On-Chain Randomness
Generating pseudo-random values in Move is similar to solutions in other languages. A Move function can create a new instance of RandomGenerator and use it for generating random values of different types, for example, generate_u128(&mut generator), generate_u8_in_range(&mut generator, 1, 6), or,
entry fun roll_dice(r: &Random, ctx: &mut TxContext): Dice {
let mut generator = new_generator(r, ctx); // generator is a PRG
Dice { value: random::generate_u8_in_range(&mut generator, 1, 6) }
}
Random has a reserved address 0x8. See random.move for the Move APIs for accessing randomness on Sui.
Although Random is a shared object, it is inaccessible for mutable operations, and any transaction attempting to modify it fails.
Having access to random numbers is only one part of designing secure applications, you should also pay careful attention to how you use that randomness. To securely access randomness:
- Define your function as (private)
entry. - Prefer generating randomness using function-local
RandomGenerator. - Make sure that the "unhappy path" of your function does not charge more gas than the "happy path".
Limited resources and Random dependent flows
Be aware that some resources that are available to transactions are limited. If you are not careful, an attacker can break or exploit your application by deliberately controlling the point where your function runs out of resources.
Concretely, gas is such a resource. Consider the following vulnerable code:
// Insecure implementation, do not use!
entry fun insecure_play(r: &Random, payment: Coin<SUI>, ...) {
...
let mut generator = new_generator(r, ctx);
let win = generator.generate_bool();
if (win) { // happy flow
... cheap computation ...
} else {
... very expensive computation ...
}
}
Observe that the gas costs of a transaction that calls insecure_play depends on the value of win.
An attacker could call this function with a gas budget that is sufficient for the "happy flow" but not the "unhappy one", resulting in it either winning or reverting the transaction (but never losing the payment).
The Random API does not automatically prevent this kind of attack, and you must be aware of this subtlety when designing your contracts.
Other limited resources per transaction that you should consider are:
- The number of new objects.
- The number of objects that can be used (including dynamic fields).
- Number of events emitted.
- Number of UIDs generated, or deleted, or transferred.
For many use cases this attack is not an issue, like when selecting a raffle winner, or lottery numbers, as the code running is independent of the randomness. However, in the cases where it can be problematic, you can consider one of the following:
- Use two steps:
Split the logic to two functions that must be called by different transactions.
The first function, called by transaction
tx1, fetches a random value and stores it in an object that is unreadable by other commands intx1(for example, by transferring the object to the caller, or, by storing the tx digest and checking it is different on read). A second function, called by transactiontx2, reads the stored value and completes the operation.tx2might indeed fail, but now the random value is fixed and cannot be modified using repeated calls. It is important that the inputs to the second function are fixed and cannot be modified aftertx1(otherwise an attacker can modify them after seeing the randomness committed bytx1). Also, it is important to gracefully handle the case in which the second step is never completed (for example, charge a fee in the first step). See this for example implementation. - Write the function in a way that the happy flow consumes more gas than the unhappy one.
- Keep in mind that external functions or native ones can change in the future, potentially resulting in different costs compared to the time you conducted your tests.
- profile-transaction can be used to profile the costs of a transaction.
Use (non-public) entry functions
While composition is very powerful for smart contracts, it opens the door to attacks on functions that use randomness. Consider for example a betting game that uses randomness for rolling dice:
module games::dice {
...
public enum Ticket has drop {
Lost,
Won,
}
public fun is_winner(t: &Ticket): bool {
match (t) {
Ticket::Won => true,
Ticket::Lost => false,
}
}
/// If you guess correctly the output, then you get a GuessedCorrectly object.
/// Otherwise you get nothing.
public fun play_dice(guess: u8, fee: Coin<SUI>, r: &Random, ctx: &mut TxContext): Ticket {
// Pay for the turn
assert!(coin::value(&fee) == 1000000, EInvalidAmount);
transfer::public_transfer(fee, CREATOR_ADDRESS);
// Roll the dice
let mut generator = new_generator(r, ctx);
if (guess == generator.generate_u8_in_range(1, 6)) {
Ticket::Won
} else {
Ticket::Lost
}
}
...
}
An attacker can deploy the next function:
public fun attack(guess: u8, r: &Random, ctx: &mut TxContext): Ticket {
let t = dice::play_dice(guess, r, ctx);
// revert the transaction if play_dice lost
assert!(!dice::is_winner(&t), 0);
t
}
The attacker can now call attack with a guess, and always revert the fee transfer if the guess is incorrect.
To protect against composition attacks, define your function as a private entry function so functions from other modules cannot call it.
The Move compiler enforces this behavior by rejecting public functions with Random as an argument.
Programmable transaction block (PTB) restrictions
A similar attack to the one previously described involves PTBs even when play_dice is defined as a private entry function.
For example, consider the entry play_dice(guess: u8, fee: Coin<SUI>, r: &Random, ctx: &mut TxContext): Ticket { … } function defined earlier, the attacker can publish the function
public fun attack(t: Ticket): Ticket {
assert!(!dice::is_winner(&t), 0);
t
}
and send a PTB with commands play_dice(...), attack(Result(0)) where Result(0) is the output of the first command.
As before, the attack takes advantage of the atomic nature of PTBs and always reverts the entire transaction if the
guess was incorrect, without paying the fee. Sending multiple transactions can repeat the attack, each one executed with
different randomness and reverted if the guess is incorrect.
To protect against PTB-based composition attacks, Sui rejects PTBs that have commands that are not TransferObjects or MergeCoins following a MoveCall command that uses Random as an input.
Instantiating RandomGenerator
RandomGenerator is secure as long as it's created by the consuming module. If passed as an argument, the caller might be able to predict the outputs of that RandomGenerator instance (for example, by calling bcs::to_bytes(&generator) and parsing its internal state).
The Move compiler enforces this behavior by rejecting public functions with RandomGenerator as an argument.
Accessing Random from TypeScript
If you want to call roll_dice(r: &Random, ctx: &mut TxContext) in module example, use the following code snippet:
const txb = new Transaction();
txb.moveCall({
target: "${PACKAGE_ID}::example::roll_dice",
arguments: [txb.object('0x8')]
});
...