How is the previous transaction hash calculated?

Question

Can anyone provide a clear example on how to calculate a previous transaction hash?

Say I want to create a new transaction, I get that the hash is calculated by doubling sha256 a message m, but I'm having a hard time understanding the structure of m.

Answer 1

You do not need to calculate previous transaction hash, because it is already known. To spend the previous output you just have to know

• tx hash
• output index in this tx
• output value
• output script

plus you should choose

• fee
• new address

then create some byte data and sign it with appropriate private key

Good manual you can find here: How to redeem a basic Tx?

Answer 2

You hash the header. Format is here: https://en.bitcoin.it/wiki/Block_hashing_algorithm The body contains the transactions. Here's the Bitcoin transaction format: https://en.bitcoin.it/wiki/Transactions