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Snowbridge

Using Snowbridge

Snowbridge can be accessed through the web app, where you may track processing times of transactions, recent transfers, and other information about the bridge's overall status.

Snowbridge by Snowfork is a general-purpose, trustless bridge between Polkadot and Ethereum. It utilizes the Bridge Hub system parachain to establish a connection to its relayers, allowing for permissionless and trustless messaging between Ethereum and Polkadot.

With Snowbridge, a sender can (but does not need to) run a relayer to ensure that their cross-chain transaction is successful. A sender is a user using relayers provided by others.

Snowbridge currently supports two-way token transfers between Ethereum and Polkadot parachain.

Random-sampling BEEFY​

A trustless bridge always has a prover (needs to compute the proof), a verifier (asks the prover to compute the proof and verifies it), and relayers to relay messages. Snowbridge prover uses BEEFY, a novel bridge protocol drastically reducing operational costs without compromising security.

Even with simplifications ushered in by BEEFY, a smart contract updating Polkadot's state on Ethereum has to perform 201 signature checks for every update since there are ~300 validators on Polkadot. This remains expensive (gas costs), especially as the validator set grows. The solution is random-sampling BEEFY that leverages the RANDAO randomness beacon as follows:

  • Commit: Relayer submits a state commitment of a recently finalized block on Polkadot and claims to have a super-majority of validator’s signatures to the light client deployed on Ethereum. It also provides one validator signature backing the commitment, which can be slashed if needed.
  • Challenge: Light client queries on-chain randomness (RANDAO) to subsample m (~25) signatures from the list Relayer’s claimed list.
  • Response: Relayer responds by sharing exactly those m signatures that were randomly sampled which the light client then verifies. If everything checks out, the finalized block is accepted.

The number of signature checks needed is significantly reduced and independent of the validator set size, making the protocol more efficient. The number of subsampled signatures, m, is the parameter that trades off security and efficiency (i.e., security parameter). The value of this parameter is derived using crypto-economic arguments.

Snowbridge Crypto-economic Security​

If up to a third of the validators are malicious, the chance that all m signatures subsampled are from these bad actors is (1/2)^m (exponentially low). Any validator who supports a malicious commitment faces severe penalties. The expected value of an attack E(A) is:

E(A) = p * V + (1 - p)*(-S)

Where V is the value of attack (bounded by market capitalization), S is the validator slashable stake, and p is the probability of a successful attack.

Snowbridge Assumptions​

Snowbridge relies on two major assumptions:

  • The crypto-economic assumption that an adversary is rational, i.e., an attack is launched only if the expected value of an attack is positive. Hence, we derive our security parameter m by ensuring the expected value of an attack is negative, i.e., E(A) < 0.

  • RANDAO unpredictability. The Web3 Foundation research team performed a thorough analysis of RANDAO bias and extended the state-of-the-art in analysing the last-revealer attack on RANDAO. Assuming 1/3rd of Ethereum validators are malicious, the effect of such bias is mitigated by proportionally increasing the security parameter.

Resources​