Data Availability Committee (DAC)

A Data Availability Committee (DAC) is a group of known, reputable entities—often including the rollup operator, foundations, or other institutional participants—that cryptographically signs attestations confirming that the data for a rollup's state transitions is available and can be retrieved. This mechanism is a trusted alternative to publishing all transaction data directly to a base layer blockchain like Ethereum, which is costly. By relying on committee signatures instead of on-chain data posting, DACs dramatically reduce transaction fees, but they introduce a trust assumption: users must trust that a majority of the committee members are honest and will not collude to withhold data.

The core function of a DAC is to provide a data availability guarantee. In a rollup system, the validity of state updates depends on the ability of any verifier to download and verify the underlying transaction data. When this data is not posted on-chain, a DAC provides off-chain attestations or signatures that serve as a promise of availability. These attestations are typically posted to the base layer or made accessible to users. If data is needed—for instance, to challenge an invalid state transition in an optimistic rollup—the signatures prove that the committee vouched for its retrievability from their designated storage systems.

DACs are most commonly associated with validium and volition scaling solutions. A validium uses a DAC (or similar proof-of-stake network) for data availability while executing transactions off-chain and proving correctness with zero-knowledge proofs (ZKPs). This combination offers high throughput and low cost but with the trust model of the DAC. The security model is often quantified by the honest majority assumption, where the system is secure as long as a threshold of committee members (e.g., a majority or supermajority) acts honestly and does not collectively withhold data.

The trade-offs of using a DAC are central to its evaluation. The primary advantage is cost efficiency, as storing data off-chain is orders of magnitude cheaper than using base layer calldata. The main disadvantage is the introduction of trust and potential censorship risk. If the committee colludes, they could withhold data, potentially freezing funds or making fraud proofs impossible. To mitigate this, some implementations use cryptographic techniques like Data Availability Sampling (DAS) or allow users to choose between DAC-based and on-chain data availability models, as seen in volition architectures.

In practice, DACs represent a pragmatic step in the evolution of scaling solutions, balancing decentralization and cost. They are often viewed as an interim or enterprise-focused solution, with the long-term goal of migrating to purely cryptoeconomic or cryptographic data availability guarantees, such as those promised by EigenDA, celestia, or Ethereum's own Proto-Danksharding (EIP-4844). These future systems aim to provide scalable data availability without trusted committees, relying instead on incentivized networks and advanced cryptographic proofs.

A Data Availability Committee (DAC) is a core component of certain optimistic rollup and validium architectures. Its primary function is to provide a cryptographic attestation, often a multi-signature, that confirms the complete transaction data for a new block is available off-chain. This attestation allows the Layer 2 network to proceed, as verifiers and users can trust the committee's signed commitment that the data exists and can be retrieved for fraud proofs or state reconstruction. Without this guarantee, a malicious sequencer could withhold data, making it impossible to detect invalid state transitions.

The operational model involves committee members, which are typically known and reputable entities, running specialized nodes. When the L2 sequencer produces a block, it disseminates the associated data to each DAC member. Each member independently verifies they have received the complete dataset and then signs a cryptographic attestation. Only when a predefined threshold of signatures (e.g., a majority or supermajority) is collected is the attestation considered valid and posted to the parent chain (Layer 1), finalizing the L2 block. This process creates a strong, collective assurance of data availability.

The security model of a DAC is based on an honest majority assumption. It is considered trust-minimized because it reduces reliance on any single entity; corruption or failure requires collusion among a threshold of members. However, it is not trustless like pure cryptographic methods such as data availability sampling (DAS) used in danksharding. DACs offer a pragmatic trade-off: they provide high throughput and low cost by keeping data off the main chain, while introducing a modest trust assumption in a known set of participants to enable practical scaling today.