L1-7: Dead Man's Switch #

1. Problem #

Create a contract where users deposit ETH and set up a periodic check-in mechanism. If a user fails to check in within the configured time interval, a designated beneficiary can withdraw their funds. The user themselves can always withdraw.

2. Why #

Real-world scenario: Cryptocurrency holders may lose access to their assets due to accidents (death, disappearance, lost private keys). A Dead Man's Switch provides a "digital inheritance" mechanism:

• The operator periodically checks in to prove they are "still active"
• Upon loss of contact, funds automatically transfer to pre-designated beneficiaries
• No need to trust a third-party custodian

3. Solution #

Core Mechanism #

• Each user sets their own checkInInterval
• deposit() / checkIn() updates lastCheckIn[user]
• The account holder can always withdraw
• Beneficiaries can withdraw only when block.timestamp > lastCheckIn[account] + checkInInterval[account]

Data Structures #

mapping(address => uint256) public balanceOf;
mapping(address => uint256) public lastCheckIn;
mapping(address => uint256) public checkInInterval;
mapping(address => address[]) public beneficiaries;

4. Pitfalls Encountered #

4.1 Beneficiaries do not need to set an interval #

If a user does not set checkInInterval, then deadline = lastCheckIn + 0 = lastCheckIn, which means block.timestamp > deadline is always true — beneficiaries can withdraw immediately.

4.2 Auto-check-in on deposit #

deposit() should also update lastCheckIn; otherwise, a new deposit could immediately be considered "inactive."

4.3 Beneficiary list management #

removeBeneficiary needs to delete an element from the array. Use the swap-and-pop pattern for O(1).

5. Why the Pitfalls Exist #

5.1 #

checkInInterval defaults to 0. lastCheckIn + 0 = lastCheckIn, so any block.timestamp > lastCheckIn satisfies the condition. An explicit check for interval == 0 is required.

5.2 #

If depositing does not update lastCheckIn, a user who just deposited a large amount of ETH could have it withdrawn by a beneficiary (because the previous interval may have already elapsed).

5.3 #

Solidity arrays have no built-in delete method. Using list[i] = list[last]; list.pop() is the optimal solution.

6. How to Solve the Pitfalls #

function withdraw(address account, uint256 amount) external {
    if (!isAccountHolder) {
        uint256 interval = checkInInterval[account];
        if (interval == 0) revert NotExpired(); // Must be explicitly set
        if (block.timestamp <= lastCheckIn[account] + interval) revert NotExpired();
    }
    // ...
}

7. Technical Takeaways #