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Module iota::random

This module provides functionality for generating secure randomness.

use iota::address;
use iota::dynamic_field;
use iota::hex;
use iota::hmac;
use iota::object;
use iota::transfer;
use iota::tx_context;
use iota::versioned;
use std::ascii;
use std::bcs;
use std::option;
use std::string;
use std::vector;

Module Functions​

prv create​

Create and share the Random object. This function is called exactly once, when the Random object is first created. Can only be called by genesis or change_epoch transactions.

fun create(ctx: &mut iota::tx_context::TxContext)
Implementation
fun create(ctx: &mut TxContext) {
    assert!(ctx.sender() == @0x0, ENotSystemAddress);
    let version = CURRENT_VERSION;
    let inner = RandomInner {
        version,
        epoch: ctx.epoch(),
        randomness_round: 0,
        random_bytes: vector[],
    };
    let self = Random {
        id: object::randomness_state(),
        inner: versioned::create(version, inner, ctx),
    };
    transfer::share_object(self);
}

Structs​

struct Random​

Singleton shared object which stores the global randomness state. The actual state is stored in a versioned inner field.

public struct Random has key
Fields

pub new_generator​

Create a generator. Can be used to derive up to MAX_U16 * 32 random bytes.

Using randomness can be error-prone if you don't observe the subtleties in its correct use, for example, randomness dependent code might be exploitable to attacks that carefully set the gas budget in a way that breaks security. For more information, see: https://docs.iota.org/developer/advanced/onchain-randomness

public fun new_generator(r: &iota::random::Random, ctx: &mut iota::tx_context::TxContext): iota::random::RandomGenerator
Implementation
public fun new_generator(r: &Random, ctx: &mut TxContext): RandomGenerator {
    let inner = r.load_inner();
    let seed = hmac_sha3_256(
        &inner.random_bytes,
        &ctx.fresh_object_address().to_bytes(),
    );
    RandomGenerator { seed, counter: 0, buffer: vector[] }
}

prv load_inner​

fun load_inner(self: &iota::random::Random): &iota::random::RandomInner
Implementation
fun load_inner(self: &Random): &RandomInner {
    let version = self.inner.version();
    // Replace this with a lazy update function when we add a new version of the inner object.
    assert!(version == CURRENT_VERSION, EWrongInnerVersion);
    let inner: &RandomInner = self.inner.load_value();
    assert!(inner.version == version, EWrongInnerVersion);
    inner
}

prv load_inner_mut​

fun load_inner_mut(self: &mut iota::random::Random): &mut iota::random::RandomInner
Implementation
fun load_inner_mut(self: &mut Random): &mut RandomInner {
    let version = versioned::version(&self.inner);
    // Replace this with a lazy update function when we add a new version of the inner object.
    assert!(version == CURRENT_VERSION, EWrongInnerVersion);
    let inner: &mut RandomInner = self.inner.load_value_mut();
    assert!(inner.version == version, EWrongInnerVersion);
    inner
}

prv update_randomness_state​

Record new randomness. Called when executing the RandomnessStateUpdate system transaction.

fun update_randomness_state(self: &mut iota::random::Random, new_round: u64, new_bytes: vector<u8>, ctx: &iota::tx_context::TxContext)
Implementation
fun update_randomness_state(
    self: &mut Random,
    new_round: u64,
    new_bytes: vector<u8>,
    ctx: &TxContext,
) {
    // Validator will make a special system call with sender set as 0x0.
    assert!(ctx.sender() == @0x0, ENotSystemAddress);
    // Randomness should only be incremented.
    let epoch = ctx.epoch();
    let inner = self.load_inner_mut();
    if (inner.randomness_round == 0 && inner.epoch == 0 && inner.random_bytes.is_empty()) {
        // First update should be for round zero.
        assert!(new_round == 0, EInvalidRandomnessUpdate);
    } else {
        // Subsequent updates should either increase epoch or increment randomness_round.
        // Note that epoch may increase by more than 1 if an epoch is completed without
        // randomness ever being generated in that epoch.
        assert!(
            (epoch > inner.epoch && new_round == 0) ||
                    (new_round == inner.randomness_round + 1),
            EInvalidRandomnessUpdate,
        );
    };
    inner.epoch = ctx.epoch();
    inner.randomness_round = new_round;
    inner.random_bytes = new_bytes;
}

struct RandomInner​

public struct RandomInner has store
Fields
version: u64
epoch: u64
randomness_round: u64
random_bytes: vector<u8>

struct RandomGenerator​

Unique randomness generator, derived from the global randomness.

