how to create txid from transaction.json

Question

Consider this is the transaction.json file:

{
  "version": 1,
  "locktime": 0,
  "vin": [
    {
      "txid": "5d1ad7d75b89e7051f17254f7e7342d70fcb502539ddb2ac1b97d5710d5fec37",
      "vout": 1,
      "prevout": {
        "scriptpubkey": "76a9140a7a059306f69dec3d4658c618d4dd33a4b288bb88ac",
        "scriptpubkey_asm": "OP_DUP OP_HASH160 OP_PUSHBYTES_20 0a7a059306f69dec3d4658c618d4dd33a4b288bb OP_EQUALVERIFY OP_CHECKSIG",
        "scriptpubkey_type": "p2pkh",
        "scriptpubkey_address": "1xPw97SFdG39TyTon7ZFeon7SgEa77HPa",
        "value": 167972
      },
      "scriptsig": "473044022073cb5faf80c08efe1dc3b2bfbabe93f42ab16aeabe3c091ddfc795a30033ee74022017ac1f308e6ec9eb8360c181f11ebfb0f23d1554baeae47a02628f4ef778a2b7012103f498484d89d98539e4260c378584f3fc41f63e627c86b7001d0845d374d80991",
      "scriptsig_asm": "OP_PUSHBYTES_71 3044022073cb5faf80c08efe1dc3b2bfbabe93f42ab16aeabe3c091ddfc795a30033ee74022017ac1f308e6ec9eb8360c181f11ebfb0f23d1554baeae47a02628f4ef778a2b701 OP_PUSHBYTES_33 03f498484d89d98539e4260c378584f3fc41f63e627c86b7001d0845d374d80991",
      "is_coinbase": false,
      "sequence": 4294967295
    }
  ],
  "vout": [
    {
      "scriptpubkey": "76a91437ca544d6159d8602780b854f196637f94a734fa88ac",
      "scriptpubkey_asm": "OP_DUP OP_HASH160 OP_PUSHBYTES_20 37ca544d6159d8602780b854f196637f94a734fa OP_EQUALVERIFY OP_CHECKSIG",
      "scriptpubkey_type": "p2pkh",
      "scriptpubkey_address": "165zYdxqvNELWdGfrSM6yrqkqBEXKeTZ5A",
      "value": 164132
    }
  ]
}

How can we calculate the txid from this json file?

Answer 1

Calculating TxID

1. convert data into Bitcoin's pre-segwit network serialisation. Taking care not to make any of the most common mistakes:

   • Encode counts/lengths using the Bitcoin variable-length Compact-Integer format (sometimes called VarInt) and
   • Take into account Bitcoin's inconsistent and non-standard use of Endianness (byte order).
   • Never convert to, or use, hexadecimal string representations (until you have a final TxId result that you want to present that way). Because f("B5") ≠ f(0xB5)

2. Calculate SHA256(SHA256(serialised-data)).

Example program

TxID  88877248B629DA0DF5A8E31FD0D1FBECAFE48A3D1FC65EAFF141DDD9BDB3361A

---

A note about Segregated Witness (Segwit)

When Segwit was introduced, it was designed to be backwards compatible with pre-Segwit nodes. So, when sending data to nodes that don't signal Segwit support, Segwit nodes send them data in the pre-Segwit network serialisation, not the Segwit serialisation used for sending data to Segwit nodes. To ensure that both Segwit and pre-Segwit nodes use the same TxID, the pre-Segwit serialisation was used to calculate the TxID even for Segwit transactions. This is what is always used in transaction input references. This makes sure that both Segwit and pre-Segwit nodes maintain the same view of Bitcoin network state, the same view of the UTXO set.

There is a separate WTxID based on a hash of the Segwit serialisation. As BIP 141 says:

Transaction ID

A new data structure, witness, is defined. Each transaction will have 2 IDs.

Definition of txid remains unchanged: the double SHA256 of the traditional serialization format: