Random Coin Toss

ℹ️ This repository contains demonstrations for safe usage of Flow's protocol-native secure randomness in both Cadence and Solidity smart contracts.

On Flow, there are two routes to get a random value. While both are backed by Flow's Random Beacon, it is important for developers to mindfully choose between revertibleRandom or seeding their own PRNG utilizing the RandomBeaconHistory smart contract:

• When using revertibleRandom a developer is relying on randomness generation controlled by the transaction, which also has the power to abort and revert based on revertibleRandom's outputs. Therefore, revertibleRandom is only suitable for smart contract functions that exclusively run within credibly-neutral transactions which a developer can trust won't revert based on undesirable random outputs.
• In contrast, using the RandomBeaconHistory allows developers to use a committed random source (or seed) that can't be reverted. The RandomBeaconHistory is key for effectively implementing a commit-and-reveal scheme which is itself a pattern that prevents users from gaming a transaction based on the result of randomness. During the commit phase, the user commits to proceed with a future source of randomness which is revealed after the commit transaction concludes. For each block, the RandomBeaconHistory automatically stores the subsequently generated source of randomness in the final transaction of that block. This source of randomness is committed by Flow's protocol service account.

🚨 A transaction can atomically revert the entirety of its action based on results exposed within the scope of that transaction. Therefore, it is possible for a transaction calling into your smart contract to post-select favorable results and revert the transaction for unfavorable results.

💡 Post-selection - the ability for transactions to reject results they don't like - is inherent to any smart contract platform that allows transactions to roll back atomically. See this very similar Ethereum example.

✅ Utilizing a commit-and-reveal scheme is important for developers to protect their smart contracts from transaction post-selection attacks.

Via a commit-and-reveal scheme, Flow's protocol-native secure randomness can be safely used within both Cadence and Solidity smart contracts when contracts are transacted on by untrusted parties. By providing examples of commit-reveal implementations we hope to foster a more secure ecosystem of decentralized applications and encourage developers to build with best practices.

Commit-Reveal Scheme

The contracts contained in this repo demonstrate how to use Flow's onchain randomness safely in contracts that are transacted on by untrusted parties. Safe randomness here meaning non-revertible randomness, i.e. mitigating post-selection attacks via a commit-and-reveal scheme.

Random sources are committed to the RandomBeaconHistory contract by the service account at the end of every block. The RandomBeaconHistory contract provides a convenient archive, where for each past block height (starting Nov 2023) the respective 'source of randomness' can be retrieved.

When used naively, revertibleRandom as well as the RandomBeaconHistory contract are subject to post-selection attacks from transactions. In simple terms, using the random source in your contract without the protection of a commit-reveal mechanism would enable non-trusted callers to condition their interactions with your contract on the random result. In the context of a random coin toss, I could revert my transaction if I didn't win - not a very fair game.

To achieve non-revertible randomness, the contract should be structured to resolve in two phases:

1. Commit - Caller commits to the resolution of their bet with some yet unknown source of randomness (i.e. in the future)
2. Reveal - Caller can then resolve the result of their bet once the source of randomness is available in the RandomBeaconHistory with a separate transaction. From a technical perspective, this could also be called a "resolving transaction", because the transaction simply executes the smart contract with the locked-in inputs, whose output all parties committed to accept in the previous phase.

Though a caller could still condition the revealing transaction on the coin flip result, all the inputs influencing the bet's outcome have already been fixed (the source of randomness being the last one that is only generated after the commit transaction concluded). Conceptually, this corresponds to owning a winning (or loosing) lottery ticket, where the numbers have already been published, but the ticket has not been handed in to the lottery company to affirm the win (or loss). All that the resolving transaction (reveal phase) is doing is affirming the win or loss. The ticket owner could revert their resolving transaction. Though that does not change whether the ticket won or lost. Furthermore, the player has already incurred the cost of their bet and gains nothing by reverting the reveal step.

Given that Flow has both Cadence and EVM runtimes, commit-reveal patterns covering Cadence and Solidity are found in this repo as well as transactions demonstrating how Flow accounts can interact with EVM implementations from the Cadence runtime via COAs.

Deployments

Contract	Testnet	Mainnet
CoinToss.cdc	0xd1299e755e8be5e7	N/A
Xorshift128plus.cdc	0xed24dbe901028c5c	0x45caec600164c9e6
CoinToss.sol	0x959922bE3CAee4b8Cd9a407cc3ac1C251C2007B1	N/A

Further Reading

• We recommend the Flow developer documentation Advanced Concepts → Flow VRF for important concepts and context on safely using Flow's VRF.
• Flow Forum post Secure random number generator for Flow’s smart contracts summarizes the weaknesses of prevalent approaches to random number generation in other blockchains (such as using block hashes) and explains the details behind Flow's secure solution.
• Pull request introducing the RandomBeaconHistory system smart contract.
• FLIP 123: On-Chain randomness history for commit-reveal schemes describes the need for a commit-and-reveal scheme and discusses ideas for additional convenience functionality to further optimize the developer experience in the future.
• For more on Cadence Arch pre-compiles and accessing random values from EVM on Flow, see documentation on the Cadence Arch precompiled contracts.