Glossary

ScriptSig

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The ScriptSig is the part of a transaction which contains the required signatures and the script which unlocks a UTXO for spending. A ScriptSig pairs with the ScriptPubKey to form a complete and valid script. The ScriptSig is only present in legacy Bitcoin transactions. In SegWit transactions, the ScriptSig is removed from the transaction body and called the Witness Script.

When bitcoin is sent, it is locked using a ScriptPubKey. The ScriptPubKey can be thought of as a riddle, solvable only by the owner of the correct private keys. To spend this bitcoin, the owner must publish the answer to the riddle by signing the transaction with his keys. The published answer to the riddle is the ScriptSig.

Every node verifies that the ScriptSig matches the ScriptPubKey for each transaction they receive, ensuring that no invalid transactions are added to the blockchain. One of the major changes implemented in SegWit was the relocation of the ScriptSig from the legacy transaction body into the Witness portion.

Related Terms
Bitcoin Script
Script is the language Bitcoin uses to determine who can spend a given piece of bitcoin (a UTXO). Script is intentionally limited in order to maintain Bitcoin's simplicity and ensure that any individual can run a node and validate the blockchain.
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Signature
A digital signature is produced by a private key and proves that the owner of the private key and corresponding public key has validated the signed data.
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ScriptPubKey
The ScriptPubKey is a script which controls how bitcoin can be spent. Bitcoin is locked by the ScriptPubKey and unlocked when the owner submits a valid ScriptSig.
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Witness Script
The Witness Script is the script which outlines the requirements to spend a Pay-to-Witness-Pubkey-Hash (P2WSH) output, much like the Redeem Script does for a P2SH output. Most commonly, Witness Scripts define the setup of a SegWit multisig wallet.
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SegWit
Segregated Witness (SegWit) is a soft-fork upgrade to Bitcoin which was activated in 2017. SegWit fixed the problem of transaction malleability, and paved the way for the implementation of the Lightning Network and many other upgrades.
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