# Oracle vs Prevrandao

## Overview

Randomness is the core of dice games. MemeDice supports two random number generation modes:**Oracle mode** and **Prevrandao mode**. The two modes each have their own advantages and disadvantages and are suitable for different scenarios.

***

## How it works

### Oracle mode

Oracle mode uses **commit-reveal** mechanism, generating randomness through an off-chain Oracle service.

```mermaid
sequenceDiagram
    participant O as Oracle service
    participant P as player
    participant C as game contract

    Note over O: ① Generate random number R
    Note over O: ② Calculate commitHash = keccak256(R)
    Note over O: ③ Sign (commitHash, deadline)
    O-->>P: Return commitHash + signature
    P->>C: placeBetAsync(commitHash, signature, bet parameters)
    Note over C: ④ Verify signature validity
    Note over C: ⑤ Lock commitHash
    Note over C: ⑥ Lock bet funds
    O->>C: fulfillRandomNumber(requestId, R)
    Note over C: ⑦ Verify keccak256(R) == commitHash ✅
    Note over C: ⑧ Use R to determine the result
    Note over C: ⑨ Settle (pay out if win, otherwise add to the pool)
```

**Key security point**:

* The Oracle has already determined the random number **before** the player places the bet (locked by commitHash)
* When the player places the bet, they submit commitHash, and the contract locks it
* The Oracle then reveals the random number, and the contract verifies `keccak256(R) == commitHash`
* Oracle **cannot** change the random number after seeing the bet
* player **cannot** predict the random number

### Prevrandao mode

Prevrandao mode uses Ethereum's `block.prevrandao`(EIP-4399) as the source of randomness, completing betting and settlement in the same transaction.

```mermaid
sequenceDiagram
    participant P as player
    participant C as game contract
    participant B as blockchain

    P->>C: placeBetSync(bet parameters)
    Note over C: ① Get block.prevrandao
    B-->>C: prevrandao value
    Note over C: ② Use prevrandao to determine the result
    Note over C: ③ Settle immediately
    C-->>P: Return result (same transaction)
```

**Key features**:

* Randomness comes from the blockchain protocol layer
* Betting and settlement are completed in **the same transaction** completed
* No need to wait for Oracle response
* The result is known immediately

***

## Comparison table

| Dimension                  | Oracle mode                                             | Prevrandao mode                         |
| -------------------------- | ------------------------------------------------------- | --------------------------------------- |
| **Source of randomness**   | Off-chain Oracle service                                | The block's `prevrandao` value          |
| **Security mechanism**     | Commit-Reveal cryptographic commitment                  | Blockchain protocol layer               |
| **Settlement speed**       | Need to wait for Oracle response (usually 3-10 seconds) | Instant (same transaction)              |
| **Number of transactions** | 2 transactions (bet + reveal)                           | 1 transaction (bet and settle)          |
| **Gas fee**                | Higher (Oracle reveal requires extra gas)               | Lower (single transaction)              |
| **Verifiability**          | ✅ Verifiable via commitHash                             | ✅ Verifiable via block explorer         |
| **Manipulation risk**      | 🟢 Very low (cryptographic guarantee)                   | 🟡 Moderate (validators can manipulate) |
| **Supported networks**     | BSC, Base, HyperEVM                                     | Ethereum only                           |
| **Contract calls**         | Allows contract calls                                   | EOA only (externally owned account)     |
| **Batch betting**          | ✅ Supported                                             | ✅ Supported                             |
| **Timeout protection**     | ✅ Can be canceled on timeout                            | Not needed (instant settlement)         |

***

## Use cases

### Scenarios recommended for the Oracle mode

* ✅ **BSC network**: BSC does not support prevrandao, so Oracle must be used
* ✅ **High security requirements**: The Oracle commit-reveal mechanism provides cryptographic-level security guarantees
* ✅ **Allow contract interaction**: If you need another contract to call the betting function
* ✅ **Large bets**: Higher security, suitable for large-bet scenarios

