Ethereum: Why is the signature hash different for each input 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 Hashes
Ethereum’s Block Producer Interface (BPI) allows for multiple inputs in a multi-input transaction. To accommodate these cases, BIP 14 defines substages, which are intermediate hashes that allow transactions to be executed efficiently.
When creating a substage, the Ethereum Virtual Machine (EVM) replaces the previous scriptPubKey for each input, resulting in different signature hashes. This replacement is necessary because the EVM must ensure that the output addresses and values of the transaction match what is expected.
Why different signatures?
Different signatures are not arbitrary; they serve a specific purpose:
- Consistency: By using different inputs with the same
scriptPubKey, we maintain consistency in the output addresses and values of the transaction.
- Efficiency: The EVM can use these substeps to optimize transaction processing, reduce computational costs, and improve overall efficiency.
- Security
: Using different inputs ensures that transactions cannot be executed maliciously by exploiting weaknesses in the system.
In conclusion
The signature hash of a multi-input transaction differs from its input hash due to the substitution of scriptPubKeys. This design choice is rooted in maintaining consistency, optimizing transaction processing, and ensuring security. At Ethereum, we rely on the EVM’s ability to correctly substitute inputs, allowing us to execute transactions efficiently and securely.
Example 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, substituting scriptPubKeys will ensure that:
- The source addresses match expectations.
- Transaction processing is efficient.
- Security is maintained.
In summary, the different signature hashes in multi-input transactions are a result of the EVM’s ability to substitute scriptPubKeys, which balances consistency, efficiency, and security.
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