The purpose of this document is to give a “mini-spec” for the beacon chain mechanism for the purpose of security analysis, safety proofs and other academic reasoning, separate from relatively irrelevant implementation details.
Beacon chain stage 1 (no justification, no dynasty changes)
Suppose there is a validator set V = {V_1 ... V_n} (we assume for simplicity that all validators have an equal amount of “stake”), with subsets S_1 .... S_{64} (no guarantee these subsets are disjoint, but we can guarantee |S_i| \ge floor(\frac{|V|}{64})), where |x| refers to set size (ie. the number of validators, or whatever other kind of object, in x). Suppose also that the system generates a random permutation of validator indices, {p_1 ... p_N}.
Note: if an attacker controls less than \frac{1}{3} of the stake, then if |S_i| \ge 892 there is a less than 2^{-80} chance that the attacker controls more than \frac{1}{2} of S_i, and there is a less than 2^{-100} chance that an attacker controls all 64 indices in a given span i_k .... i_{k+63}. We can assume that it is certain that neither of these things will happen (that is, we can assume there exists a substring of validator indices p_{i_1}, p_{i_2} ... with p_{i_{k+1}} - p_{i_k} < 64 and that every S_i is majority honest).
We divide time into slots; if the genesis timestamp of the system is T_0, then slot i consists of the time period [T_0 + 8i, T_0 + 8(i+1)). When slot i begins, validator V_{p_{i\ mod\ N}} is expected to create (“propose”) a block, which contains a pointer to some parent block that they perceive as the “head of the chain”, and includes all of the attestations that they know about that have not yet been included into that chain. After 4 seconds, validators in S_{i\ mod\ 64} are expected to take the newly published block (if it has actually been published) into account, determine what they think is the new “head of the chain” (if all is well, this will generally be the newly published block), and publish a (signed) attestation, [current\_slot, h_1, h_2 .... h_{64}], where h_1 ... h_{64} are the hashes of the ancestors of the head up to 64 slots (if a chain has missing slots between heights a and b, then use the hash of the block at height a for heights a+1 .... b-1), and current\_slot is the current slot number.
The fork choice used is “recursive maximum attestation”. The mechanism is as follows:
- Set H to equal the genesis block.
- Choose the descendant of H such that the highest number of validators attests to H (ie. published an attestation where H \in {h_1 ... h_{64}}).
- Repeat (2) until H is a block with no descendants.
Claims:
- Safety: assuming the attacker controls less than \frac{1}{3} of V, and selected the portion of V to control before the validators were randomly sorted, the chain will never revert (ie. once a block is part of the canonical chain, it will be part of the canonical chain forever).
- Incentive-compatibility: assume that there is a reward for including attestations, and for one’s attestation being included in the chain (and this reward is higher if the attestation is included earlier). Proposing blocks and attesting to blocks correctly is incentive-compatible.
- Randomness fairness: in the long run, the attacker cannot gain by manipulating the randomness
Beacon chain stage 2 (add justification and finalization)
As the chain receives attestations, it keeps track of the total set of validators that attest to each block. The chain keeps track of a variable, last\_justified\_slot, which starts at 0. If, for some block B in the chain, a set of validators V_B attest to it, with |V_B| \ge |V| * \frac{2}{3}, then last\_justified\_slot is increased to the maximum of its previous value and that block’s slot number. Attestations are required to state the last\_justified\_slot in the chain they are attesting to to get included in the chain.
If a span of blocks (in the same chain) with slots s, s+1 … s+64 (65 slots altogether) all get justified, then the block at slot s is finalized.
We change the fork choice rule above so that instead of starting H from the genesis block, it starts from the justified block with the highest slot number.
We then add two slashing conditions:
- A validator cannot make two distinct attestations in the same slot
- A validator cannot make two attestations with slot numbers t1, t2 and last justified slots s1, s2 such that s1 < s2 < t2 < t1
Claims:
- Safety: once a block becomes finalized, it will always be part of the canonical chain as seen by any node that has downloaded the chain up to the block and the evidence finalizing the block, unless at least a set of validators V_A with |V_A| \ge |V| * \frac{1}{3} violated one of the two slashing conditions (possibly a combination of the two).
- Plausible liveness: given an “honest” validator set V_H with |V_H| \ge |V| * \frac{2}{3}, V_H by itself can always finalize a new block without violating slashing conditions.
Beacon chain stage 3: adding dynamic validator sets
Every block B comes with a subset of validators S_B, with the following restrictions:
- Define the dynasty of a block recursively: dynasty(genesis) = 0, generally dynasty(B) = dynasty(parent(B)) except when (i) B's 128th ancestor was finalized (and this fact is known based on what is included in the chain before B) and (ii) a dynasty transition has not taken place within the last 256 ancestors of B, in which case dynasty(B) = dynasty(parent(B)) + 1.
- Define the local validator set of B as LVS(B) = S_B \cup S_{parent(B)}\ \cup\ ... \ \cup\ S_{parent^{63}(B)}
- Suppose for two blocks in the chain, B_1 and B_2, dynasty(B_2) - dynasty(B_1) = k. Then, |LVS(B_1)\ \cap\ LVS(B_2)| \ge LVS(B_1) * (1 - \frac{k}{60}) (and likewise wrt LVS(B_2)). That is, at most \frac{1}{60} of the local validator set changes with each dynasty.
Claims:
- All of the above claims hold, with appropriate replacements of V with LVS(...), except with fault tolerance possibly reduced from \frac{1}{3} to 30%.