### Recommended scenarios for using Prevrandao mode

* ✅ **Ethereum mainnet**: Native support for prevrandao
* ✅ **Pursuing speed**: Instant settlement, no waiting required
* ✅ **Save Gas**: Completed in a single transaction
* ✅ **Simple experience**: Bet and get results immediately, smoother experience

***

## Oracle mode details

### Commit-Reveal mechanism

Commit-Reveal is a classic cryptographic protocol used to fairly generate random numbers in untrusted environments:

1. **Commit stage**:
   * Oracle generates a random number `R`
   * Compute the hash `commitHash = keccak256(R)`
   * send `commitHash` and the signature to the player
   * The player will `commitHash` submit to the contract
2. **Reveal stage**:
   * The Oracle reveals the original random number `R` submit to the contract
   * The contract verifies `keccak256(R) == commitHash`
   * If the verification passes, use `R` to determine the result

### Why is it secure?

```
                    Timeline
    ─────────────────────────────────────►
    
    Oracle generates R     Player bets      Oracle reveals R
    Compute commitHash   Submit commitHash   Contract verification
         │               │               │
         ▼               ▼               ▼
    R is determined         R is locked in        R is verified
    (Oracle knows)    (cannot be modified)      (publicly verifiable)
```

* **Oracle cannot cheat**: commitHash is locked in before the bet, so the Oracle cannot modify R after seeing the bet
* **Player cannot cheat**: The player does not know the value of R and cannot predict the result
* **Anyone can verify**: Everyone can verify `keccak256(R) == commitHash`

### Timeout protection

If the Oracle does not reveal the random number within the specified time (for example, if the Oracle service goes down), the player can:

1. Wait for the timeout period (`BET_TIMEOUT`, usually a few minutes)
2. Call the cancel function
3. Retrieve the betting funds

This ensures that even if the Oracle does not respond, players’ funds will not be locked up.

### Oracle Fee

The Oracle mode requires an additional Oracle service fee to cover:

* Gas fees for Oracle reveal transactions
* Operating costs of the Oracle service

***

## Prevrandao Mode Explained

### What is Prevrandao?

`prevrandao`(EIP-4399) is a block-level pseudorandom value introduced in Ethereum after The Merge. It replaced the original `block.difficulty` field.

* Each block has a unique `prevrandao` value
* This value is determined by the RANDAO mix of the previous block
* Before a block is proposed,`prevrandao` the value of is unpredictable (for ordinary users)

### Security Limitations

Prevrandao mode has some security limitations:

1. **Validator Manipulation**: Block validators can selectively include or exclude transactions, indirectly affecting the outcome
2. **EOA Only**: Contracts cannot call the betting function in Prevrandao mode, preventing flash loan attacks
3. **Network Limitations**: BSC does not support prevrandao

For a detailed security analysis, see [Prevrandao Security Analysis](/dapp.meme-en/technical-overview/prevrandao-security-analysis.md).

***

## How to choose?

```mermaid
graph TD
    A[Choose random number mode] --> B{Which network are you using?}
    B -->|BSC| C[Only Oracle can be chosen ✅]
    B -->|Ethereum| D{What do you prioritize?}
    D -->|Security| E[Choose Oracle ✅]
    D -->|Speed and cost| F[Choose Prevrandao ✅]
    D -->|Need contract calls| G[Choose Oracle ✅]
    
    style C fill:#4F46E5,color:#fff
    style E fill:#4F46E5,color:#fff
    style F fill:#059669,color:#fff
    style G fill:#4F46E5,color:#fff
```

**Simple recommendation**:

* If you're not sure, choose **Oracle mode**(default option)
* If you're on BSC, you can only choose **Oracle mode**
* If you're on Ethereum and want the utmost speed, you can choose **Prevrandao mode**

***

## Related links

* ⬅️ Back [Technical Deep Dive Overview](/dapp.meme-en/technical-overview/technical-deep-dive-overview.md)
* 🛡️ Learn More [Prevrandao Security](/dapp.meme-en/technical-overview/prevrandao-security-analysis.md)
* 🔍 Use [Fairness Verification Tool](/dapp.meme-en/leaderboard-and-logs/fairness-verification.md)


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