13 Feb 2025

Ethereum: Is there any way to make an UTXO that cannot be spent until a certain block #?

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Unlocking Potential in Ethereum: Is There Any Way to Create UTXOs with Time-Dependent Spending Restrictions?

Ethereum, like its predecessor Bitcoin, utilizes a unique set of cryptographic techniques and smart contracts to facilitate secure and transparent transactions. Two of the most interesting aspects of Ethereum are the use of Unspent Transaction Outputs (UTXOs) and time-dependent spending restrictions, particularly in regards to UTXO creation with specific block numbers.

What is a UTXO?

In Bitcoin, each transaction consists of multiple inputs that together form a UTXO. These inputs are then combined into a single output using the recipient’s public key. UTXOs have several advantages: they offer a high degree of security since each input and output is unique; they can be easily verified and validated by nodes on the network; and, most importantly for this discussion, they can be spent only when their corresponding inputs are included in a transaction.

The NLockTime Feature

In Bitcoin, transactions have an nLockTime field that specifies the earliest block number/time at which the transaction can be added to the blockchain. This field is used by miners to optimize block creation and ensure consistency across different forks of the blockchain. In Ethereum, UTXOs also incorporate this feature; each UTXO has a timestamp (timestamp) associated with it.

Time-Dependent Spending Restrictions

To create a UTXO that cannot be spent until a certain block number is reached, developers can utilize the concept of nLockTime and time-dependent spending restrictions. Here’s how:

UTXO Creation: When an Ethereum smart contract creates a new UTXO, it sets its timestamp to the current timestamp.

Addition to the Ledger: The newly created UTXO is then added to the UTXO ledger.

Spending Restriction
: To create a UTXO with time-dependent spending restrictions, developers can set an additional field called blockHash, which specifies the block number at which the UTXO should be spent.

Here’s an example of how this could work:


pragma solidity ^0.8.0;


contract Example {

    // UTXO creation

    struct UTXO {

        address public recipient;

        uint256 timestamp;

        uint256 nLockTime; // Block number (time)

        bool spent; // Whether the UTXO has been spent or not

    }


    constructor() public {

        require(0 < blockHash, "Block hash must be provided");

    }


    function createUTXO() public {

        UTXO memory utxo = UTXO(

            address(0x123456789),

            block.timestamp,

            100, // time before the UTXO can be spent

            false // not yet spent

        );

    }

}

Spending the UTXO: To spend the newly created UTXO, you would need to find its corresponding input in your transaction’s blockchain and update the nLockTime field of that input.

In Conclusion

Creating a UTXO with time-dependent spending restrictions is not as straightforward as creating an empty UTXO. Developers must carefully consider the block number at which they want the UTXO to be spent, and ensure that this value does not exceed the current timestamp or fall outside the valid range for UTXO usage.

This example demonstrates how nLockTime can be used in Ethereum smart contracts to create UTXOs with specific spending restrictions. However, it is essential to remember that this feature requires careful consideration of blockchain security and optimization strategies.

Additional Resources: