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

From: Christian Decker <[email-masked]>
Date: Wed, 21 Oct 2015 07:39:45 +0000

On Wed, Oct 21, 2015 at 8:19 AM Luke Dashjr <[email-masked]> wrote:

> On Monday, October 19, 2015 2:01:04 PM Christian Decker via bitcoin-dev
> wrote:
> > The proposal is implemented (see below), by computing the normalized
> > transaction ID when adding them to the UTXO and storing them along with
> the
> > coin state. OP_CHECKSIGEX mostly duplicates OP_CHECKSIG and
> > OP_CHECKMULTISIG, but I'm hoping somebody can give me some pointers into
> > how to best refactor the common functionality into reusable blocks. And
> the
> > annotating incoming transactions with their normalized inputs is a bit
> > cumbersome, maye somebody has some pointers here as well?

> 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?

>
> Additionally, you have a flag to control whether the opcode behaves as
> VERIFY
> or not. Non-VERIFY is not possible as a softfork (without doing a
> second/new
> P2SH) since it can be negated.
>

Yes, this is my mistake and has been pointed out in the PR, I will amend
the PR to make the verify flag mandatory, which also guarantees that the
top of the stack contains a non-null element, thus resulting in a
successful evaluation on non-updated clients.

> Luke