What is the difference between a UTXO's "scriptSig" and "scriptPubKey"?

Understanding the Role of scriptPubKey

1. The scriptPubKey is a locking mechanism attached to a UTXO that defines the conditions under which the output can be spent. It typically includes a public key hash or address and requires a valid signature to unlock.

2. When a user sends cryptocurrency, they create a new UTXO with a scriptPubKey that only the recipient can satisfy using their private key. This ensures that only the rightful owner can spend those funds in a future transaction.

3. The structure of a scriptPubKey often follows standard formats such as P2PKH (Pay-to-Public-Key-Hash) or P2SH (Pay-to-Script-Hash), each enforcing different spending rules through Bitcoin's scripting language.

4. Miners validate incoming transactions by checking whether the provided unlocking data matches what the scriptPubKey demands. If the criteria are not met, the transaction is rejected from the blockchain.

5. Since it resides within the output of a transaction, the scriptPubKey remains immutable once confirmed on-chain, forming a permanent part of the ledger’s state.

The Functionality of scriptSig

1. The scriptSig appears in the input section of a transaction and serves as the unlocking component for a specific UTXO. It contains data like digital signatures and public keys needed to meet the scriptPubKey's requirements.

2. Each time someone spends a UTXO, they must construct a valid scriptSig that, when combined with the referenced scriptPubKey, results in a successful script evaluation.

3. In a P2PKH transaction, the scriptSig usually consists of two elements: the ECDSA signature and the spender’s full public key, both pushed onto the stack during script execution.

4. Unlike scriptPubKey, the scriptSig is temporary and only exists within the context of a spending transaction. Once validated, it disappears from future considerations after confirmation.

5. Malformed or incorrect scriptSig scripts lead to immediate rejection by network nodes, preventing unauthorized access to funds and maintaining system integrity.

Interaction Between scriptSig and scriptPubKey

1. During transaction validation, nodes concatenate the scriptSig from the input and the scriptPubKey from the referenced output, executing them sequentially in Bitcoin’s stack-based environment.

2. The combination allows values pushed by the scriptSig—such as signatures and public keys—to be consumed and verified by operations defined in the scriptPubKey, like OP_CHECKSIG.

3. A correctly formed pair ensures that ownership is provable without exposing private keys, leveraging cryptographic principles to secure value transfer across decentralized networks.

4. Segregated Witness (SegWit) altered this model slightly by moving signature data out of the scriptSig field into a separate witness structure, reducing malleability risks while improving scalability.

5. Despite architectural changes like SegWit, the conceptual relationship between unlocking and locking scripts remains central to how Bitcoin enforces spend conditions at the protocol level.

Frequently Asked Questions

Can a scriptPubKey be modified after a transaction is confirmed?No, once a transaction is included in a block, its outputs—and therefore the associated scriptPubKey—become fixed parts of the blockchain. Any attempt to alter it would break consensus rules and invalidate the chain.

Is scriptSig used in all types of Bitcoin transactions?Traditional non-SegWit transactions use scriptSig to provide unlocking data. However, in native SegWit transactions (Bech32 addresses), the scriptSig is empty, and witness data replaces its role entirely.

What happens if the scriptSig does not match the scriptPubKey requirements?The transaction fails validation immediately. Nodes will reject it, refusing to propagate or include it in blocks. This protects against theft and enforces strict adherence to predefined spending conditions.

Are there any privacy implications related to scriptSig and scriptPubKey?Yes, because both scripts are publicly visible on-chain, patterns in their usage can be analyzed to link addresses and trace fund flows. Techniques like address reuse amplify these privacy leaks significantly.