SIGHASHNONE

SIGHASHNONE is a signature hashing type used in Bitcoin and other cryptocurrencies that employ similar scripting systems. It is one of several options available when creating a transaction that dictates which parts of the transaction are included in the signature. Specifically, SIGHASHNONE signifies that only the input being spent is intended to be signed. This means that the signature generated does not cover any of the other inputs or outputs within the transaction.

When a transaction uses SIGHASHNONE, the signer is essentially asserting their ownership of a specific unspent transaction output (UTXO) and authorizing its expenditure. However, they are not authorizing any other spending within that same transaction. This has significant implications for transaction malleability and security. Because the outputs are not covered by the signature, a third party could potentially alter the recipient address or amount of the outputs without invalidating the signature.

This malleability means that a transaction signed with SIGHASHNONE might have its output details changed before it is confirmed on the blockchain. While this might seem like a vulnerability, it has been addressed by various mechanisms in Bitcoin's development, such as Segregated Witness (SegWit). SegWit separates the signature data from the transaction data, effectively mitigating the malleability issues associated with SIGHASHNONE and other signature hashing types.

In practice, SIGHASHNONE is rarely used on its own for standard Bitcoin transactions due to the malleability concerns. It is more commonly encountered in conjunction with other SIGHASH flags or in specific smart contract scenarios where controlled malleability is either understood or mitigated by the protocol. Understanding SIGHASHNONE is crucial for developers working with Bitcoin's transaction structure and scripting capabilities.