Agreed.
> =20
> - "Combiner can detect inconsistencies"
> =20
> Added in response to this comment [1], the current wording looks like
> =20
> it's describing what the Combiner is capable of, as opposed to
> =20
> prescribing what the combiner is allowed to do.
> =20
> We suggest changing to something like:
> =20
> "For every field type that the Combiner understands, it MAY also refu=
se
> =20
> to combine PSBTs that have inconsistencies in that field, or cause a
> =20
> conflict when combined."
Agreed.
Andrew
|
