How to Understand tx.origin and msg.sender in Solidity

Developers need to understand the difference between tx.origin and msg.sender in Solidity. These two global variables often need clarification with each other despite their fundamental differences. While they may appear similar at first glance, tx.origin and msg.sender represent distinct addresses in the context of a transaction.

In this blog post, we will delve deeper into the meanings of each of these variables.

What Is tx.origin?

In Solidity, tx.origin identifies the original sender of a transaction. It points to the external account initiating the transaction and remains constant throughout subsequent smart contract interactions (full call chain).

When a transaction is initiated through the MetaMask wallet, the address of the user's MetaMask wallet is stored in tx.origin. This address remains the same, even if the transaction passes through multiple contracts. The consistency of this address is essential for tracing the initial sender of the transaction.

What Is msg.sender?

In smart contract development, msg.sender identifies the sender of the current call. This variable is dynamic and can change throughout the transaction process.

When a transaction moves through several smart contracts, the msg.sender value changes to indicate the most recent contract address in the call chain. For example, if Contract A calls Contract B, then the msg.sender value within Contract B will be recognized as Contract A.

Coding It Up

To demonstrate how the tx.origin and msg.sender change between smart contract calls, we will create an EntryContract smart contract that references the UnderlyingContract contract.

Let's add a printTxOriginAndMsgSender function that prints each address.

Here, we have the Entry smart contract:

contract EntryContract {
  IUnderlyingContract private underlyingContract;

  constructor(IUnderlyingContract _underlyingContract) {
    underlyingContract = _underlyingContract;
  }

  function printTxOriginAndMsgSender() public view {
    console.log("tx.origin", tx.origin);
    console.log("msg.sender", msg.sender);
  }

  function callUnderlyingContract() external {
    underlyingContract.printTxOriginAndMsgSender();
  }
}

Now, let's define the UnderlyingContract and its interface:

interface IUnderlyingContract {
  function printTxOriginAndMsgSender() external ;
}

contract UnderlyingContract is IUnderlyingContract {
  function printTxOriginAndMsgSender() external view {
    console.log("tx.origin", tx.origin);
    console.log("msg.sender", msg.sender);
  }
}

To execute the test, we must first deploy the UnderlyingContract, and use its address when deploying the EntryContract.

When we call the printTxOriginAndMsgSender function of the EntryContract contract, we see that both addresses are the same.

Let's call the callUnderlyingContract function on the EntryContract contract. We can see that tx.origin and msg.sender are different now. tx.origin is the original caller's address and msg.sender is the EntryContract smart contract’s address.

TL;DR

In Solidity, tx.origin and msg.sender are two variables that serve different but crucial purposes. tx.origin always refers to the address that initially initiated the transaction and remains constant throughout the transaction chain.

On the other hand, msg.sender represents the sender of the current message or contract interaction and changes with each call.

It is crucial to be cautious when using msg.sender as it may not always represent the initial caller of the transaction.

Links

• Demo code
• Solidity documentation: Special Variables and Functions
• Solidity documentation: Block and Transaction Properties
• Unraveling Solidity Mysteries: Demystifying tx.origin and msg.sender