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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get the address of the immediate caller.

Blockchain / Solidity

address caller = [1];

Drag options to blanks, or click blank then click option'

Amsg.sender

Btx.origin

Cthis

Daddress(this)

Attempts:

3 left

💡 Hint

Common Mistakes

Using tx.origin instead of msg.sender when you want the immediate caller.

msg.sender is the address of the immediate caller of the function, which is what you want here.

2fill in blank

medium

Complete the code to get the original external account that started the transaction.

Blockchain / Solidity

address origin = [1];

Drag options to blanks, or click blank then click option'

Amsg.sender

Baddress(this)

Ctx.origin

Dblock.coinbase

Attempts:

3 left

💡 Hint

Common Mistakes

Using msg.sender instead of tx.origin to get the original transaction sender.

tx.origin is the original external account that started the transaction, even through multiple calls.

3fill in blank

hard

Fix the error in the require statement to check the immediate caller is the owner.

Blockchain / Solidity

require([1] == owner, "Not owner");

Drag options to blanks, or click blank then click option'

Amsg.sender

Baddress(this)

Cowner

Dtx.origin

Attempts:

3 left

💡 Hint

Common Mistakes

Using tx.origin in authorization checks, which can be exploited.

You should use msg.sender to check the immediate caller, not tx.origin.

4fill in blank

hard

Fill both blanks to create a function that returns the original transaction sender and the immediate caller.

Blockchain / Solidity

function getSenders() public view returns (address origin, address sender) {
    origin = [1];
    sender = [2];
}

Drag options to blanks, or click blank then click option'

Atx.origin

Bmsg.sender

Caddress(this)

Dblock.coinbase

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping tx.origin and msg.sender in the return values.

This function returns tx.origin as the original transaction sender and msg.sender as the immediate caller.

5fill in blank

hard

Fill all three blanks to create a function that returns the original sender, immediate caller, and contract address.

Blockchain / Solidity

function getContext() public view returns (address origin, address sender, address contractAddr) {
    origin = [1];
    sender = [2];
    contractAddr = [3];
}

Drag options to blanks, or click blank then click option'

Atx.origin

Bmsg.sender

Caddress(this)

Dblock.coinbase

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing contract address with sender addresses.

This function returns tx.origin as the original transaction sender, msg.sender as the immediate caller, and address(this) as the contract's own address.

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