What information are visible in transaction after a call to a payable function?

Question

I was wondering what information are publicly visible and intelligible when an EOA (external ownable address) calls a payable function on a smart contract, with some parameters and a return value.

Let us say I have the smart contract function below, the question is: 'is the return value visible somewhere on the blockchain?'. I am using Solidity 0.8.12.

function askForSecretCode(uint time) external payable returns (bytes32) {
    require(msg.value == 42, 'Need to pay 42 wei.');
    secretCodes[secretCodesNum] = keccak256(abi.encodePacked(msg.sender, time);
    return keccak256(abi.encodePacked(msg.sender, time);
}

Answer 1

Anyone can see the input time param value as a part of a transaction invoking the askForSecretCode function.

Even though if you don't publish the contract source code, the bytecode is public, and there are decompilers that can (with some error rate) help generate source code back from the bytecode. So let's assume that the source code is available as well.

From the source code (or usually even from the pseudocode generated by the decompiler), anyone can determine the storage slot of secretCodesNum (which from the context seems to be a property of the contract) and retrieve its value using the eth_getStorageAt RPC method, including historical values.

Using these secretCodesNum values, they can use the same method to determine storage slots of any secretCodes[secretCodesNum] and their values.

TLDR: Never ever store private data on the blockchain.