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.

Use Case 1 - crowdfunding

Use Case 2 - blank check

Use Case 3 - Modular Transaction

References

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