Ethereum: Why is the signature hash different for each entry in a multi-input transaction?
In Ethereum, when a multi-input transaction is executed, the transaction signature hash is different from the input hash. This may seem counterintuitive at first, but it is important to understand why this design choice was made.
BIP 14: Substages and Signature Scattering
Ethereum’s Block Producer Interface (BPI) allows for multiple entries in a multi-input transaction. To accommodate these cases, BIP 14 defines substages, which are intermediate hashes that allow for efficient transaction execution.
When creating a substage, the Ethereum Virtual Machine (EVM) replaces the previous scriptPubKey for each entry, resulting in different signature hashes. This replacement is necessary because the EVM must ensure that the output addresses and transaction values match what is expected.
Why different signatures?
The different signatures are not arbitrary; serve a specific purpose:
- Consistency: By using different inputs with the same
scriptPubKey, we maintain consistency in the output addresses and transaction values.
- Efficiency
: The EVM can use these substages to optimize transaction processing, reducing computational costs and improving overall efficiency.
- Security: Using different inputs ensures that transactions cannot be maliciously executed by exploiting weaknesses in the system.
Conclusion
Multi-input transaction signature hashing is different from hashing the inputs due to overriding scriptPubKeys. This design choice is based on maintaining consistency, optimizing transaction processing, and ensuring security. In Ethereum, we rely on the EVM’s ability to correctly swap inputs, which allows us to execute transactions efficiently and securely.
Use case
Consider a multi-input transaction where two users want to send funds from one account to another using different scriptPubKeys. To execute this transaction, overriding scriptPubKeys would ensure the following:
- Output addresses match expectations.
- Transaction processing is efficient.
- Security is maintained.
In short, the different signature hashes in multi-input transactions are a result of the EVM’s ability to replace scriptPubKeys, which balances consistency, efficiency, and security.