Data Availability Committee (DAC)

A DAC's primary role is to provide a data availability guarantee for off-chain transactions. Instead of posting all transaction data directly to the Layer 1 blockchain, a rollup or validium can send data to the committee members. The DAC cryptographically signs attestations that the data is available and can be provided upon request, enabling secure state transitions without full on-chain data publication.

A DAC operates on a trusted, permissioned model, contrasting with the trustless nature of pure Layer 1 blockchains. Security depends on the committee's honesty and liveness. Users must trust that a supermajority of committee members (e.g., 5 of 7) will not collude to withhold data. This model trades some decentralization for significant cost savings in data publication.

DACs are a foundational component of Validium scaling solutions. In a Validium, transaction execution and proof generation happen off-chain, and only a validity proof (like a ZK-SNARK) is posted on-chain. The corresponding transaction data is held by the DAC. This architecture provides high throughput and low fees but relies on the DAC for data availability, unlike rollups which post data to Layer 1.

Members are typically well-known, reputable entities like exchanges, foundations, or institutional validators. Selection aims for geographic and organizational diversity to reduce collusion risk. Members are often bonded or staked and can be slashed for malicious behavior. Their incentive is to maintain the network's integrity to preserve the value of the ecosystem they serve and their reputational capital.

If a user suspects data is being withheld, they can challenge the DAC. The system includes mechanisms for users to request specific data from committee members. Failure to provide data within a challenge period can trigger a fraud proof or slashing condition on-chain. This creates a cryptographic-economic deterrent against data withholding by the committee.

• vs. On-Chain Data (Rollups): DACs are cheaper but introduce a trust assumption; rollups are trustless but have higher fees.
• vs. Pure Data Availability Layers: DACs are a centralized, committee-based interim solution, whereas networks like Celestia or EigenDA provide decentralized, cryptoeconomically secured data availability.
• vs. Centralized Sequencers: A DAC secures data after sequencing; it is a complementary, not competing, component.

A validium is a general scaling pattern where execution proofs are verified on-chain, but data is kept off-chain by a DAC. This is a primary architectural use case for DACs.

• Key Trade-off: Achieves high scalability by removing data from the base layer.
• Security Consideration: Introduces a data availability risk dependent on the committee's honesty.
• Examples: Besides StarkEx and zkPorter, other Layer 2s and application-specific chains use this model.

A cryptographic technique that allows light nodes to probabilistically verify data availability by downloading small, random samples of block data. This is the core innovation behind data availability layers like Celestia, enabling secure scaling without requiring any single node to download all data.

• Key Benefit: Enables trust-minimized scaling where nodes can verify large blocks without full downloads.
• How it works: Nodes request multiple random chunks; if data is withheld, sampling will eventually detect its absence.

A data redundancy method used to reconstruct original data from fragments, crucial for both DACs and Data Availability Sampling (DAS). Data is expanded with parity chunks, so the original can be recovered even if a significant portion (e.g., 50%) is missing.

• Role in DAS: Makes sampling possible by ensuring any 50% of the coded data is sufficient for full reconstruction.
• Fault Tolerance: Provides resilience against data withholding attacks by malicious block producers.

A Layer 2 scaling solution where transaction data is stored off-chain by a Data Availability Committee (DAC) or a similar operator set, while proofs of validity are posted on-chain. This offers high throughput and low fees but introduces a data availability trust assumption.

• Trade-off: Sacrifices some decentralization for scalability and cost.
• Use Case: Suitable for applications where high transaction volume is prioritized over pure cryptographic security guarantees.

A hybrid data availability model, pioneered by StarkEx, that gives users a per-transaction choice between Validium mode (data off-chain with a DAC) and ZK-Rollup mode (data on-chain). This provides a flexible security-scalability trade-off.

• User Sovereignty: Users can opt for higher security (on-chain data) for valuable assets or higher throughput (off-chain data) for less critical transactions.
• Architecture: Highlights the DAC as one configurable component within a broader system.

A specialized blockchain layer, like Celestia or EigenDA, whose primary function is to order and guarantee the availability of transaction data for other execution layers (rollups). It provides a neutral, scalable foundation for modular blockchain architectures.

• Contrast with DAC: Offers cryptoeconomic security via a decentralized network of nodes, replacing the need for a trusted committee.
• Core Service: Provides data availability proofs that rollups can use to verify their data was published.

The cryptographic mechanisms that allow a Data Availability Committee to collectively attest to data availability. This often involves threshold signatures (e.g., BLS signatures) where a predefined subset of members must sign to produce a valid attestation.

• Security Model: Relies on the assumption that the threshold of honest members will not collude to sign for unavailable data.
• Implementation: Enables a single, verifiable on-chain proof representing the committee's consensus.