Epoch change proof

The Aptos consensus has (at any given time) a set of validators that are charged with executing blocks and sign them to append them to the chain state.

A set of given validators is updated at the last block of every epoch on the chain. An epoch on the Aptos chain has a duration of 2 hours. The light client needs to keep track of the current trusted validator set hash, which is updated on every epoch change.

For a given epoch \(N\) with a set of validators \(V_n\), it is expected to have a block for the transition to epoch \(N+1\) signed by \(V_n\) containing the new validator set \(V_{\text{n+1}}\).

It is the job of the light client to produce a proof at every epoch change to verify the signature on the validators for the new epoch. This is handled by the Epoch Change program.

Epoch Change program IO

Program reference

Inputs

The following data structures are required for proof generation (detailed data structure references can be found at the end of this document):

• Latest Known TrustedState: The most recent known state, representing the trusted state for the current epoch.
  • ValidatorVerifier: Validator set information for epoch N, provided by the user.
• EpochChangeProof: Proof structure required to transition to the next epoch.
  • LedgerInfoWithSignatures: Signed ledger info that includes the new validator set for epoch N+1, provided by the user.

Outputs

• Previous ValidatorVerifier Hash: The previous validator verifier hash, used for comparison.
• Ratcheted ValidatorVerifier Hash: The hash representing the new validator set for epoch N+1.