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
SIGHASH_ALL signs all inputs and outputs used to build the transaction. Once an input signed with SIGHASH_ALL is added to a transaction it cannot be changed without that signature being invalidated.
| 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