Struct pallet_babe::CurrentBlockRandomness

pub struct CurrentBlockRandomness<T>(_);

Randomness produced semi-freshly with each block, but inherits limitations of RandomnessFromTwoEpochsAgo from which it derives.

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.

As with RandomnessFromTwoEpochsAgo, all input commitments combined with CurrentBlockRandomness should come from at least two epoch ago, except preferably not near epoch ending, and thus ideally three epochs ago.

Almost all users learn this randomness for a block when the block producer announces the block, which makes this randomness appear quite fresh. Yet, the block producer themselves learned this randomness at the beginning of epoch current_epoch - 2, at the same time as they learn RandomnessFromTwoEpochsAgo.

Aside from just biasing RandomnessFromTwoEpochsAgo, adversaries could also bias CurrentBlockRandomness by never announcing their block if doing so yields an unfavorable randomness. As such, CurrentBlockRandomness should be considered weaker than both other randomness sources provided by BABE, but CurrentBlockRandomness remains constrained by declared staking, while a randomness source like block hash is only constrained by adversaries’ unknowable computational power.

As an example use, parachains could assign block production slots based upon the CurrentBlockRandomness of their relay parent or relay parent’s parent, provided the parachain registers collators but avoids censorship sensitive functionality like slashing. Any parachain with slashing could operate BABE itself or perhaps better yet a BABE-like approach that derives its CurrentBlockRandomness, and authorizes block production, based upon the relay parent’s CurrentBlockRandomness or more likely the relay parent’s RandomnessFromTwoEpochsAgo.

Trait Implementations

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

fn random(subject: &[u8]) -> (Option<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.