TXID The hash of a transaction's data. A TXID (Transaction ID) is basically an identification number for a bitcoin transaction. Examples: f4184fc596403b9d638783cf57adfe4c75c605f6356fbc91338530e9831e9e16 - First ever Bitcoin transaction to Hal Finney in 2010.

- First ever Bitcoin transaction to Hal Finney in 2010. a1075db55d416d3ca199f55b6084e2115b9345e16c5cf302fc80e9d5fbf5d48d - Pizza transaction for 10,000 BTC in 2010.

- Pizza transaction for 10,000 BTC in 2010. 4ce18f49ba153a51bcda9bb80d7f978e3de6e81b5fc326f00465464530c052f4 - The transaction containing the first donation I received for making this website. A TXID is always 32 bytes (64 characters) and hexadecimal. Creating a TXID You get a TXID by hashing transaction data through SHA256 twice. 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 169e1e83e930853391bc6f35f605c6754cfead57cf8387639d3b4096c54f18f4 (Note: reverse the byte order first if you want to find this transaction in the blockchain…)

Searching for TXIDs in the blockchain. If you’ve just hashed some transaction data and want to search for a TXID in the blockchain, you have to search for it in reverse byte order. txid (original): 169e1e83e930853391bc6f35f605c6754cfead57cf8387639d3b4096c54f18f4 txid (searching): f4184fc596403b9d638783cf57adfe4c75c605f6356fbc91338530e9831e9e16 Why? Because welcome to Bitcoin. Due to historical accident, the tx and block hashes that bitcoin core uses are byte-reversed. I’m not entirely sure why. May be something like using openssl bignum to store hashes or something like that, then printing them as a number. – Wladimir van der Laan (Bitcoin Core developer) In other words, this was a slight oversight in the early development of Bitcoin that has now become a standard.

Where are TXIDs used?

1. Searching the blockchain.

If you’ve just made a transaction, you can use the TXID to find it in the blockchain. For example:

bitcoin-cli getrawtransaction 0e3e2357e806b6cdb1f70b54c3a3a17b6714ee1f0e68bebb44a74b1efd512098 getrawtransaction 0e3e2357e806b6cdb1f70b54c3a3a17b6714ee1f0e68bebb44a74b1efd512098

The console window in the Bitcoin Core Wallet

If you have been given a TXID by your bitcoin wallet, it’s probably already in its “searchable” format (reverse byte order).

2. Spending outputs.

You use a TXID when you want to use an existing output as an input in a new transaction.

To refer to an existing output, you use the txid it was created in, along with the vout number for that transaction.

Because after all, a TXID is a unique identifier1 for a transaction.

Notes

Why hash twice? Why not once? Things often get hashed twice in bitcoin for super-extra security. SHA-256(SHA-256(x)) was proposed by Ferguson and Schneier in their excellent book “Practical Cryptography” (later updated by Ferguson, Schneier, and Kohno and renamed “Cryptography Engineering”) as a way to make SHA-256 invulnerable to “length-extension” attack. They called it “SHA-256d”. –



https://crypto.stackexchange.com/questions/779/hashing-or-encrypting-twice-to-increase-security/884#884

Hash functions like SHA256 are great for creating identification numbers, because they will take in any string of data and always spit out a short yet unique result.