public struct RandomGenerator has drop
Fields
seed: vector<u8>
counter: u16
buffer: vector<u8>

pub generate_bool​

Generate a boolean.

public fun generate_bool(g: &mut iota::random::RandomGenerator): bool
Implementation
public fun generate_bool(g: &mut RandomGenerator): bool {
    (uint_from_bytes!(g, 1) & 1) == 1
}

pub generate_bytes​

Generate n random bytes.

public fun generate_bytes(g: &mut iota::random::RandomGenerator, num_of_bytes: u16): vector<u8>
Implementation
public fun generate_bytes(g: &mut RandomGenerator, num_of_bytes: u16): vector<u8> {
    let mut result = vector[];
    // Append RAND_OUTPUT_LEN size buffers directly without going through the generator's buffer.
    let num_of_blocks = num_of_bytes / RAND_OUTPUT_LEN;
    num_of_blocks.do!(|| result.append(g.derive_next_block()));
    // Fill the generator's buffer if needed.
    let num_of_bytes = num_of_bytes as u64;
    let remaining = num_of_bytes - result.length();
    if (g.buffer.length() < remaining) {
        let next_block = g.derive_next_block();
        g.buffer.append(next_block);
    };
    // Take remaining bytes from the generator's buffer.
    remaining.do!(|| result.push_back(g.buffer.pop_back()));
    result
}

pub generate_u128​

Generate a u128.

public fun generate_u128(g: &mut iota::random::RandomGenerator): u128
Implementation
public fun generate_u128(g: &mut RandomGenerator): u128 {
    uint_from_bytes!(g, 16)
}

pub generate_u128_in_range​

Generate a random u128 in [min, max] (with a bias of 2^-64).

public fun generate_u128_in_range(g: &mut iota::random::RandomGenerator, min: u128, max: u128): u128
Implementation
public fun generate_u128_in_range(g: &mut RandomGenerator, min: u128, max: u128): u128 {
    uint_in_range!(g, min, max, 24)
}

pub generate_u16​

Generate a u16.

public fun generate_u16(g: &mut iota::random::RandomGenerator): u16
Implementation
public fun generate_u16(g: &mut RandomGenerator): u16 {
    uint_from_bytes!(g, 2)
}

pub generate_u16_in_range​

Generate a random u16 in [min, max] (with a bias of 2^-64).

public fun generate_u16_in_range(g: &mut iota::random::RandomGenerator, min: u16, max: u16): u16
Implementation
public fun generate_u16_in_range(g: &mut RandomGenerator, min: u16, max: u16): u16 {
    uint_in_range!(g, min, max, 10)
}

pub generate_u256​

Generate a u256.

public fun generate_u256(g: &mut iota::random::RandomGenerator): u256
Implementation
public fun generate_u256(g: &mut RandomGenerator): u256 {
    uint_from_bytes!(g, 32)
}

pub generate_u32​

Generate a u32.

public fun generate_u32(g: &mut iota::random::RandomGenerator): u32
Implementation
public fun generate_u32(g: &mut RandomGenerator): u32 {
    uint_from_bytes!(g, 4)
}

pub generate_u32_in_range​

Generate a random u32 in [min, max] (with a bias of 2^-64).

public fun generate_u32_in_range(g: &mut iota::random::RandomGenerator, min: u32, max: u32): u32
Implementation
public fun generate_u32_in_range(g: &mut RandomGenerator, min: u32, max: u32): u32 {
    uint_in_range!(g, min, max, 12)
}

pub generate_u64​

Generate a u64.

public fun generate_u64(g: &mut iota::random::RandomGenerator): u64
Implementation
public fun generate_u64(g: &mut RandomGenerator): u64 {
    uint_from_bytes!(g, 8)
}

pub generate_u64_in_range​

public fun generate_u64_in_range(g: &mut iota::random::RandomGenerator, min: u64, max: u64): u64
Implementation
public fun generate_u64_in_range(g: &mut RandomGenerator, min: u64, max: u64): u64 {
    uint_in_range!(g, min, max, 16)
}

pub generate_u8​

Generate a u8.

