Ethereum: Why is the signature hash different for each input in a multi-input transaction?
In the case of Ethereum, when a multi-input transaction is executed, the transaction signature hash is different from the input hash. At first glance, this may seem illogical, but it is important to understand why this design decision was made.
BIP 14: Sub-stages and signature hashes
The Ethereum Block Producer Interface (BPI) allows for multiple inputs to be used in a multi-input transaction. To accommodate such cases, BIP 14 defines sub-hashes, which are intermediate hashes that allow for efficient transaction execution.
When creating a sub-stage, the Ethereum Virtual Machine (EVM) replaces the previous scriptPubKey for each input, resulting in different signature hashes. This replacement is necessary because the EVM needs to ensure that the transaction addresses and outputs match what is expected.
Why different signatures?
Different signatures are not arbitrary; they have a specific purpose:
- Consistency
: By using different inputs with the same
scriptPubKey, we maintain consistency of the addresses and outputs of the transaction.
- Efficiency
: EVM can use these sub-stages to optimize transaction processing, reducing computational load and increasing overall performance.
- Security: Using different inputs ensures that transactions cannot be executed maliciously by exploiting weaknesses in the system.
To summarize
The hash of a multi-input transaction signature is different from the input hash due to the substitution of scriptPubKeys. The choice of this solution is based on the need for consistency, optimization of transaction processing, and security. In Ethereum, we rely on the EVM’s ability to correctly swap inputs, which allows us to perform transactions efficiently and securely.
Usage Example
Consider a multi-input transaction where two users want to transfer funds from one account to another using different scriptPubKeys. To perform this transaction, replacing scriptPubKeys will result in:
- Output addresses are as expected.
- Transaction processing is efficient.
- Security is preserved.
In short, different signature digests in multi-input transactions are a result of the EVM’s ability to substitute scriptPubKeys, which balances consistency, efficiency, and security.