Is the value of a contract's method's parameter visible on blockchain? On Etherscan, in the tx information there is an Input Data field, which details says

The binary data that formed the input of the transaction, either the input data if it was a message call or the contract initialisation if it was a contract creation

Is this what I am looking for?

up vote 2 down vote accepted

Yes, that information is publicly available and is generated like this:

Mastering Ethereum - Transmitting a Data Payload to an EOA or Contract

When your transaction contains data, it is most likely addressed to a contract address. That doesn’t mean you cannot send a data payload to an EOA—that is completely valid in the Ethereum protocol. However, in that case, the interpretation of the data is up to the wallet you use to access the EOA. It is ignored by the Ethereum protocol. Most wallets also ignore any data received in a transaction to an EOA they control. In the future, it is possible that standards may emerge that allow wallets to interpret data the way contracts do, thereby allowing transactions to invoke functions running inside user wallets. The critical difference is that any interpretation of the data payload by an EOA is not subject to Ethereum’s consensus rules, unlike a contract execution.

For now, let’s assume your transaction is delivering data to a contract address. In that case, the data will be interpreted by the EVM as a contract invocation. Most contracts use this data more specifically as a function invocation, calling the named function and passing any encoded arguments to the function.

The data payload sent to an ABI-compatible contract (which you can assume all contracts are) is a hex-serialized encoding of:

A function selector

The first 4 bytes of the Keccak-256 hash of the function’s prototype. This allows the contract to unambiguously identify which function you wish to invoke.

The function arguments

The function’s arguments, encoded according to the rules for the various elementary types defined in the ABI specification.

In solidity_faucet_example, we defined a function for withdrawals:

function withdraw(uint withdraw_amount) public {

The prototype of a function is defined as the string containing the name of the function, followed by the data types of each of its arguments, enclosed in parentheses and separated by commas. The function name here is withdraw and it takes a single argument that is a uint (which is an alias for uint256), so the prototype of withdraw would be:

withdraw(uint256)

Let’s calculate the Keccak-256 hash of this string:

> web3.sha3("withdraw(uint256)"); 
'0x2e1a7d4d13322e7b96f9a57413e1525c250fb7a9021cf91d1540d5b69f16a49f'

The first 4 bytes of the hash are 0x2e1a7d4d. That’s our "function selector" value, which will tell the contract which function we want to call.

Next, let’s calculate a value to pass as the argument withdraw_amount. We want to withdraw 0.01 ether. Let’s encode that to a hex-serialized big-endian unsigned 256-bit integer, denominated in wei:

> withdraw_amount = web3.toWei(0.01, "ether");
'10000000000000000'
> withdraw_amount_hex = web3.toHex(withdraw_amount);
'0x2386f26fc10000'

Now, we add the function selector to the amount (padded to 32 bytes):

2e1a7d4d000000000000000000000000000000000000000000000000002386f26fc10000

That’s the data payload for our transaction, invoking the withdraw function and requesting 0.01 ether as the withdraw_amount.

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