On Tue, May 19, 2015 at 9:28 AM, Christian Decker <decker.christian@gmail.com> wrote:

Thanks Stephen, I hadn't thought about BIP 34 and we need to address this in both proposals. If we can avoid it I'd like not to have one transaction hashed one way and other transactions in another way.

The normalized TXID cannot depend on height for other transactions. Otherwise, it gets mutated when been added to the chain, depending on height.

An option would be that the height is included in the scriptSig for all transactions, but for non-coinbase transctions, the height used is zero.

I think if height has to be an input into the normalized txid function, the specifics of inclusion don't matter.

The previous txid for coinbases are required to be all zeros, so the normalized txid could be to add the height to the txids of all inputs. Again, non-coinbase transactions would have heights of zero.

Is there a specific reason why that was not chosen at the time?

I assumed that since the scriptSig in the coinbase is specifically intended to be "random" bytes/extra nonce, so putting a restriction on it was guaranteed to be backward compatible.