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Transaction ID (txid)

A txid or Transaction ID is a string of letters and numbers that identifies a specific transaction on the blockchain. The string is simply the double SHA-256 hash of a transaction. This hash can be used to look up a transaction on a node or block explorer.

When signing a transaction, it is in fact the txid that is signed. Signing the txid ensures that if any part of the transaction changes, the transaction ID changes, and the signature is rendered invalid.

Technically, a txid is not always a hash of the entire transaction. Since a signature cannot sign itself, signatures are not included in the txid, and thus signatures can be changed after their creation, sometimes without being invalidated. Before SegWit, this allowed a txid to change after the transaction was signed, a problem called transaction malleability.