Ethereum: Transaction with slightly modified script never commits
While playing around with Bitcoin transactions recently, I came across some strange behavior regarding the output script in Ethereum transactions. The traditional way to represent transactions on the Ethereum network uses the following format:
OP_DUP OP_HASH160 (hash) OP_EQUALVERIFY OP_CHECKSIG
This structure allows for the hash-based checking that is necessary to ensure the integrity and authenticity of digital signatures.
However, there is an interesting twist to this setup. If you want to add more data or slightly modify the transaction, 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 how it interacts with scripts. Specifically, it has something to do with the concept of “transaction fees” and how they affect transaction execution.
When a transaction is executed on the Ethereum network, the EVM first checks 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 another instruction to a transaction that interacts with the modified script (e.g., calling a function or updating a variable), the EVM checks for any transaction fees associated with that interaction. Transaction fees are used to offset the network’s gas costs when executing complex transactions.
Unfortunately, in most cases, adding another 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 This means that all subsequent checks using the modified transaction will fail because the hash values are no longer valid.
As a result, transactions with slightly modified scripts (i.e., those that contain new instructions or modify existing ones) may never be transmitted on the Ethereum network. This does not mean that you cannot use such modifications in your applications; it is just that they may not work as expected due to the way the EVM interacts with scripts and transmission fees.
In short, 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 is essential to understand the basic mechanics of the EVM and its interactions with scripts before attempting to use such modifications in your applications.