Problem Statement: Cross-Chain Asset Bridge
Problem Statement: Cross-Chain Asset Bridge — cross-chain bridge interview depth
Problem Statement: Cross-Chain Asset Bridge
A cross-chain bridge moves value between chains that can't natively talk to each other. The dominant design is lock-and-mint: lock collateral in a vault on the source chain, prove the deposit was finalized, and mint a wrapped representation on the destination chain under a quorum-gated authorization; the reverse (burn wrapped → unlock native) returns the original asset. The reference systems — Wormhole, Axelar, LayerZero — differ on how they verify the source event (guardian multisig, validator set, light client) but share the same collateral-accounting discipline.
The framing that wins the interview: a bridge is a state machine with safety invariants, not an "API between chains." Every transfer walks DEPOSITED → PROVEN → MINTED (and BURNED → UNLOCKED on the way back), and a mint is authorized only against a finalized inclusion proof plus a signature quorum. Treat it as plumbing and you ship the next exploit.
The three invariants to state up front. Safety: no mint without a finalized inclusion proof + quorum signatures. Accounting: locked native on source ≥ wrapped outstanding on destination (the conservation law). Liveness: an honest minority of relayers can still progress transfers after an outage.
Why bridges are the most-exploited thing in crypto. Over $2.5B has been stolen from bridges: Ronin (~$625M, validator keys compromised), Wormhole (~$326M, a signature-verification bug), Nomad (~$190M, a replayable-proof bug anyone could copy-paste), plus Harmony Horizon and Poly Network. Each maps to one invariant failing — which is why the smoking-gun detail interviewers probe is your reorg-rewind policy: a reorg on Ethereum can invalidate a proof that already minted on Polygon, leaving wrapped surplus with no backing. Every failure drill (forged quorum, relayer censorship, destination congestion, post-exploit surplus) follows the same loop: detect → pause → reconcile.
Scale to anchor on: ~$2B peak TVL, ~80k transfers/day across top corridors, Ethereum source finality ~15 minutes. TVL is the sum of locked native assets backing wrapped supply — and it is exactly the number an attacker is trying to break free from its backing.
Key Highlights
- •lock assets on source chain; mint wrapped representation on destination
- •relayers or guardians observe source finality before authorizing mint
- •withdraw burns wrapped tokens then unlocks native collateral on source
- •bridge TVL is the sum of locked native assets backing wrapped supply
Section Rescue Kit
Buzzwords to use:
Safe statements:
- "If Problem Statement: Cross-Chain Asset Bridge gets fuzzy, I'll redraw source vault → relayer proof → quorum → destination minter."
- "I'll restate the TVL invariant before discussing scaling or cloud choices."