The ScriptPubKey is a script which controls how bitcoin can be spent. In english, most bitcoin scripts translate as follows: “In order to spend this bitcoin, one must produce a signature belonging to this public key…”, followed by a public key. More commonly, public key hashes are used instead of the public keys themselves, but the concept is the same. A ScriptPubKey is often called a locking script because it locks the bitcoin until someone can provide an answer to unlock the bitcoin. The unlocking script, called the ScriptSig in legacy transactions and the Script Witness in SegWit transactions, is provided when this bitcoin is spent in a future transaction.
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