Why is the field 'hash' necessary in a bitcoin transaction?

2

Using bitcoin-cli decoderawtransaction I see results like...

{
  "txid": "d2183b3ea763f41a30ec5ba6fbf6e5f32e0ca6a5e8761249c533fe22190f71f1",
  "hash": "d2183b3ea763f41a30ec5ba6fbf6e5f32e0ca6a5e8761249c533fe22190f71f1",
  "version": 2,
  "size": 225,
  "vsize": 225,
   etc...

Why is the "hash" field necessary if the txid field is already a hash? (more specifically a double-pass through SHA-256 of the transaction's data)

I'm noticing in some transactions that txid and hash are equal, but in others they're not. Appreciate any explanation on the difference since I'm struggling to figure it out through Google and the source code comments.

d3wannabe

Posted 2019-07-19T01:38:52.000

Reputation: 159

Answers

4

Note that Bitcoin transactions don't actually contain the txid or hash within them. This is only part of the decoding output from Bitcoin Core.

The hash is there because of segwit. Segwit specifies that witness data for a segwit input (i.e. signatures, scripts) are not part of the txid. Those are in a separate area in the transaction. You can see this information decoded in the txinwitness field. So the hash is over the entire transaction, including this witness data, but the txid is not. Since transactions that do not have any segwit inputs do not have witness fields for their inputs, the txid and hash are the same.

Andrew Chow

Posted 2019-07-19T01:38:52.000

Reputation: 40 910

thank you - that makes sense regarding the hash field. But if txid is not included in the transaction, is it also not included in the vin/vout fields within a transaction? – d3wannabe – 2019-07-19T01:53:13.480

2No, the txids for the outputs being spent are included. Just the txid of the transaction itself is not since you can calculate the txid from it. – Andrew Chow – 2019-07-19T01:55:31.460

Asked: 2019-07-19T01:38:52.000

Viewed: 51 times

Active: 2019-07-19T01:46:23.040