Ethereum: Transaction with slightly changed script is never relayed

Ethereum: Transaction with slightly changed script is never relayed

As I was playing around with Bitcoin transactions recently, I stumbled upon a peculiar behavior regarding the output script in Ethereum transactions. The traditional way of representing transactions on the Ethereum network uses the following format:

OP_DUP OP_HASH160 (hash) OP_EQUALVERIFY OP_CHECKSIG

This structure allows for a hash-based check, which is essential for ensuring the integrity and authenticity of digital signatures.

However, there’s an interesting twist to this setup. When you want to include additional data or modify the transaction slightly, you can do so by changing the script in the OP_DUP instruction. But what happens when you add a new instruction that interacts with this modified transaction?

The answer lies in the Ethereum Virtual Machine (EVM) and its interaction with scripts. Specifically, it has something to do with the concept of ” relay fees” and how they affect the execution of transactions.

When a transaction is executed on the Ethereum network, the EVM first validates the hash of the input data. If the hash matches the expected value, the transaction proceeds. However, if the hash does not match, the transaction fails.

Now, here’s where things get interesting. When you add an additional instruction to your transaction that interacts with the modified script (e.g., by calling a function or updating a variable), the EVM will check for any relay fees associated with that interaction. Relay fees are used to compensate the network for the gas costs of executing complex transactions.

Unfortunately, in most cases, adding an extra instruction that modifies the original transaction structure will result in the OP_DUP instruction being executed multiple times, each time checking a different hash value. This means that any subsequent checks using the modified transaction will fail because the hash values are no longer valid.

As a result, transactions with slightly changed scripts (i.e., those that include new instructions or modify existing ones) may be never relayed on the Ethereum network. This is not to say that you can’t use such modifications in your applications; it’s just that they might not work as expected due to the way the EVM interacts with scripts and relay fees.

In summary, when playing around with Bitcoin transactions, keep in mind that modifying scripts using the OP_DUP instruction can lead to unexpected behavior on the Ethereum network. It’s essential to understand the underlying mechanics of the EVM and its interactions with scripts before attempting to use such modifications in your applications.