TXID

The hash of a transaction ‘s data .

A TXID ( Transaction ID ) is basically an recognition number for a bitcoin transaction .

Examples:

• f4184fc596403b9d638783cf57adfe4c75c605f6356fbc91338530e9831e9e16 – First ever Bitcoin transaction to Hal Finney in 2010.
• a1075db55d416d3ca199f55b6084e2115b9345e16c5cf302fc80e9d5fbf5d48d – Pizza transaction for 10,000 BTC in 2010.
• 4ce18f49ba153a51bcda9bb80d7f978e3de6e81b5fc326f00465464530c052f4 – The transaction containing the first donation I received for making this website.

A TXID is constantly 32 bytes ( 64 characters ) and hexadecimal .

Creating a TXID

You get a TXID by hashing transaction data through SHA256 doubly.

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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 texas and block hashes that bitcoin congress of racial equality uses are byte-reversed. I ’ meter not entirely certain why. May be something like using openssl bignum to store hashes or something like that, then printing them as a count. – Wladimir van five hundred Laan ( Bitcoin Core developer )

In early words, this was a little 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 case :

 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 ’ second credibly 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 remark 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 alone identifier 1 for a transaction.

Notes

Why hash twice? Why not once?

Things frequently 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 ” ( by and by 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 ”. –

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

Hash functions like SHA256 are great for creating recognition numbers, because they will take in any string of data and constantly spit out a brusque so far unique result .