Ethereum: Getting the wrong signer address when verifying a signature with a generated hash of a signed message in trust

Incorrect Signer Address on Verifying Signature in Ethereum Solidity

Ethereum is a decentralized, open-source blockchain platform that enables developers to build smart contracts and decentralized applications (dApps). One of the key aspects of building reliable and secure smart contracts is verifying the signature of transactions using digital signatures.

In this article, we’ll explore why you might be experiencing incorrect signer addresses on verifying the signature with the generated hash of the signed message in Solidity, a programming language used to build Ethereum smart contracts.

The Problem: Incorrect Signer Addresses

When building a smart contract on Ethereum, the signer variable is expected to hold the private key of the account that is signing the transaction. However, when verifying the signature with the generated hash of the signed message using the Keccak-256 hash function (the default hash function used in Ethereum), you might be encountering incorrect signer addresses.

Why is this happening?

The issue arises from the way the signer variable is initialized and its relationship to the transaction data. In Solidity, when a contract calls a function that updates the signer variable, it should also update the rewards variable to point to the new signer.

However, in some cases, this can lead to incorrect signer addresses being stored in the rewards variable. This is particularly problematic when verifying signatures with the generated hash of the signed message using the Keccak-256 hash function.

JS Code (According to Keir Finlow-Bates’ suggestion)

To illustrate this issue, let’s consider a simple example:

pragma solidity ^0.8.0;


contract MyContract {

    address public signer; // Initialize with a default value

    uint256 public rewards;


    function updateSigner(address _signer) public {

        signer = _signer;

        rewards = 0;

    }


    function verifySignature() public view returns (bool) {

        bytes memory msg = abi.encodePacked("Hello, World!");

        uint256 hash = keccak256(msg);

        return signers[hash] == signer; // Incorrectly stores the wrong signer address

    }

}

In this example, we define a contract MyContract with a default value for the signer variable. When updating the signer variable using the updateSigner function, it also updates the rewards variable to point to the new signer.

However, when verifying signatures with the generated hash of the signed message using the Keccak-256 hash function (the default hash function used in Ethereum), we incorrectly store the wrong signer address. This is because the signers mapping in Solidity stores a mapping from hashes to the corresponding signers, but our updateSigner function only updates the rewards variable.

Solution

To resolve this issue, you can use the following approach:

• Update the rewards variable whenever the signer changes.

• Use the correct signers mapping in Solidity to access the correct signer address.

Here’s an updated example:

pragma solidity ^0.8.0;


contract MyContract {

    address public signer;

    uint256 public rewards;


    function updateSigner(address _signer) public {

        signer = _signer;

        rewards = 0;

    }


    function verifySignature() public view returns (bool) {

        bytes memory msg = abi.encodePacked("Hello, World!");

        uint256 hash = keccak256(msg);

        return signers[hash].address == signer; // Correctly stores the correct signer address

    }

}

In this updated example, we’ve added a new variable rewards to store the current signer’s private key. Whenever the signer changes, we update the rewards variable.