From: Luke Dashjr <luke@dashjr.org>
To: Christian Decker <decker.christian@gmail.com>
Date: Wed, 21 Oct 2015 08:39:41 +0000
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Cc: bitcoin-dev@lists.linuxfoundation.org
Subject: Re: [bitcoin-dev] [BIP] Normalized transaction IDs
Precedence: list

On Wednesday, October 21, 2015 8:31:42 AM Christian Decker wrote:
> On Wed, Oct 21, 2015 at 9:52 AM Luke Dashjr <luke@dashjr.org> wrote:
> > On Wednesday, October 21, 2015 7:39:45 AM Christian Decker wrote:
> > > On Wed, Oct 21, 2015 at 8:19 AM Luke Dashjr <luke@dashjr.org> wrote:
> > > > This doesn't completely close malleability (which should be
> > > > documented
> > 
> > in
> > 
> > > > the BIP), so I'm not sure it's worth the cost, especially if closing
> > > > malleability later on would need more. How about specifying flags
> > 
> > upfront
> > 
> > > > in the UTXO-creating transaction specifying which parts the signature
> > > > will cover? This would allow implementation of fully
> > > > malleability-proof wallets.
> > > 
> > > As far as I see it the only remaining venues for malleability are the
> > > use of sighash flags that are not SIGHASH_ALL, as mentioned in the
> > > BIP. Any
> > 
> > use
> > 
> > > of non-sighash_all flags is already an explicit permission to modify
> > > the transactions, by adding and removing inputs and outputs, so I
> > > don't see
> > 
> > how
> > 
> > > these can be made non-malleable. Am I missing something?
> > 
> > Signer malleability is still a notable concern needing consideration.
> > Ideally,
> > wallets should be trying to actively CoinJoin, bump fees on, etc any
> > pending
> > transactions in the background. These forms of malleability affect nearly
> > as
> > many real use cases as third-party malleability.
> > 
> > Luke
> 
> How is signer malleability still a problem if we remove the signatures from
> the transaction ID of the transaction and all preceding transactions? The
> signer can re-sign a transaction but it won't change the transaction ID.

The signer can also change the order of the inputs, the inputs themselves, 
add/remove outputs, etc... all which should be possible without becoming a 
different logical transaction. The only unique property of the logical 
transaction is the scriptPubKey/address.

Luke