SIGHASH flags

A SIGHASH flag is used to indicate which part of the transaction is signed by the ECDSA signature. The mechanism provides a flexibility in constructing transactions. There are in total 6 different flags that can be added to a digital signature in a transaction.

Flag	Value	Functional Meaning
SIGHASH_ALL	0x00000001	Sign all inputs and outputs
SIGHASH_NONE	0x00000002	Sign all inputs and no output
SIGHASH_SINGLE	0x00000003	Sign all inputs and the output with the same index
SIGHASH_ALL | ANYONECANPAY	0x00000081	Sign its own input and all outputs
SIGHASH_NONE | ANYONECANPAY	0x00000082	Sign its own input and no output
SIGHASH_SINGLE | ANYONECANPAY	0x00000083	Sign its own input and the output with the same index

The table below illustrates what is signed and what is not signed in a more granulated detail by an ECDSA siganture in the Input 1 in a transaction.

	ALL	NONE	SINGLE	ALL | ANYONECANPAY	NONE | ANYONECANPAY	SINGLE | ANYONECANPAY
Version	Yes	Yes	Yes	Yes	Yes	Yes
Locktime	Yes	Yes	Yes	Yes	Yes	Yes
Input 1 outpoint	Yes	Yes	Yes	Yes	Yes	Yes
Input 1 scriptSig	No	No	No	No	No	No
Input 1 sequence number	Yes	Yes	Yes	Yes	Yes	Yes
Input 2 outpoint	Yes	Yes	Yes	No	No	No
Input 2 scriptSig	No	No	No	No	No	No
Input 2 sequence number	Yes	Yes	Yes	No	No	No
Output 1	Yes	No	Yes	Yes	No	Yes
Output 2	Yes	No	No	Yes	No	No

Note that the version number and locktime are always signed, and the scriptSigs are never signed.

SIGHASH flags are handy when constructing smart contracts and negotiable transactions in payment channels.

Use Case 1 - crowdfunding

Using ALL | ANYONECANPAY allows a transaction to have a fixed output or fixed outputs while keep the input list open. That is, anyone can add their input with their signature to the transaction without invalidating all existing signatures.

Use Case 2 - blank check

Using NONE allows any miner to add their desired outputs to the transaction to claim the fund in the input.

Use Case 3 - Modular Transaction

Using SINGLE | ANYONECANPAY modularises a transaction. Any number of these transactions can be combined into one transaction.

References

https://github.com/bitcoin-sv/bitcoin-sv/blob/master/src/script/sighashtype.h