Permissionless Relay

A permissionless relay operates without a central gatekeeper, meaning any user or application can submit a cross-chain message without requiring approval. This is enforced by cryptographic proofs (like Merkle proofs) that any network participant can verify, ensuring the relay cannot arbitrarily block or censor transactions. This is a fundamental departure from permissioned, multi-sig bridges which have centralized points of control.

Security is derived from the underlying blockchain's consensus and a system of cryptoeconomic incentives. Key mechanisms include:

• Bonding/Staking: Relayers or provers must stake (bond) native tokens, which can be slashed for malicious behavior.
• Fraud Proofs: Observers can challenge invalid state transitions, with slashed funds rewarding the challenger.
• Cost of Attack: The security budget is tied to the total value staked, making attacks economically irrational.

Instead of trusting a committee, message validity is proven via light client verification or zero-knowledge proofs (zk-SNARKs/STARKs). A light client on the destination chain cryptographically verifies block headers and Merkle proofs from the source chain. This allows the destination chain to autonomously verify the origin and integrity of incoming messages, removing trusted intermediaries.

The relay network itself is permissionless. Anyone can run a relayer node to listen for events, generate proofs, and submit transactions, earning fees for service. This creates a competitive, decentralized marketplace for relay services, improving liveness and redundancy. Examples include the p2p networks of relayers in protocols like Axelar and the open prover sets in zkBridge designs.

Designed to connect a wide array of blockchains, not just EVM-compatible ones. A permissionless relay uses general-purpose message passing and agnostic verification to facilitate communication between heterogeneous chains (e.g., Cosmos, Solana, Bitcoin). This is achieved through abstracted adapter contracts and chain-agnostic proof formats like ICS (Interchain Standards).

The ultimate goal is to reduce trust assumptions to the security of the connected blockchains themselves. This is achieved through a combination of:

• On-chain verification of cryptographic proofs.
• Economic penalties for provers.
• Decentralized watchtowers for fraud detection. This creates a security model where users do not need to trust the relay operators, only the mathematical guarantees of the protocol.

A permissionless relay operates as a peer-to-peer (P2P) network of independent relay nodes. These nodes listen for transaction bundles from users or builders, propagate them, and submit them to the target blockchain. The architecture must be fault-tolerant and resistant to censorship, with no single point of failure or control. Key components include:

• Node Client Software: Open-source software that any participant can run.
• Gossip Protocol: For propagating transaction data across the network.
• Blockchain RPC Endpoints: Connections to one or more execution and consensus layer clients.

To ensure liveness and honest participation, a permissionless relay requires a robust cryptoeconomic model. This typically involves:

• Staking/Slashing: Relay operators may be required to post a bond (stake) that can be slashed for malicious behavior (e.g., censorship, withholding blocks).
• Fee Mechanism: Operators earn fees (e.g., a percentage of priority fees or MEV) for successfully relaying blocks, aligning their incentive with network utility.
• Cost Recovery: The model must cover operational costs like gas fees for submission and infrastructure overhead.

The core technical guarantee of a permissionless relay is credible neutrality. Implementation focuses on making transaction censorship technically difficult and economically costly. Mechanisms include:

• First-Come-First-Served (FCFS) Ordering: Processing transactions in the order they are received.
• Commit-Reveal Schemes: Hiding transaction content until it's too late to censor.
• Redundancy: Multiple independent nodes ensure if one refuses a valid transaction, another can include it.
• Transparent Logs: Publicly verifiable logs of all received transactions to prove censorship attempts.

The relay acts as a trust-minimized communication layer between block builders and block proposers (validators). Technical integration requires:

• Standardized APIs: Such as the Builder API (mev-boost) for Ethereum, defining how builders submit execution payloads and bids.
• Bid Authentication: Verifying the cryptographic signatures of builders on their bids.
• Payload Delivery: Securely delivering the winning block payload to the proposer just in time for inclusion, often using TLS-encrypted connections.

Relays must provide cryptographic proofs that the data they handle is available and correct. This involves:

• Header Broadcast: Distributing the block header (including commitments like the transactions root) immediately upon receipt.
• Data Hosting: Making the full block body available for download by validators and other nodes for a sufficient time window.
• Attestation Signing: In some designs, relays may provide signed attestations that a valid payload was delivered, which can be used in slashing protocols.

Implementing a secure relay requires guarding against specific attack vectors:

• DoS Protection: Rate-limiting and sybil resistance to prevent network flooding.
• MEV Extraction by Relay: Preventing the relay itself from frontrunning or stealing MEV from user transactions.
• Timing Attacks: Ensuring the proposer receives the block with enough time to publish it, mitigating time-bandit attacks.
• Software Vulnerabilities: Regular audits of the relay client code and dependency management are critical.

A permissionless relay operator can technically censor transactions by refusing to include them in a block, even if they are valid. This is distinct from validator-level censorship. Mitigations include:

• Relay Diversity: Users and builders can submit transactions to multiple relays.
• Incentive Alignment: Relays are economically motivated to include profitable transactions (e.g., those with high MEV).
• Fallback Mechanisms: Builders can be configured with a list of backup relays.

Since anyone can run a relay, the network must be resilient to relays that are buggy, malicious, or go offline. Key risks include:

• Data Availability: A relay could withhold block data after winning an auction.
• Invalid Headers: Submitting blocks with invalid execution payloads.
• Liveness Failures: A relay crashing can disrupt block production for dependent builders. Defenses rely on builder reputation systems, slashing conditions in some designs, and builders quickly switching to reliable relays.

Relays are central hubs for MEV (Maximal Extractable Value) flow. This creates attack vectors:

• Transaction Ordering: A relay can reorder transactions within a block to extract value, potentially harming users.
• Frontrunning & Sandwich Attacks: Relays have privileged view of pending transactions from builders.
• Collusion: A relay could collude with a specific builder or validator to capture MEV unfairly. Solutions include encrypted mempools and fair ordering protocols to reduce the relay's exploitable information.

Despite being permissionless, relay operation tends to centralize due to economies of scale, high-performance requirements, and the need for validator trust. Challenges:

• De Facto Centralization: A few dominant relays create systemic risk; if compromised, they could halt the chain.
• Trusted Setup: Validators must explicitly trust a relay's list to receive blocks, creating a social consensus layer.
• Data Relay Reliance: The ecosystem depends on relays for fast, reliable block data propagation.

Relay software is complex, handling sensitive signing operations, network communication, and cryptographic proofs. Bugs can be catastrophic:

• Signature Verification Flaws: Could allow invalid blocks to be accepted.
• Logic Errors: In auction mechanisms or slashing conditions.
• Network Attacks: DoS attacks against a relay's public API. Rigorous auditing, bug bounty programs, and implementation diversity are critical for security.

Operating a permissionless relay may expose the operator to legal scrutiny, as they facilitate all transactions on-chain, including those from sanctioned addresses or involving illicit activities. This creates tension between censorship resistance and compliance, potentially forcing relays to geofilter or block certain transactions to avoid legal liability, undermining the permissionless ideal.