Data Availability Committee (DAC)

A Data Availability Committee (DAC) is a permissioned set of reputable, often independent, organizations that cryptographically attest to the availability of transaction data for a layer-2 blockchain. In scaling architectures like validiums or certain optimistic rollups, transaction data is stored off-chain to maximize throughput and reduce costs. The DAC's primary role is to sign attestations, or data availability certificates, confirming they possess the complete data and can provide it upon request for fraud proofs or chain reconstruction. This model trades the full data availability guarantee of posting all data directly to a base layer like Ethereum for greater scalability, relying on the committee's collective honesty and liveness.

The operational model of a DAC involves several key technical components. Members typically run a node that monitors the layer-2 sequencer. When a new batch of transactions is produced, the sequencer sends the data to each DAC member. Each member then generates a cryptographic signature, often a BLS signature, over the data's root hash (like a Merkle root). A threshold of these signatures is aggregated into a single certificate, which is posted to the main chain. This certificate acts as a compact proof for users and verifiers that the underlying data is held by the committee and is retrievable, enabling light clients to trust the state of the layer-2 chain without downloading all the data themselves.

The security and trust assumptions of a DAC are distinct from purely cryptographic data availability solutions like data availability sampling (DAS) used in danksharding. A DAC is a trusted model, where users must trust that a majority of the committee members are honest and will not collude to withhold data. To mitigate this risk, committees are often composed of well-known entities with established reputations, and their actions are publicly verifiable. If a DAC fails—meaning a threshold of members refuses to provide data—the layer-2 chain may halt, but user funds typically remain safe as the security of withdrawals is still anchored to the underlying layer-1 blockchain.

Prominent examples of DAC usage include early versions of StarkEx validiums (powering applications like dYdX and Immutable X) and Polygon Avail. The specific implementation details, such as the committee size, signature threshold, and slashing conditions for misbehavior, vary by project. While DACs provide a pragmatic path to high transaction throughput, the blockchain community continues to develop trust-minimized alternatives, with the long-term goal of achieving scalable data availability through cryptographic proofs and decentralized sampling, as envisioned by Ethereum's proto-danksharding roadmap.

A Data Availability Committee (DAC) operates as a multi-signature group of reputable, known entities. When a rollup's sequencer batches transactions, it sends the raw transaction data—the data availability (DA)—to the committee members. Each member cryptographically signs a commitment, typically a hash of the data, attesting they have received and stored it. This set of signatures is then posted on the parent chain (e.g., Ethereum) as proof that the data is available for a defined period. The system's security relies on the assumption that at least one honest committee member will make the data public if others withhold it.

The primary function of the DAC is to provide a data availability guarantee without publishing all data directly to the layer-1 blockchain, which is costly. Instead of using the L1 for full data storage, the rollup uses it as a bulletin board for the committee's attestations. This model, often called validium or a variant of a zk-rollup with off-chain data, significantly reduces transaction fees. However, it introduces a trust assumption: users must trust that the committee is honest and will not collude to withhold data, which would prevent the generation of fraud proofs or the reconstruction of the chain state.

The committee's workflow is continuous and automated. Members run specialized nodes that listen for data from the sequencer, verify its integrity, and promptly return signatures. Slashing conditions or bonding mechanisms are often implemented to penalize members for non-compliance or malicious behavior. If a user or a watchdog suspects data is being withheld, they can challenge the committee by requesting the data directly from its members using the posted attestations as a verifiable record of their commitment.

Compared to other DA solutions, a DAC represents a trusted model versus the trustless model of data availability sampling (DAS) used by celestia or Ethereum's danksharding. It is a pragmatic choice for enterprise or permissioned use cases where high throughput and low cost are prioritized, and a known consortium can be trusted. The security model is explicitly defined: compromise requires collusion among a threshold of committee members, making the security quantifiable based on the committee's size and the reputation of its participants.

In practice, a DAC's performance is measured by its liveness (availability of signatures) and reconstruction latency (time to retrieve data). To enhance resilience, data is often redundantly stored across members and can be propagated through peer-to-peer networks. The evolution of pure cryptographic DA solutions may reduce reliance on committees, but DACs remain a critical bridging technology, enabling scalable applications today by providing a clear, auditable, and economically efficient data availability layer.

A Data Availability Committee (DAC) is a trusted, permissioned set of entities responsible for cryptographically attesting that transaction data for a rollup or Layer 2 is available and can be reconstructed, serving as a more scalable but less decentralized alternative to full data availability sampling on a base layer like Ethereum. Initially popularized by solutions like Validium, this model allows for extremely high transaction throughput by moving data off-chain while relying on committee signatures—stored on-chain—as a guarantee of data availability. The core trade-off is between the scalability benefits of not posting all data to a costly base layer and the trust assumption that a majority of the committee members will remain honest and available.

The evolution of DACs is marked by a push toward reducing trust assumptions and increasing robustness. Early implementations relied on a small, known set of institutional validators. Modern iterations, often called Data Availability Layers or Networks, incorporate cryptographic techniques like dispersal and erasure coding to make the system tolerant to faults among members. Projects like Celestia and EigenDA exemplify this shift, transforming the committee model into a dedicated, scalable network of nodes that provide data availability as a service. This progression mirrors the broader modular blockchain thesis, where specialized layers handle execution, settlement, consensus, and data availability independently.

The future trajectory of DACs involves their convergence with and potential obsolescence by fully trust-minimized protocols. Volitions, hybrid systems that let users choose between a DAC (Validium) mode and a fully on-chain ZK-rollup mode, represent an intermediate step. Long-term, advances in data availability sampling (DAS) and proofs of data availability aim to eliminate the need for trusted committees entirely, enabling light clients to verify data availability with high probability. However, DACs will likely persist for high-performance, enterprise-grade applications where defined legal liability and regulatory compliance of known entities are prioritized over pure cryptographic guarantees, cementing their role as a vital option in a diversified scaling landscape.