Solidity ABI – Understanding ‘Warning: This Function Only Accepts a Single “bytes” Argument’

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In this question the OP is running into a warning upon using the keccak256 function in Solidity:

Warning: This function only accepts a single "bytes" argument. Please use "abi.encodePacked(…)" or a similar function to encode the data.

uint256 _unixTimestamp;
uint256 _timeExpired;

bytes32 output = keccak256(msg.sender, _unixTimestamp, _timeExpired);

What is the rationale behind this error – why do I need to use abi.encodePacked and what does it do in comparison to using it just like shown above?

Best Answer

Prior to Solidity 0.5.0, both functioned the same way. The warning is part of the process of deprecating the variable-argument versions of keccak256 and other hash functions.

As of Solidity 0.5.0, sha3 has been removed. See https://github.com/ethereum/solidity/issues/3955 for details.

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[Ethereum] What does Solidity’s “call” function mean

Solidity's call is a low-level interface for sending a message to a contract. It returns false if the subcall encounters an exception, otherwise it returns true. There is no notion of a legal call, if it compiles, it's valid Solidity.

nameReg.call("register", "MyName") is a message that passes certain bytes to nameReg. For the bytes, see: Understanding nameReg.call("register", "MyName") style call between contracts
nameReg.call(bytes4(sha3("fun(uint256)")), a) is a message that would invoke a function named fun (if nameReg adheres to the ABI) and pass it the raw, unpadded data a (you need to correctly pad a to 32 bytes first if you want behavior to match the ABI. For uint256 use left-padding.).

For 3, contract.call.value(...)(...) is a way to add Ether when invoking a contract. if(!nameReg.call.value(10)()){throw;} is an example of handling the failure case of the subcall. Note the extra parentheses value(10)() which invokes the fallback function.

call is a low-level interface, and it is simpler to invoke a function directly, nameReg.fun(a) instead of the second example. The direct invocation is also type-safe, and allows the return value of fun to be used.

[Ethereum] SOLIDITY: How to get a bytes32 keccak256 hash of an address and 2 uint256 variables

As reported in this thread: https://github.com/ethereum/solidity/issues/3955

The new abi.encode*() methods offer a replacement to perform packing, which then can be passed to all these hashing functions.

Bottom line: As @mirg commented, just use abi.encodePacked() inside keccak256(). That is:

keccak256(abi.encodePacked(msg.sender, _unixTimestamp, _timeExpired))

Best Answer

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[Ethereum] SOLIDITY: How to get a bytes32 keccak256 hash of an address and 2 uint256 variables

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