[Bitcoin-development] [BIP] Normalized Transaction IDs

Christian Decker decker.christian at gmail.com
Wed May 13 18:04:54 UTC 2015

If the inputs to my transaction have been long confirmed I can be
reasonably safe in assuming that the transaction hash does not change
anymore. It's true that I have to be careful not to build on top of
transactions that use legacy references to transactions that are
unconfirmed or have few confirmations, however that does not invalidate the
utility of the normalized transaction IDs.

The resource doubling is not optimal, I agree, but compare that to dragging
around malleability and subsequent hacks to sort-of fix it forever.
Additionally if we were to decide to abandon legacy transaction IDs we
could eventually drop the legacy index after a sufficient transition period.

I remember reading about the SIGHASH proposal somewhere. It feels really
hackish to me: It is a substantial change to the way signatures are
verified, I cannot really see how this is a softfork if clients that did
not update are unable to verify transactions using that SIGHASH Flag and it
is adding more data (the normalized hash) to the script, which has to be
stored as part of the transaction. It may be true that a node observing
changes in the input transactions of a transaction using this flag could
fix the problem, however it requires the node's intervention.

Compare that to the simple and clean solution in the proposal, which does
not add extra data to be stored, keeps the OP_*SIG* semantics as they are
and where once you sign a transaction it does not have to be monitored or
changed in order to be valid.

There certainly are merits using the SIGHASH approach in the short term (it
does not require a hard fork), however I think the normalized transaction
ID is a cleaner and simpler long-term solution, even though it requires a


On Wed, May 13, 2015 at 7:14 PM Pieter Wuille <pieter.wuille at gmail.com>

> Normalized transaction ids are only effectively non-malleable when all
> inputs they refer to are also non-malleable (or you can have malleability
> in 2nd level dependencies), so I do not believe it makes sense to allow
> mixed usage of the txids at all. They do not provide the actual benefit of
> guaranteed non-malleability before it becomes disallowed to use the old
> mechanism. That, together with the +- resource doubling needed for the UTXO
> set (as earlier mentioned) and the fact that an alternative which is only a
> softfork are available, makes this a bad idea IMHO.
> Unsure to what extent this has been presented on the mailinglist, but the
> softfork idea is this:
> * Transactions get 2 txids, one used to reference them (computed as
> before), and one used in an (extended) sighash.
> * The txins keep using the normal txid, so not structural changes to
> Bitcoin.
> * The ntxid is computed by replacing the scriptSigs in inputs by the empty
> string, and by replacing the txids in txins by their corresponding ntxids.
> * A new checksig operator is softforked in, which uses the ntxids in its
> sighashes rather than the full txid.
> * To support efficiently computing ntxids, every tx in the utxo set
> (currently around 6M) stores the ntxid, but only supports lookup bu txid
> still.
> This does result in a system where a changed dependency indeed invalidates
> the spending transaction, but the fix is trivial and can be done without
> access to the private key.
> On May 13, 2015 5:50 AM, "Christian Decker" <decker.christian at gmail.com>
> wrote:
>> Hi All,
>> I'd like to propose a BIP to normalize transaction IDs in order to
>> address transaction malleability and facilitate higher level protocols.
>> The normalized transaction ID is an alias used in parallel to the current
>> (legacy) transaction IDs to address outputs in transactions. It is
>> calculated by removing (zeroing) the scriptSig before computing the hash,
>> which ensures that only data whose integrity is also guaranteed by the
>> signatures influences the hash. Thus if anything causes the normalized ID
>> to change it automatically invalidates the signature. When validating a
>> client supporting this BIP would use both the normalized tx ID as well as
>> the legacy tx ID when validating transactions.
>> The detailed writeup can be found here:
>> https://github.com/cdecker/bips/blob/normalized-txid/bip-00nn.mediawiki.
>> @gmaxwell: I'd like to request a BIP number, unless there is something
>> really wrong with the proposal.
>> In addition to being a simple alternative that solves transaction
>> malleability it also hugely simplifies higher level protocols. We can now
>> use template transactions upon which sequences of transactions can be built
>> before signing them.
>> I hesitated quite a while to propose it since it does require a hardfork
>> (old clients would not find the prevTx identified by the normalized
>> transaction ID and deem the spending transaction invalid), but it seems
>> that hardforks are no longer the dreaded boogeyman nobody talks about.
>> I left out the details of how the hardfork is to be done, as it does not
>> really matter and we may have a good mechanism to apply a bunch of
>> hardforks concurrently in the future.
>> I'm sure it'll take time to implement and upgrade, but I think it would
>> be a nice addition to the functionality and would solve a long standing
>> problem :-)
>> Please let me know what you think, the proposal is definitely not set in
>> stone at this point and I'm sure we can improve it further.
>> Regards,
>> Christian
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