Based Contestable Rollup (BCR)

A Based Contestable Rollup is a rollup that features contestation and employs based sequencing. To illustrate how contestation operates within Taiko's framework, consider this example:

  1. Alice proposes a new block.

  2. Bob submits a proof for the state transition H1→H2, with H1 being the parent hash and H2 being the hash of the new block. Bob places a 10,000 TKO validity bond. His proof then enters a cooldown period.

  3. Bob's proposed state transition and accompanying proof are publicly visible.

  4. Cindy, upon noticing an error in Bob's transition, contends it should be H1→H3, not H1→H2. Cindy challenges Bob's proof during the cooldown period by posting her 10,000 TKO contestation bond, yet she does NOT provide an alternative proof or declare the correct transition explicitly.

  5. The contested transition is now awaiting a new, higher-tier proof. Both Bob and any other provers have the opportunity to provide this.

Scenario 1:

  • David presents a tier-3 proof for H1→H2, affirming Bob's original claim. David earns a 2,500 TKO reward and becomes the current prover, also posting a 20,000 TKO validity bond.

  • Cindy forfeits her entire contestation bond.

  • Bob is reimbursed his 10,000 TKO validity bond plus a 2,500 TKO reward.

  • David's proof initiates a new cooldown period.

Scenario 2:

  • David provides a tier-3 proof for a transition from H1→H4, which indicates Bob’s transition was incorrect. David receives a 2,500 TKO reward and secures his position with a 20,000 TKO validity bond.

  • Cindy retrieves her 10,000 TKO contestation bond and an additional 2,500 TKO reward.

  • Bob’s validity bond is seized.

  • A new cooldown period commences for the fresh proof.

Each proof in Taiko, except for the highest tier, requires a validity bond paid in Taiko tokens by the original prover. This proof enters a cooldown window, during which it can be contested by others who do not need to provide any fraud/validity proofs but must also pay a contestation bond in Taiko tokens. If contested, a higher-tier proof is required to resolve the dispute before this block can be verified.