Expand description

BABE (Blind Assignment for Blockchain Extension)

BABE is a slot-based block production mechanism which uses a VRF PRNG to randomly perform the slot allocation. On every slot, all the authorities generate a new random number with the VRF function and if it is lower than a given threshold (which is proportional to their weight/stake) they have a right to produce a block. The proof of the VRF function execution will be used by other peer to validate the legitimacy of the slot claim.

The engine is also responsible for collecting entropy on-chain which will be used to seed the given VRF PRNG. An epoch is a contiguous number of slots under which we will be using the same authority set. During an epoch all VRF outputs produced as a result of block production will be collected on an on-chain randomness pool. Epoch changes are announced one epoch in advance, i.e. when ending epoch N, we announce the parameters (randomness, authorities, etc.) for epoch N+2.

Since the slot assignment is randomized, it is possible that a slot is assigned to multiple validators in which case we will have a temporary fork, or that a slot is assigned to no validator in which case no block is produced. Which means that block times are not deterministic.

The protocol has a parameter c [0, 1] for which 1 - c is the probability of a slot being empty. The choice of this parameter affects the security of the protocol relating to maximum tolerable network delays.

In addition to the VRF-based slot assignment described above, which we will call primary slots, the engine also supports a deterministic secondary slot assignment. Primary slots take precedence over secondary slots, when authoring the node starts by trying to claim a primary slot and falls back to a secondary slot claim attempt. The secondary slot assignment is done by picking the authority at index:

blake2_256(epoch_randomness ++ slot_number) % authorities_len.

The secondary slots supports either a SecondaryPlain or SecondaryVRF variant. Comparing with SecondaryPlain variant, the SecondaryVRF variant generates an additional VRF output. The output is not included in beacon randomness, but can be consumed by parachains.

The fork choice rule is weight-based, where weight equals the number of primary blocks in the chain. We will pick the heaviest chain (more primary blocks) and will go with the longest one in case of a tie.

An in-depth description and analysis of the protocol can be found here: https://research.web3.foundation/en/latest/polkadot/block-production/Babe.html



  • BABE authority selection and slot claiming.
  • Schema for BABE epoch changes in the aux-db.



  • An consensus log item for BABE.
  • Errors encountered by the babe authorship task.
  • Information about the next epoch config, if changed. This is broadcast in the first block of the epoch, and applies using the same rules as NextEpochDescriptor.
  • A BABE pre-runtime digest. This contains all data required to validate a block and for the BABE runtime module. Slots can be assigned to a primary (VRF based) and to a secondary (slot number based).




  • API necessary for block authorship with BABE.
  • A digest item which is usable with BABE consensus.
  • An oracle for when major synchronization work is being undertaken.


  • Produce a BABE block-import object to be used later on in the construction of an import-queue.
  • Read configuration from the runtime state at current best block.
  • Extract the BABE pre digest from the given header. Pre-runtime digests are mandatory, the function will return Err if none is found.
  • Start an import queue for the BABE consensus algorithm.
  • Reverts protocol aux data to at most the last finalized block. In particular, epoch-changes and block weights announced after the revert point are removed.
  • Start the babe worker.

Type Definitions

  • A Babe authority identifier. Necessarily equivalent to the schnorrkel public key used in the main Babe module. If that ever changes, then this must, too.
  • A Babe authority keypair. Necessarily equivalent to the schnorrkel public key used in the main Babe module. If that ever changes, then this must, too.
  • A Babe authority signature.
  • The weight of an authority.
  • The cumulative weight of a BABE block, i.e. sum of block weights starting at this block until the genesis block.
  • Randomness type required by BABE operations.