We can’t “just compute the Transaction ID the same way the hash for signing the transaction is computed” because with different SIGHASH flags, there are 6 (actually 256) ways to hash a transaction.

Also, changing the definition of TxID is a hardfork change, i.e. everyone are required to upgrade or a chain split will happen.

It is possible to use “normalised TxID” (BIP140) to fix malleability issue. As a softfork, BIP140 doesn’t change the definition of TxID. Instead, the normalised txid (i.e. txid with scriptSig removed) is used when making signature. Comparing with segwit (BIP141), BIP140 does not have the side-effect of block size increase, and doesn’t provide any incentive to control the size of UTXO set. Also, BIP140 makes the UTXO set permanently bigger, as the database needs to store both txid and normalised txid

On 21 Nov 2017, at 1:24 AM, Praveen Baratam via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:

Bitcoin Noob here. Please forgive my ignorance.

From what I understand, in SegWit, the transaction needs to be serialized into a data structure that is different from the current one where signatures are separated from the rest of the transaction data.

Why change the format at all? Why cant we just compute the Transaction ID the same way the hash for signing the transaction is computed?

--
Dr. Praveen Baratam

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