Subject: Re: [bitcoin-dev] [BIP] Normalized transaction IDs

From: Christian Decker <[email-masked]>
Date: Tue, 03 Nov 2015 21:44:02 +0000

On Tue, Nov 3, 2015 at 9:49 PM Luke Dashjr <[email-masked]> wrote:

> On Tuesday, November 03, 2015 8:37:44 PM Christian Decker wrote:
> > I am still very much intrigued by Luke's idea of having empty scriptsigs
> > and ship the signatures in external scripts, however the proposal uses
> the
> > on-the-fly normalization because we have no good way of relaying the
> > external scripts. Since we are still in the drafting phase I am open to
> > suggestions and if there is a good/working solution I can amend/withdraw
> > the proposal.
>
> Changing the network protocol is trivial in comparison to making a
> permanent
> increase in UTXO set costs.
>

Ok, so assuming we can get a connected component of upgraded nodes that
relay both the transaction and the associated external scripts then we
could just piggyback the external scripts on top of the normal messages.
Non-upgraded nodes will read the entire two-part message but only parse the
classical transaction, dropping the external script. Validation rules for
upgraded nodes are the same as before: if the attached signatures are
invalid the entire TX is dropped. We have to commit to the external scripts
used during the creation of a block. I think the easiest way to add this
commitment is the coinbase input I guess, and following the transaction
list a new list of signature lists is shipped with the rest of the block.
Non-upgraded will ignore it as before.

Would that work? It all hinges on having upgraded miners in a connected
component otherwise non-upgraded nodes will drop the external scripts on
the way (since they parse and then reconstruct the messages along the
path). But if it works this could be a much nicer solution.

>
> > As for open venues for malleability, I'm not sure we can fix them at all,
> > after all the ability of a single signer to doublespend by
> > appending/replacing inputs/outputs in an arbitrary fashion is not fixable
> > IMHO and will cause any future transaction building on its outputs to be
> > orphaned. What would the perfect properties for such a fix be?
>
> The problem isn't changing inputs/outputs, but that such changes invalidate
> later spends. In particular, note that wallets *should ideally* be actively
> trying to make transfers using multiple malleated versions of the same
> payment.
>

So this is indeed a form of desired malleability we will likely not be able
to fix. I'd argue that this goes more into the direction of double-spending
than a form of malleability, and is mostly out of scope for this BIP. As
the abstract mentions this BIP attempts to eliminate damage incurred by
malleability in the third party modification scenario and in the multisig
scenario, with the added benefit of enabling transaction templating. If we
can get the segregated witnesses approach working all the better, we don't
even have the penalty of increased UTXO size. The problem of singlesig
users doublespending their outputs to update transactions remains a problem
even then.

>
> So the way to make an anti-malleable wallet, would be to strictly enforce
> the
> no-address-reuse rule on payments received (note this has no effect on
> other/current wallets) and rely only on the hash of that scriptPubKey+value
> for the input in subsequent transactions. This way, no matter what inputs
> or
> other outputs the transaction paying the address/invoice uses, the
> subsequent
> transaction ignores them and remains valid. (I am not suggesting this as a
> mandatory change that all wallets must adopt to receive the current semi-
> malleability protection you propose - only that it be *possible* for
> wallets
> to upgrade to or offer in the future.)
>

Sounds very interesting. That would then be a new signature checking opcode
I guess that would allow the transaction hash in the input be replaced by
the hash of the serialized output it is spending? That way the transaction
would not be detached from the coins unless the amount or the scriptpubkey
(containing the address) is modified. So a user may add new outputs and
inputs to an existing transaction like you mentioned. This does not help
someone receiving funds from a sender to build new transactions on top
since the sender may simply doublespend its output before it is confirmed.
I think this is probably best addressed in a separate proposal.

> Luke