Transaction context (tx.origin vs msg.sender) in Blockchain / Solidity

Introduction

Understanding tx.origin and msg.sender helps you know who started a transaction and who called a function in smart contracts.

When you want to check who originally started a transaction across multiple contract calls.

When you want to know the immediate caller of a function in a smart contract.

When building security checks to prevent unauthorized access in smart contracts.

When debugging or logging the flow of transactions in decentralized applications.

Syntax

Blockchain / Solidity

address txOrigin = tx.origin;
address msgSender = msg.sender;

tx.origin is the original external account that started the transaction.

msg.sender is the immediate caller of the current function.

Examples

This function returns the original address that started the transaction.

Blockchain / Solidity

function checkOrigin() public view returns (address) {
    return tx.origin;
}

This function returns the address that called this function directly.

Blockchain / Solidity

function checkSender() public view returns (address) {
    return msg.sender;
}

This example shows how tx.origin stays the same across calls, but msg.sender changes to the immediate caller.

Blockchain / Solidity

contract Caller {
    function callOther(address callee) public {
        OtherContract(callee).calledFunction();
    }
}

contract OtherContract {
    function calledFunction() public view returns (address origin, address sender) {
        return (tx.origin, msg.sender);
    }
}

Sample Program

This program has two contracts. Caller calls OtherContract. When callOther runs, tx.origin is the original user who started the transaction, and msg.sender inside calledFunction is the Caller contract.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract OtherContract {
    function calledFunction() public view returns (address origin, address sender) {
        return (tx.origin, msg.sender);
    }
}

contract Caller {
    OtherContract other;

    constructor(address otherAddress) {
        other = OtherContract(otherAddress);
    }

    function callOther() public view returns (address origin, address sender) {
        return other.calledFunction();
    }
}

Important Notes

Never use tx.origin for authorization checks because it can be tricked by phishing attacks.

Use msg.sender to check who called your function directly for better security.

Remember, tx.origin is always an external account, never a contract.

Summary

tx.origin is the original external account that started the transaction.

msg.sender is the immediate caller of the current function.

Use msg.sender for secure access control in smart contracts.

Practice

1. What does tx.origin represent in a blockchain transaction?

easy

A. The original external account that started the transaction

B. The immediate caller of the current function

C. The contract address executing the function

D. The miner who included the transaction in a block

Solution

1. Step 1: Understand the meaning of tx.origin

   tx.origin always points to the external account that initiated the transaction, regardless of how many contracts are called.

2. Step 2: Differentiate from msg.sender

   msg.sender is the immediate caller, which can be a contract or an account, but tx.origin is always the original external user.

3. Final Answer:

   The original external account that started the transaction -> Option A

4. Quick Check:

   tx.origin = original external account [OK]

Hint: Remember: tx.origin is the original user, msg.sender is immediate caller [OK]

Common Mistakes:

• Confusing tx.origin with msg.sender
• Thinking tx.origin changes in internal calls
• Assuming tx.origin is always a contract

2. Which of the following is the correct way to check if the caller is the owner using msg.sender in Solidity?

easy

A. require(owner == block.coinbase);

B. require(tx.origin == owner);

C. require(owner == tx.origin);

D. require(msg.sender == owner);

Solution

1. Step 1: Identify the correct variable for immediate caller

   To check the caller of the function, msg.sender is used because it represents the immediate caller.

2. Step 2: Verify the syntax of the require statement

   The correct syntax is require(msg.sender == owner); to ensure only the owner can proceed.

3. Final Answer:

   require(msg.sender == owner); -> Option D

4. Quick Check:

   Use msg.sender for permission checks [OK]

Hint: Use msg.sender for permission checks, not tx.origin [OK]

Common Mistakes:

• Using tx.origin for permission checks
• Swapping owner and caller in comparison
• Using block.coinbase which is miner address

3. Consider this Solidity code snippet:

contract Test {
    address owner;
    constructor() {
        owner = msg.sender;
    }
    function checkOrigin() public view returns (bool) {
        return tx.origin == owner;
    }
}

If an external user calls checkOrigin() directly, what will be the returned value?

medium

A. false

B. true

C. Compilation error

D. Revert due to permission error

Solution

1. Step 1: Understand owner assignment in constructor

   The owner is set to msg.sender when the contract is deployed, which is the external user deploying it.

2. Step 2: Analyze checkOrigin() call by external user

   When the external user calls checkOrigin(), tx.origin is the same external user who deployed the contract (owner), so the comparison returns true.

3. Final Answer:

   true -> Option B

4. Quick Check:

   Direct call: tx.origin equals owner [OK]

Hint: Direct calls: tx.origin equals msg.sender equals owner [OK]

Common Mistakes:

• Assuming tx.origin differs from msg.sender on direct calls
• Confusing owner initialization
• Expecting errors from this code

4. Given this Solidity code:

contract Caller {
    address owner;
    function callCheck(address callee) public view returns (bool) {
        (bool success, ) = callee.call(abi.encodeWithSignature("checkOrigin()"));
        return success;
    }
}

contract Callee {
    address owner;
    constructor() {
        owner = msg.sender;
    }
    function checkOrigin() public view returns (bool) {
        return tx.origin == owner;
    }
}

What is the main issue with the callCheck function in Caller?

medium

A. It causes a compilation error due to missing payable keyword

B. It compares tx.origin instead of msg.sender inside checkOrigin

C. It uses low-level call incorrectly and ignores return value unpacking

D. It sets owner incorrectly in Callee constructor

Solution

1. Step 1: Analyze the use of low-level call in callCheck

   The call returns (bool, bytes), but the code only returns the bool from call without decoding the actual function's return value.

2. Step 2: Understand the consequence of ignoring return data

   Because the function ignores the returned bytes, it cannot get the actual boolean result from checkOrigin(), leading to incorrect or misleading results.

3. Final Answer:

   It uses low-level call incorrectly and ignores return value unpacking -> Option C

4. Quick Check:

   Low-level call needs return data decoding [OK]

Hint: Low-level call returns bytes; decode to get function result [OK]

Common Mistakes:

• Blaming tx.origin usage instead of call misuse
• Ignoring that call returns (bool, bytes)
• Assuming owner is set wrongly

5. You want to design a contract that only allows the original external user who started a transaction to execute a sensitive function, even if called through multiple contracts. Which is the safest way to implement this check?

hard

A. Use require(tx.origin == owner); to check the original external account

B. Use require(msg.sender == owner); to check the immediate caller

C. Use require(tx.origin == msg.sender); to ensure both are equal

D. Use require(owner == address(this)); to check contract address

Solution

1. Step 1: Understand the difference between msg.sender and tx.origin

   msg.sender is the immediate caller, which can be a contract, while tx.origin is the original external account that started the transaction.

2. Step 2: Choose the check that ensures original user authorization

   To ensure only the original external user can execute the function, use require(tx.origin == owner); because it tracks the transaction origin through multiple calls.

3. Final Answer:

   Use require(tx.origin == owner); to check the original external account -> Option A

4. Quick Check:

   Original user check uses tx.origin [OK]

Hint: Use tx.origin to track original external user through calls [OK]

Common Mistakes:

• Using msg.sender which can be a contract
• Comparing tx.origin to msg.sender incorrectly
• Checking contract address instead of user