public fun generate_u8(g: &mut iota::random::RandomGenerator): u8
Implementation
public fun generate_u8(g: &mut RandomGenerator): u8 {
    uint_from_bytes!(g, 1)
}

pub generate_u8_in_range​

Generate a random u8 in [min, max] (with a bias of 2^-64).

public fun generate_u8_in_range(g: &mut iota::random::RandomGenerator, min: u8, max: u8): u8
Implementation
public fun generate_u8_in_range(g: &mut RandomGenerator, min: u8, max: u8): u8 {
    uint_in_range!(g, min, max, 9)
}

pub shuffle​

Shuffle a vector using the random generator (Fisher–Yates/Knuth shuffle).

public fun shuffle<T>(g: &mut iota::random::RandomGenerator, v: &mut vector<T>)
Implementation
public fun shuffle<T>(g: &mut RandomGenerator, v: &mut vector<T>) {
    let n = v.length();
    if (n == 0) return;
    assert!(n <= U16_MAX, EInvalidLength);
    let n = n as u16;
    let end = n - 1;
    end.do!(|i| {
        let j = g.generate_u16_in_range(i, end);
        v.swap(i as u64, j as u64);
    });
}

prv derive_next_block​

Get the next block of 32 random bytes.

fun derive_next_block(g: &mut iota::random::RandomGenerator): vector<u8>
Implementation
fun derive_next_block(g: &mut RandomGenerator): vector<u8> {
    g.counter = g.counter + 1;
    hmac_sha3_256(&g.seed, &bcs::to_bytes(&g.counter))
}

prv uint_from_bytes (macro)​

macro fun uint_from_bytes<$T: drop>($g: &mut iota::random::RandomGenerator, $num_of_bytes: u8): $T
Implementation
macro fun uint_from_bytes<$T: drop>($g: &mut RandomGenerator, $num_of_bytes: u8): $T {
    let g = $g;
    let num_of_bytes = $num_of_bytes;
    if (g.buffer.length() < num_of_bytes as u64) {
        let next_block = g.derive_next_block();
        g.buffer.append(next_block);
    };
    // TODO: why regression test fails if we use $T instead of u256
    let mut result: u256 = 0;
    num_of_bytes.do!(|_| {
        let byte = g.buffer.pop_back() as u256;
        result = (result << 8) + byte;
    });
    result as $T
}

prv uint_in_range (macro)​

Helper macro to generate a random uint in [min, max] using a random number with num_of_bytes bytes. Assumes that the caller verified the inputs, and uses num_of_bytes to control the bias (e.g., 8 bytes larger than the actual type used by the caller function to limit the bias by 2^-64).

macro fun uint_in_range<$T: drop>($g: &mut iota::random::RandomGenerator, $min: $T, $max: $T, $num_of_bytes: u8): $T
Implementation
macro fun uint_in_range<$T: drop>(
    $g: &mut RandomGenerator,
    $min: $T,
    $max: $T,
    $num_of_bytes: u8,
): $T {
    let min = $min;
    let max = $max;
    assert!(min <= max, EInvalidRange);
    if (min == max) return min;
    // Pick a random number in [0, max - min] by generating a random number that is larger than max-min, and taking
    // the modulo of the random number by the range size. Then add the min to the result to get a number in
    // [min, max].
    let range_size = (max - min) as u256 + 1;
    let rand = uint_from_bytes!($g, $num_of_bytes);
    min + (rand % range_size as $T)
}

Constants​

err ENotSystemAddress​

const ENotSystemAddress: u64 = 0;

err EWrongInnerVersion​

const EWrongInnerVersion: u64 = 1;

err EInvalidRandomnessUpdate​

const EInvalidRandomnessUpdate: u64 = 2;

err EInvalidRange​

const EInvalidRange: u64 = 3;

err EInvalidLength​

const EInvalidLength: u64 = 4;

const CURRENT_VERSION​

const CURRENT_VERSION: u64 = 1;

const RAND_OUTPUT_LEN​

const RAND_OUTPUT_LEN: u16 = 32;

const U16_MAX​

const U16_MAX: u64 = 65535;