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From: matejcik <jan.matejek@satoshilabs.com>
To: Pieter Wuille <pieter.wuille@gmail.com>
Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: [bitcoin-dev]  BIP 174 thoughts
Date: Tue, 10 Jul 2018 14:10:10 +0200	[thread overview]
Message-ID: <797d9751-9795-55e6-35c9-61532e067d27@satoshilabs.com> (raw)
In-Reply-To: <CAPg+sBi1Rt_V1V0K50RN-c6wr8hW+5OYWx4aR-Kh8Dp-U0LLdA@mail.gmail.com>


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On 6.7.2018 00:06, Pieter Wuille wrote:> The only case where "malicious"
conflicting values can occur is when
> one of the Signers produces an invalid signature, or modifies any of
> the other fields already present in the PSBT for consumption by
> others. If this were an issue, it would be an issue regardless of the
> Combiner's operation, as in topology A no Combiner is even present.
> This is generally true I think - Combiners can always be replaced with
> just a different (and possibly less parallel) topology of data flow.

This is an interesting thesis, and also an unspoken assumption ISTM. It
seems worth adding something like this to the spec:
"""
In general, the result of Combiner combining two PSBTs from independent
participants A and B should be functionally equivalent to a result
obtained from processing the original PSBT by A and then B in a sequence.
or, for participants performing fA(psbt) and fB(psbt):
Combine(fA(psbt), fB(psbt)) == fA(fB(psbt)) == fB(fA(psbt))
"""

(...)

> The bottom line is that a Combiner which picks arbitrarily in case of
> conflicts will never end up with something worse than what you already
> need to deal with. If you disregard the case of invalid fields
> (because the result will just be an invalid transaction), then any
> choice the Combiner makes is fine, because all the values it can pick
> from are valid.

This sounds reasonable and IMHO it would be good to have a summary of
this argument in the Rationale section.


> If you're worried about attack surface, I don't believe rejecting
> invalid fields ever matters. An attacker can always drop the fields
> you don't understand before giving you the PSBT, making your behavior
> identical to one where you'd have ignore those fields in the first
> place.

Modifying the PSBT requires an active attacker. A passive attacker could
possibly sniff the invalid signatures and misuse them.
Where an active attacker can likely do more than drop fields.


In general, this comes down to a philosophical difference again. I'm
reluctant to sign an input with unknown data, on the premise that there
could be *anything* in that data; the fact that right now I can't come
up with a field that would be problematic does not mean that tomorrow
won't bring one. (in particular, a potential failure here is silent,
invisible to the user)

We are most likely to implement the "do not sign with unknown fields"
rule in any case (technically a whitelist of "known OK" field types),
and resolve potential problems as they arise. I raised this point mainly
because I think discussing this explicitly in the spec is beneficial: a
distinction between mandatory and optional fields is one way, mentioning
or prescribing possible signing strategies is another.

regards
m.


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  reply	other threads:[~2018-07-10 12:10 UTC|newest]

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-06-15 23:34 [bitcoin-dev] BIP 174 thoughts Pieter Wuille
2018-06-16 15:00 ` Peter D. Gray
2018-06-19  9:38 ` Jonas Schnelli
2018-06-19 14:20 ` matejcik
2018-06-19 15:20   ` Jonas Schnelli
2018-06-21 20:28     ` Peter D. Gray
2018-06-19 17:16   ` Pieter Wuille
2018-06-21 11:29     ` matejcik
2018-06-21 17:39       ` Pieter Wuille
2018-06-21 11:44     ` Tomas Susanka
2018-06-19 14:22 ` matejcik
2018-06-21  0:39 ` Achow101
2018-06-21 14:32   ` Tomas Susanka
2018-06-21 15:40     ` Greg Sanders
2018-06-21 19:56     ` Peter D. Gray
2018-06-21 21:39       ` Gregory Maxwell
2018-06-22 19:10       ` Pieter Wuille
2018-06-22 22:28         ` Achow101
2018-06-23 17:00           ` William Casarin
2018-06-23 20:33             ` Andrew Chow
2018-06-24  8:19               ` Andrea
2018-06-24  8:28                 ` Andrew Chow
2018-06-24  9:00                   ` Andrea
2018-06-23 18:27           ` Peter D. Gray
2018-06-25 19:47           ` Tomas Susanka
2018-06-25 20:10             ` Jonas Schnelli
2018-06-25 20:30             ` Achow101
2018-06-26 15:33               ` matejcik
2018-06-26 16:58                 ` William Casarin
2018-06-26 17:11                   ` Marek Palatinus
2018-06-27 14:11                   ` matejcik
2018-06-26 20:30                 ` Pieter Wuille
2018-06-27 14:04                   ` matejcik
2018-06-27 15:06                     ` Pieter Wuille
2018-06-29  9:53                       ` matejcik
2018-06-29 19:12                         ` Achow101
2018-06-29 20:31                           ` Peter D. Gray
2018-07-04 13:19                           ` matejcik
2018-07-04 18:35                             ` Achow101
2018-07-05 17:23                               ` Jason Les
2018-07-04 19:09                             ` Pieter Wuille
2018-07-05 11:52                               ` matejcik
2018-07-05 22:06                                 ` Pieter Wuille
2018-07-10 12:10                                   ` matejcik [this message]
2018-07-11 18:27                                     ` Pieter Wuille
2018-07-11 20:05                                       ` Gregory Maxwell
2018-07-11 20:54                                         ` [bitcoin-dev] BIP 174 thoughts on graphics vv01f
2018-06-26 21:56                 ` [bitcoin-dev] BIP 174 thoughts Achow101
2018-06-27  6:09                   ` William Casarin
2018-06-27 13:39                     ` Andrea
2018-06-27 17:55                     ` Achow101
2018-06-28 20:42                       ` Rodolfo Novak
2018-07-05 19:20                       ` William Casarin
2018-07-06 18:59                         ` Achow101
2018-06-20  0:39 Jason Les

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