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Date: Thu, 5 Nov 2015 16:27:37 +0100
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From: =?UTF-8?B?Sm9yZ2UgVGltw7Nu?= <jtimon@jtimon.cc>
To: Luke Dashjr <luke@dashjr.org>
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Cc: Bitcoin Dev <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] [BIP] Normalized transaction IDs
Precedence: list

On Tue, Nov 3, 2015 at 11:01 PM, Luke Dashjr via bitcoin-dev
<bitcoin-dev@lists.linuxfoundation.org> wrote:
> On Tuesday, November 03, 2015 9:44:02 PM Christian Decker wrote:
>> 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.
>
> I don't know what you're trying to say here. Double spending to the same
> destination(s) and malleability are literally the same thing. Things affected
> by malleability are still just as broken even with this BIP - whether it is
> triggered by a third-party or not is not very relevant.

I think this is just a terminology confusion.
There's conflicting spends of the same outputs (aka unconfirmed
double-spends), and there's signature malleability which Segregated
Witnesses solves.
If we want to define malleability as signature malleability +
conflicting spends, then that's fine.
But it seems Christian is mostly interested in signature malleability,
which is what SW can solve.
In fact, creating conflicting spends is sometimes useful for some
contracts (ie to cancel the contract when that's supposed to be
allowed).
Maybe it is "incorrect" that people use "malleability" when they're
specifically talking about "signature malleability", but I think that
in this case it's clear that we're talking about transactions having
an id that cannot be changed just by signing with a different nonce
(what SW provides).

Please, Christian, correct me if you mean something else.