Verifiable Delay Function (VDF)

What Is a Verifiable Delay Function?

A Verifiable Delay Function (VDF) is designed to force a time delay into a digital process. Unlike standard mathematical problems that can be solved faster by adding more computers (parallel processing), a VDF is strictly sequential. This means that no matter how much hardware an attacker has, they cannot skip ahead or finish the calculation faster than the time dictated by the function’s parameters.

In the blockchain ecosystem, VDFs are primarily used to ensure fairness. They act as a time-lock that prevents participants from seeing the outcome of a process, such as a lottery winner or a block producer selection, until it is too late to manipulate the inputs.

How Does a Verifiable Delay Function Work?

The mechanics of a VDF rely on three core properties:

• Sequentiality: The function consists of steps that depend entirely on the result of the previous step. Due to this dependency, the work cannot be split across multiple processors. The total time taken is determined by the speed of a single processing thread.
• Efficient Verifiability: While the prover must spend a significant amount of time and effort to find the answer, any verifier can check the correctness of that answer in a fraction of a second using a small cryptographic proof.
• Evaluation Uniqueness: For any input/output, only one valid proof passes verification. This prevents malicious actors from providing multiple valid-looking results to confuse the network.

Why VDFs Matter in Blockchain

While VDFs are mathematically complex, they are essential for preventing human or bot-driven manipulation in decentralized networks.

For one thing, VDFs solve a fundamental problem in distributed systems: the Last Revealer attack. In many decentralized protocols, a random number is generated by combining contributions from several people. However, the last person to submit their data can often calculate the final result ahead of time and decide whether or not to participate based on whether the outcome favors them.

By adding a VDF to the process, the final random number is not revealed until long after the submission window has closed. This ensures that no one can predict the outcome in time to influence it.