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Transaction ID (TXID) | Bitcoin Glossary | Mapping Bitcoin

Transaction ID (TXID)

Protocol

Also known as: TXID, txid, transaction hash

A unique 256-bit hash that identifies a specific Bitcoin transaction, computed by double SHA-256 hashing the serialized transaction data. TXIDs are used to reference transactions in inputs, block explorers, and application programming interfaces.

Overview

A Transaction ID (TXID) is a unique identifier assigned to every Bitcoin transaction. It is computed by performing a double SHA-256 hash on the serialized transaction data and is represented as a 64-character hexadecimal string. TXIDs serve as the primary reference for locating and linking transactions throughout the Bitcoin protocol and ecosystem.

How TXIDs Are Computed

Raw Transaction Data
       |
  SHA-256(data)
       |
  SHA-256(result)
       |
  Reverse byte order
       |
  TXID (64 hex chars)

Example TXID:
a1075db55d416d3ca199f55b6084e2115b9345e16c5cf302fc80e9d5fbf5d48d

The double SHA-256 operation (sometimes written as SHA256d or HASH256) is the same hashing scheme used throughout Bitcoin, including block headers and Merkle trees.

TXIDs in the Protocol

TXIDs play several critical roles in Bitcoin:

  • Input references: Each transaction input specifies the TXID and output index of the UTXO it is spending. This creates the chain of ownership that constitutes Bitcoin's transaction graph.
  • Merkle tree leaves: TXIDs are hashed together to form the Merkle root in each block header.
  • Block explorers: Users look up transactions on block explorers by TXID.
  • Payment confirmation: Merchants and services track TXIDs to monitor payment status.

SegWit and Malleability

Before SegWit, Bitcoin suffered from transaction malleability, where signature data could be modified without invalidating the transaction, causing the TXID to change. SegWit fixed this by excluding witness data from the TXID calculation.

SegWit also introduced the wtxid (witness TXID), which includes witness data in the hash. The wtxid is used in the witness commitment within the coinbase transaction, while the traditional TXID remains the primary identifier for referencing outputs.

Common Misconceptions

  • TXIDs are displayed in reverse byte order compared to the raw hash output. This is a historical convention that can cause confusion when working at the protocol level.
  • Having a TXID does not mean a transaction is confirmed — unconfirmed transactions in the mempool also have TXIDs, though they can change if the transaction is replaced via RBF.

Related Terms

TransactionProtocol

A signed data structure that transfers bitcoin from one or more inputs to one or more outputs. Transactions reference previous UTXOs, include scripts or witness data proving authorization, and are broadcast to the network for inclusion in a block.

HashCryptography

A fixed-length output produced by a hash function from an arbitrary input. In Bitcoin, hashes serve as unique identifiers for blocks and transactions, form the basis of proof of work, and link blocks together in the blockchain.

Block ExplorerDevelopment

A web-based tool that allows users to browse and search the Bitcoin blockchain, including viewing transaction details, block contents, address balances, and network statistics. Popular explorers include mempool.space and blockstream.info.

Transaction MalleabilitySecurity

A vulnerability where a transaction's ID could be changed without invalidating the transaction by modifying the signature data. SegWit fixed this issue by moving witness data outside the transaction hash calculation, enabling reliable transaction chaining for the Lightning Network.