Signature Aggregator

The Boneh-Lynn-Shacham (BLS) signature scheme is the predominant cryptographic primitive for aggregation. Its key properties enable efficient, non-interactive aggregation:

• Non-interactivity: Signatures can be combined by anyone after they are created.
• Determinism: The same message signed by multiple parties produces a unique, aggregatable signature.
• Size Efficiency: The aggregated signature is the same size as a single signature (e.g., ~96 bytes for BLS12-381). This is foundational for Ethereum 2.0's consensus and many Layer-2 rollups.

Signature aggregators transform the user experience for multi-sig wallets (e.g., Gnosis Safe). Instead of submitting N separate signatures in a transaction:

• Old Method: Transaction data includes multiple ECDSA (v, r, s) tuples, increasing gas costs linearly.
• With Aggregation: A single aggregated BLS signature is submitted, drastically reducing calldata and verification gas. This enables more signers per wallet without prohibitive on-chain costs.

Layer-2 solutions like zkRollups and Optimistic Rollups use signature aggregation to batch and prove transaction validity.

• State Transition Proofs: Validators aggregate signatures for all transactions in a batch into one proof.
• Data Availability: Aggregated signatures for data availability committees (DACs) confirm data has been received.
• Fraud Proofs: In optimistic systems, a single aggregated signature can challenge a state root, representing many disputed transactions. This is critical for reducing the on-chain footprint of L2 operations.

In Proof-of-Stake (PoS) networks, signature aggregators are essential for scalability. Ethereum 2.0 uses BLS aggregation in its consensus mechanism:

• Attestation Aggregation: Thousands of validator signatures for a block attestation are combined into a single signature in the Beacon Chain.
• Sync Committee Aggregation: Light clients verify a single aggregated signature from a committee to follow the chain head. Without aggregation, the consensus overhead would be unsustainable, requiring gigabytes of signature data per epoch.

Signature aggregators enable threshold signature schemes (TSS), where a predefined subset (e.g., 5-of-10) of participants must sign.

• Distributed Key Generation (DKG): Participants collaboratively generate a single public key and individual secret shares.
• Aggregation of Partial Signatures: Each participant creates a partial signature; a combiner aggregates them into a single, valid signature for the group's public key. This provides enhanced security for institutional custody and decentralized oracles (e.g., Chainlink CCIP) without a single point of failure.

A signature aggregator acts as a centralized coordinator in a decentralized system. If the aggregator node is compromised, goes offline, or becomes malicious, it can:

• Censor transactions from specific users.
• Fail to include signatures, causing valid transactions to be dropped.
• Become a target for Denial-of-Service (DoS) attacks, halting the entire batched transaction flow.

This is a fundamental cryptographic attack against naive multi-signature and aggregation schemes. An attacker can generate their public key in a way that allows them to forge a group signature for any message. Mitigations require:

• Proof of Possession (PoP): Requiring users to prove they possess the private key for their claimed public key during setup.
• Specific aggregation protocols like BLS signatures with secure parameterized curves, which are designed to be resistant to this attack.

The complexity of aggregation logic creates a large attack surface. Critical risks include:

• Cryptographic library bugs in elliptic curve operations or pairing functions.
• Timing side-channels that leak private key material during signature generation or verification.
• Incorrect handling of signature malleability, where a valid signature can be altered without invalidating it, potentially causing replay issues.

Different aggregation models carry varying trust assumptions:

• Semi-Trusted Aggregator: Users must trust the aggregator not to steal funds but can trust it to correctly aggregate (common in rollups).
• Trustless Aggregator: The protocol cryptographically guarantees correct aggregation (e.g., via zk-SNARKs).
• Committee-Based: Security relies on the honesty of a supermajority of aggregator nodes, introducing Byzantine Fault Tolerance assumptions.

Aggregation protocols often force a choice between liveness (the chain always progresses) and safety (transactions are always valid).

• Waiting for more signatures improves safety but reduces liveness.
• A fast, optimistic aggregator improves liveness but may include invalid signatures, requiring complex fraud proofs or dispute resolution mechanisms to ensure safety after the fact.

Improper incentive design can undermine security:

• Staking/Slashing: If aggregators are required to stake collateral, insufficient slash amounts may not deter malicious behavior.
• MEV Extraction: Aggregators can reorder or exclude transactions within a batch to extract Maximal Extractable Value (MEV), harming user fairness.
• Freezing Attacks: A malicious aggregator could "freeze" funds by repeatedly including a user's signature in failed batches, blocking their transactions.

In Ethereum and EVM chains, a Signature Aggregator is often implemented as a smart contract (e.g., a Singleton). This contract:

• Receives a batch of user operations and their individual signatures.
• Validates the aggregated signature against the aggregated public keys of the involved accounts.
• Returns a single boolean result to the EntryPoint, confirming all signatures in the batch are valid.
• This moves costly signature verification off the critical path of individual transaction execution.

Signature aggregation extends to Paymasters, entities that sponsor gas fees for users. An aggregator can batch and verify:

• Paymaster signatures authorizing gas sponsorship for multiple user operations.
• Paymaster data (like token exchange rates) used across a batch.
• This allows a single Paymaster transaction to cover gas for hundreds of user ops, drastically reducing overhead and cost for sponsored transactions.

The Bundler is the network actor that interacts directly with the Signature Aggregator. Its role includes:

• Collecting UserOperations from the mempool.
• Calling the aggregator contract to verify the signature batch.
• Submitting a bundled transaction containing only the aggregated proof to the blockchain.
• This architecture is central to Account Abstraction, separating signature logic from transaction execution.

The primary value proposition is massive gas savings and throughput scaling. For a batch of N operations:

• Without aggregation: O(N) signature verification gas costs.
• With aggregation: O(1) verification cost plus minor overhead.
• Example: Verifying 100 ECDSA signatures may cost ~3M gas, while verifying one BLS aggregate may cost ~150k gas, offering ~95% savings. This enables viable micro-transactions and high-frequency dApp interactions.

Using an aggregator introduces specific security considerations:

• Trusted Setup: Some BLS schemes require a one-time trusted ceremony to generate public parameters.
• Aggregator Honesty: Users must trust the bundler/aggregator to correctly include their signature in the batch and not censor it.
• Rogue-Key Attacks: The aggregation protocol must be designed to prevent attackers from creating a public key that enables forging aggregate signatures.