[Bitcoin-development] [BIP] Normalized Transaction IDs

[Christian Decker]

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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