[Bitcoin-development] [RFC] [BIP proposal] Dealing with malleability

Mike Hearn mike at plan99.net
Thu Feb 20 14:08:59 UTC 2014

We've done forking changes before much faster than a year and that was with
less experience. If we want, we can get this done within months.
On 20 Feb 2014 16:30, "Michael Gronager" <gronager at mac.com> wrote:

> As I see the BIP it is basically stressing that ver 1 transactions are
> malleable.
> It then addresses the need for unmalleable transactions for e.g. spending
> unconfirmed outputs in a deterministic way (i.e. no 3rd party can sabotage)
> - this transaction type is defined as ver 3.
> A lot of clients today spend unconfirmed outputs (even bitcoin-qt) and as
> such make an implicit assumption that this is kind of safe, which it is not
> - it can be intervened and sabotaged through tx malleability.
> What I suggested was to ensure that a subclass of version 1 transactions
> become unmalleable - namely those with sighash=all. Note that only the
> sender can modify the sighash as it is part of the hash signed. So instead
> of defining a version 3, we could constrain version 1 txes with sighash=all
> to have a unmalleable hash. If you e.g. would like to still have a
> sighash=all type of transaction with malleable features you can simply use
> that sighash=all today is checked for using sighash&0x1f=sighash_all, so
> just OR'ing with 0x20 or 0x40 will get you the 'old' feature.
> I do however buy the argument of Peter and Gregory that there might exist
> unpublished transactions out there that does not even conform to the DER
> rules etc, and as such we cannot forbid them from being mined, nor can we
> timestamp them and include 'only the old ones'. Hence we cannot change the
> consensus rule for version 1 transactions - and only changing the relay
> rules will not provide a certain guarantee.
> So, I think the two line argument for the BIP is as follows:
> 1. We cannot change the consensus rules for version 1 transactions as that
> might invalidate unpublished non-standard transactions (= voiding peoples
> money, which is a line we don't want to cross)
> 2. The prime usecase for unmalleable transactions is being able to spend
> unconfirmed outputs - this is done today by almost all clients, but it is
> really broken. Hence a need for a fix asap.
> I am all in favor for the BIP, but I expect the realistic timeline for
> enforced version 3 transactions is roughly one year, which is better than
> two, but it would have been nice to get it faster...
> /M
> On Feb 19, 2014, at 10:11 PM, Pieter Wuille <pieter.wuille at gmail.com>
> wrote:
> > On Wed, Feb 19, 2014 at 9:28 PM, Michael Gronager <gronager at mac.com>
> wrote:
> >> I think that we could guarantee fewer incidents by making version 1
> transactions unmalleable and then optionally introduce a version 3 that
> supported the malleability feature. That way most existing problematic
> implementations would be fixed and no doors were closed for people
> experimenting with other stuff - tx v 3 would probably then be called
> experimental transactions.
> >
> > Just to be clear: this change is not directly intended to avoid
> > "incidents". It will take way too long to deploy this. Software should
> > deal with malleability. This is a longer-term solution intended to
> > provide non-malleability guarantees for clients that a) are upgraded
> > to use them  b) willing to restrict their functionality. As there are
> > several intended use cases for malleable transactions (the sighash
> > flags pretty directly are a way to signify what malleabilities are
> > *wanted*), this is not about outlawing malleability.
> >
> > While we could right now make all these rules non-standard, and
> > schedule a soft fork in a year or so to make them illegal, it would
> > mean removing potential functionality that can only be re-enabled
> > through a hard fork. This is significantly harder, so we should think
> > about it very well in advance.
> >
> > About new transaction and block versions: this allows implementing and
> > automatically scheduling a softfork without waiting for wallets to
> > upgrade. The non-DER signature change was discussed for over two
> > years, and implemented almost a year ago, and we still notice wallets
> > that don't support it. We can't expect every wallet to be instantly
> > modified (what about hardware wallets like the Trezor, for example?
> > they may not just be able to be upgraded). Nor is it necessary: if
> > your software only spends confirmed change, and tracks all debits
> > correctly, there is no need.
> >
> > --
> > Pieter
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