TXID
A TXID (Transaction ID) is a unique 64-character hexadecimal hash that identifies a specific Bitcoin transaction. It is computed by double-SHA256 hashing the serialized transaction data and serves as the permanent reference for that transaction on the blockchain.
How It Works
Every Bitcoin transaction has a unique TXID generated by hashing the complete transaction data (inputs, outputs, amounts, scripts) with SHA-256 twice. The result is a 32-byte hash displayed as a 64-character hexadecimal string. This hash is deterministic — the same transaction always produces the same TXID — and it is practically impossible for two different transactions to have the same TXID.
Before SegWit, a third party could modify the signature (scriptSig) data in a transaction without invalidating it, producing a different TXID for the same logical transaction. This "transaction malleability" was a major problem for systems that tracked transactions by TXID. SegWit fixed this by moving signatures into a separate witness structure excluded from the TXID hash. It also introduced the witness TXID (wtxid), which commits to the full transaction including witness data.
TXIDs are how you look up transactions on block explorers, share payment proofs, and track confirmation status. When you send someone bitcoin, sharing the TXID lets them verify the transaction independently. However, be aware that TXIDs link to your addresses on the public blockchain. Sharing a TXID with a third party reveals your addresses and can be used for chain analysis. Use your own full node to verify transactions privately.
Key Points
- Unique 64-character hex hash identifying a specific Bitcoin transaction
- Computed by double-SHA256 of the serialized transaction data
- SegWit fixed transaction malleability by separating witness data from the TXID
- Used to look up transactions on block explorers and verify payment status
- Sharing TXIDs reveals your addresses — verify privately using your own full node