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. Note that different inputs can use different SIGHASH flags enabling complex compositions of spending conditions.

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 tables below illustrate what is signed and what is not signed in an ECDSA siganture depending on the SIGHASH type used.

Items that are always signed

The signature on any input always signs the TXID and VOUT that comprise the Outpoint being spent.

TxID
Version
Locktime
Inputs		Outputs
	Unlocking Script		Locking Script
Outpoint A	.....Sig Pa, Tx.....	Xa BSV	[ChecksigP1]
Outpoint B	.....Sig Pb, Tx.....	Xb BSV	[ChecksigP2]
...		...
Outpoint N	.....Sig PN, Tx.....	XN BSV	[ChecksigPm]

Items that are never signed

Unlocking scripts are never signed

TxID
Version
Locktime
Inputs		Outputs
	Unlocking Script		Locking Script
Outpoint A	.....Sig Pa, Tx.....	Xa BSV	[ChecksigP1]
Outpoint B	.....Sig Pb, Tx.....	Xb BSV	[ChecksigP2]
...		...
Outpoint N	.....Sig PN, Tx.....	XN BSV	[ChecksigPm]

SIGHASH_ALL

TxID
Version
Locktime
Inputs		Outputs
	Unlocking Script		Locking Script
Outpoint A	.....Sig Pa, Tx.....	Xa BSV	[ChecksigP1]
Outpoint B	.....Sig Pb, Tx.....	Xb BSV	[ChecksigP2]
...		...
Outpoint N	.....Sig PN, Tx.....	XN BSV	[ChecksigPm]

SIGHASH flags are useful 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