SIGHASHANYONECANPAY

SIGHASHANYONECANPAY is a flag used in Bitcoin transaction signing to alter which parts of a transaction are covered by a signature. It is represented by the bit 0x80 and can be combined with the base sighash types (SIGHASH_ALL, SIGHASH_NONE, or SIGHASH_SINGLE) to modify the signing scope.

How it works: When the ANYONECANPAY flag is set, the signature is created with respect to only the input being signed, rather than all inputs in the transaction. The outputs that are signed still follow the rules of the chosen base sighash type: with SIGHASH_ALL, all outputs are signed; with SIGHASH_NONE, no outputs are signed; with SIGHASH_SINGLE, only the corresponding output is signed. Other inputs remain unsigned in this signature, meaning their inclusion or amounts do not affect the validity of the signature for the signed input.

Implications and use cases: ANYONECANPAY enables multi-party signing workflows where multiple participants each sign their own inputs, allowing others to add additional inputs to the transaction without invalidating the existing signatures. This is helpful in scenarios such as cooperative funding, multi-signature arrangements, or coinjoin-like constructs where flexibility to assemble a transaction from multiple sources is desirable.

Limitations and considerations: Because the signature does not commit to other inputs, it does not fully bind the transaction, potentially affecting security guarantees that rely on the entire transaction being signed. Parties must understand the specific combination of sighash type and ANYONECANPAY being used. This flag is one of several sighash options available to tailor transaction signing behavior.

See also: SIGHASH types, SIGHASH_ALL, SIGHASH_NONE, SIGHASH_SINGLE.