SIGHASH

Sighash, short for signature hash, is a term used in Bitcoin and related blockchains to describe the type of hash that is signed to authorize a transaction input. The sighash defines which parts of the transaction are covered by the digital signature, thereby restricting how the transaction can be altered after signing. This helps prevent certain forms of transaction malleability and supports flexible spending conditions in complex scripts.

When signing a transaction, the signer creates a cryptographic hash of a modified form of the transaction and then signs that hash with the private key. The sighash type is included with the signature so that verifiers know which portions of the transaction were signed and how to validate it. In SegWit and other modern formats, the same general concept applies with rules adapted for witness data.

Common sighash types are SIGHASH_ALL, SIGHASH_NONE, SIGHASH_SINGLE, and the flag SIGHASH_ANYONECANPAY. SIGHASH_ALL (0x01) signs every input and every output; SIGHASH_NONE (0x02) signs the inputs but not the outputs; SIGHASH_SINGLE (0x03) signs the input with the corresponding indexed output; SIGHASH_ANYONECANPAY (0x80) signs only the input being spent when combined with other types. Types can be combined via bitwise OR, enabling examples such as SIGHASH_ALL | SIGHASH_ANYONECANPAY.

In practice, SIGHASH_ALL is the default for standard transactions, while other types enable more flexible spending or partial-signature scenarios. The sighash mechanism remains a core aspect of how transactions are validated and how spending conditions are expressed in Bitcoin-like systems.