Struct pallet_babe::RandomnessFromTwoEpochsAgo

pub struct RandomnessFromTwoEpochsAgo<T>(_);

Randomness usable by consensus protocols that depend upon finality and take action based upon on-chain commitments made during the epoch before the previous epoch.

An off-chain consensus protocol requires randomness be finalized before usage, but one extra epoch delay beyond RandomnessFromOneEpochAgo suffices, under the assumption that finality never stalls for longer than one epoch.

All randomness is relative to commitments to any other inputs to the computation: If Alice samples randomness near perfectly using radioactive decay, but then afterwards Eve selects an arbitrary value with which to xor Alice’s randomness, then Eve always wins whatever game they play.

All input commitments used with RandomnessFromTwoEpochsAgo should come from at least three epochs ago. We require BABE session keys be registered at least three epochs before being used to derive ParentBlockRandomness for example.

All users learn RandomnessFromTwoEpochsAgo when epoch current_epoch - 1 starts, although some learn it a few block earlier inside epoch current_epoch - 2.

Adversaries with enough block producers could bias this randomness by choosing upon what their block producers build at the end of epoch current_epoch - 2 or the beginning epoch current_epoch - 1, or skipping slots at the end of epoch current_epoch - 2.

Adversaries should not possess many block production slots towards the beginning or end of every epoch, but they possess some influence over when they possess more slots.

Trait Implementations

impl<T: Config> Randomness<<T as Config>::Hash, <T as Config>::BlockNumber> for RandomnessFromTwoEpochsAgo<T>

fn random(subject: &[u8]) -> (T::Hash, T::BlockNumber)

Get the most recently determined random seed, along with the time in the past since when it was determinable by chain observers.

fn random_seed() -> (Output, BlockNumber)

Get the basic random seed.