Ethereum: Why is the signature hash different for each input in a multi-input transaction?

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 signature hash of the transaction differs from the input hash. This may seem counterintuitive at first, but it is essential 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) overwrites the previous scriptPubKey for each input, resulting in different signature hashes. This overwrite is necessary because the EVM needs to ensure that the transaction’s output addresses and values ​​match what it expects.

Why Different Signatures?

The different signatures are not arbitrary; they serve a specific purpose:

• Consistency: By using different inputs with the same scriptPubKey, we maintain consistency in transaction addresses and output values.

• Efficiency

  

  : The EVM can use these substages to optimize transaction processing, reducing computational overhead and improving overall efficiency.

• Security: Using different inputs ensures that transactions cannot be executed maliciously by exploiting weaknesses in the system.

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. In 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, replacing scriptPubKeys would ensure that:

• The output 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 replace scriptPubKeys, which balances consistency, efficiency, and security.