Merge pull request #902 from onflow/vrf-doc

create vrf documentation

docs/evm/guides/vrf.md

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+---
+title: VRF (Randomness) in Solidity
+sidebar_label: VRF (Randomness) in Solidity
+sidebar_position: 6
+---
+
+## **Introduction**
+
+Flow provides secure, native on-chain randomness that developers can leverage through Cadence Arch, a precompiled contract available on the Flow EVM environment. This guide will walk through how Solidity developers can use Cadence Arch to access Flow’s verifiable randomness using Solidity.
+
+### **What is Cadence Arch?**
+
+[Cadence Arch](https://github.com/onflow/flips/blob/main/protocol/20231116-evm-support.md#cadence-arch) is a precompiled smart contract that allows Solidity developers on Flow EVM to interact with Flow’s randomness and other network features like block height. This contract can be accessed using its specific address, and Solidity developers can make static calls to retrieve random values and other information.
+
+---
+
+## **Prerequisites**
+
+- Basic Solidity knowledge.
+- Installed Metamask extension.
+- Remix IDE for compilation and deployment.
+- Flow EVM Testnet setup in Metamask.
+
+## **Network Information for Flow EVM**
+
+| **Parameter**       | **Value**                                                                     |
+| ------------------- | ----------------------------------------------------------------------------- |
+| **Network Name**    | EVM on Flow Testnet                                                           |
+| **RPC Endpoint**    | [https://testnet.evm.nodes.onflow.org](https://testnet.evm.nodes.onflow.org/) |
+| **Chain ID**        | 545                                                                           |
+| **Currency Symbol** | FLOW                                                                          |
+| **Block Explorer**  | [https://evm-testnet.flowscan.io](https://evm-testnet.flowscan.io/)           |
+
+## **Steps to Connect Flow EVM Testnet to Metamask**
+
+1. Open Metamask and click **Networks** -> **Add Network**.
+2. Enter the following details:
+   - **Network Name**: EVM on Flow Testnet
+   - **RPC URL**: `https://testnet.evm.nodes.onflow.org`
+   - **Chain ID**: `545`
+   - **Currency Symbol**: `FLOW`
+   - **Block Explorer**: `https://evm-testnet.flowscan.io`
+3. Click **Save** and switch to the Flow EVM Testnet.
+
+![image.png](./vrf-1.png)
+
+## **Obtaining FLOW for Testnet**
+
+You can fund your account with testnet FLOW using the [Flow Faucet](https://testnet-faucet.onflow.org/fund-account). Enter your Flow-EVM testnet address, and you’ll receive testnet FLOW tokens to interact with smart contracts.
+
+---
+
+## **Solidity Code Example: Retrieving Random Numbers**
+
+Below is a simple Solidity contract that interacts with the Cadence Arch contract to retrieve a pseudo-random number.
+
+```solidity
+// SPDX-License-Identifier: GPL-3.0
+pragma solidity >=0.7.0 <0.9.0;
+
+contract CadenceArchCaller {
+    // Address of the Cadence Arch contract
+    address constant public cadenceArch = 0x0000000000000000000000010000000000000001;
+
+    // Function to fetch a pseudo-random value
+    function revertibleRandom() public view returns (uint64) {
+        // Static call to the Cadence Arch contract's revertibleRandom function
+        (bool ok, bytes memory data) = cadenceArch.staticcall(abi.encodeWithSignature("revertibleRandom()"));
+        require(ok, "Failed to fetch a random number through Cadence Arch");
+        uint64 output = abi.decode(data, (uint64));
+        // Return the random value
+        return output;
+    }
+}
+
+```
+
+### **Explanation of the Contract**
+
+1. **Cadence Arch Address**:
+
+   The `cadenceArch` variable stores the address of the Cadence Arch precompiled contract (`0x0000000000000000000000010000000000000001`), which is constant across Flow EVM.
+
+2. **Revertible Random**:
+
+   The `revertibleRandom()` function makes a static call to the `revertibleRandom()` function to fetch a pseudo-random number. If the call is successful, it decodes the result as a `uint64` random value.
+
+---
+
+## **Deploying and Testing the Contract**
+
+### Compile and Deploy the Contract
+
+1. Open Remix IDE.
+2. Create a new file and paste the Solidity code above.
+
+![image.png](./vrf-2.png)
+
+3. Compile the contract by selecting the appropriate Solidity compiler version (0.8.x).
+
+![image.png](./vrf-3.png)
+
+4. Connect Remix to your Metamask wallet (with Flow EVM testnet) by selecting **Injected Web3** as the environment.
+
+![image.png](./vrf-4.png)
+
+5. Deploy the contract.
+
+![image.png](./vrf-5.png)
+
+### Call revertibleRandom
+
+After deployment, you can interact with the contract to retrieve a random number.
+
+Call the `revertibleRandom()` function in the left sidebar on the deployed contract. This will fetch a pseudo-random number generated by Flow’s VRF.
+
+![image.png](./vrf-6.png)
+
+The result will be a `uint64` random number generated on Flow EVM.
+
+---
+
+## **Generating Random Numbers in a Range**
+
+For use-cases like games and lotteries, it’s useful to generate a random number within a specified range, the following example shows how to get a value between a min and max number.
+
+```solidity
+// SPDX-License-Identifier: GPL-3.0
+pragma solidity >=0.7.0 <0.9.0;
+
+contract RandomInRange {
+address constant public cadenceArch = 0x0000000000000000000000010000000000000001;
+
+// Generate a random number between min and max
+function getRandomInRange(uint64 min, uint64 max) public view returns (uint64) {
+    // Static call to the Cadence Arch contract's revertibleRandom function
+    (bool ok, bytes memory data) = cadenceArch.staticcall(abi.encodeWithSignature("revertibleRandom()"));
+    require(ok, "Failed to fetch a random number through Cadence Arch");
+    uint64 randomNumber = abi.decode(data, (uint64));
+
+    // Return the number in the specified range
+    return (randomNumber % (max + 1 - min)) + min;
+	}
+}
+```
+
+## **Secure Randomness with Commit-Reveal Scheme in Solidity**
+
+The **`revertibleRandom()`** function can be directly used to generate a pseudo-random number. However, in certain situations, especially involving untrusted users, this function exposes a vulnerability: the ability of a transaction to **revert after seeing the random result**.
+
+**The Issue with Using `revertibleRandom()` Directly:**
+
+- When an untrusted party calls a contract function that uses `revertibleRandom()`, they receive the random number **during the transaction execution**.
+- **Post-selection** is the ability of the caller to abort the transaction if the random outcome is unfavorable. In this case, the user could choose to revert the transaction (for example, if they lose a bet) and attempt to call the function again in hopes of a better outcome.
+- This can lead to a form of **transaction reversion attack**, where the randomness can be exploited to repeatedly attempt transactions until a favorable result is obtained.
+
+## Read More
+
+For further details on Flow’s randomness and secure development practices, check out the [Flow Randomness Documentation](https://developers.flow.com/build/advanced-concepts/randomness). You can also view an exammple in both Solidity and Cadence of a [cointoss implentation](https://github.com/onflow/random-coin-toss/tree/add-solidity-impl) using the VRF.
+
+_This documentation was contributed by Noah Naizir, a community developer._