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Date: Wed, 12 Feb 2014 10:03:21 -0800
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From: Gregory Maxwell <gmaxwell@gmail.com>
To: =?UTF-8?Q?Rune_Kj=C3=A6r_Svendsen?= <runesvend@gmail.com>
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Cc: Bitcoin Dev <bitcoin-development@lists.sourceforge.net>
Subject: Re: [Bitcoin-development] [RFC] [BIP proposal] Dealing with
	malleability
Precedence: list

On Wed, Feb 12, 2014 at 7:12 AM, Rune Kj=C3=A6r Svendsen <runesvend@gmail.c=
om> wrote:
> Instead of trying to remove the possibility of transaction
> malleability, would it make sense to define a new, "canonical
> transaction hash/ID" (cTxID), which would be a hash of the part of the
> transaction data which we know is not malleable, and have clients use
> this cTxID internally, thus making the traditional transaction hash
> irrelevant for a client to function correctly?

This is fine and good. But it only scratches the surface of the
problems created by malleability, especially for fancier transaction
protocols.

Mutation allows you to invalidate a chain of unconfirmed transaction
by mutating the parent. This breaks any protocol which depends on
creating a precomputed nlocked time refund transaction.

So a canonical ID can be used to prevent some buggy behavior it
doesn't actually fix the problem. Fortunately the non-fixed parts
aren't too critical today.

On Wed, Feb 12, 2014 at 8:22 AM, Alan Reiner <etotheipi@gmail.com> wrote:
> I think the solution is simply to encourage Bitcoin software developers t=
o
> design their software to use this static ID, instead of the full transact=
ion
> hash.    If MtGox had talked those IDs instead of the TX ID, their softwa=
re
> would've correctly identified the mutated transactions and there would be
> no problem.

This is incorrect.  MtGox was automatically issuing replacement
transactions resulting in double payments.

When you attempt to replace/reissue/cancel a transaction you __MUST__
double-spend the original transaction. If the original transaction has
not been conflicted then it is possible someone will pull the original
transaction out of a hat and both your replacement and the original
will be confirmed.  It is not safe at any time to look to see if the
original has been confirmed yet, and if not reissue=E2=80=94 not because
mutation may mean you're looking in the wrong place=E2=80=94 but because th=
e
state of the world could change nano-seconds after you looked.

If you do double-spend the original then there is no chance that both
will go through, you'll have atomic exclusion and only one transaction
or the other will be confirmed.