Data Availability Layer

Avail’s core innovation is its data availability layer built on Kate polynomial commitments (also known as KZG commitments). This enables light clients to verify data availability through efficient random sampling without downloading entire blocks.

Overview

The data availability layer ensures that block data is available and can be retrieved, even if you don’t download the entire block. This is critical for:

Rollups: Publishing transaction data to Avail
Validiums: Storing data off-chain with DA guarantees
Light Clients: Verifying data availability without full nodes
Modular Blockchains: Separating data availability from execution

Data Matrix Construction

Extrinsic to Matrix Transformation

Transactions are arranged into a 2D matrix for erasure coding (runtime/src/kate/native.rs:27-54):

Collect Extrinsics

Block author collects transactions from the mempool, tagged with their AppId.

Matrix Dimensions

Determine matrix size from DynamicBlockLength:

let (max_width, max_height) = to_width_height(&block_length);
// Default: 256 cols × 256 rows
// Max: 1024 cols × 1024 rows

Arrange into Grid

Extrinsics are serialized and placed into the matrix cell-by-cell:

runtime/src/kate/native.rs:39-41

let grid = EGrid::from_extrinsics(
    submitted,
    MIN_WIDTH,     // Minimum 32
    max_width,     // Configured width
    max_height,    // Configured height
    seed           // Randomness for row assignments
)?;

Erasure Coding Extension

The grid is extended by 2x in the column dimension:

runtime/src/kate/native.rs:40-41

.extend_columns(NonZeroU16::new(2).expect("2>0"))

This creates redundancy - only 50% of cells are needed to reconstruct the full data.

Cell Structure

Each cell in the matrix contains:

Original Cells: Actual transaction data (first 50% of columns)
Extended Cells: Erasure-coded redundancy (second 50% of columns)
Cell Size: 31 bytes per cell (fits in a BLS12-381 scalar field element)

The chunk size is 32 bytes, but only 31 bytes are used for data to ensure the value fits within the scalar field: BLOCK_CHUNK_SIZE = 32

Kate Polynomial Commitments

Kate commitments are KZG (Kate-Zaverucha-Goldberg) polynomial commitments over the BLS12-381 elliptic curve.

How It Works

Polynomial Commitment Basics

Step 1: Interpolate PolynomialEach row of the matrix is interpolated as a polynomial:

row[0], row[1], ..., row[n-1] → polynomial P(x)

Step 2: Commit to PolynomialUsing a trusted setup SRS (Structured Reference String), compute:

commitment = P(τ) · G

where τ is a secret value from the trusted setup, and G is a generator point.Step 3: Generate ProofsFor any cell at position i, create a proof:

proof = (P(x) - P(i)) / (x - i) · G

Step 4: VerifyAnyone can verify that a cell value v is correct at position i:

e(commitment - v·G, G) == e(proof, τ·G - i·G)

using elliptic curve pairings.

Implementation

The Kate implementation uses the kate crate with runtime interfaces (runtime/src/kate/native.rs):

fn proof(
    extrinsics: Vec<AppExtrinsic>,
    block_len: BlockLength,
    seed: Seed,
    cells: Vec<(u32, u32)>,  // (row, col) positions
) -> Result<Vec<GDataProof>, Error> {
    let srs = SRS.get_or_init(multiproof_params);
    let (max_width, max_height) = to_width_height(&block_len);
    
    // Build and extend grid
    let grid = EGrid::from_extrinsics(extrinsics, MIN_WIDTH, max_width, max_height, seed)?
        .extend_columns(NonZeroU16::new(2).expect("2>0"))?;
    
    // Create polynomial grid
    let poly = grid.make_polynomial_grid()?;
    
    // Generate proof for each cell
    let proofs = cells
        .into_par_iter()
        .map(|(row, col)| {
            let data = grid.get(row, col)?;
            let cell = Cell::new(BlockLengthRows(row), BlockLengthColumns(col));
            let proof = poly.proof(srs, &cell)?;
            Ok((data, proof))
        })
        .collect()?;
    
    Ok(proofs)
}

Proof Types

Avail supports two proof types:

Single Cell Proof
Multiproof

GDataProof: Proves a single cell’s correctness

runtime/src/kate/mod.rs:24

pub type GDataProof = (GRawScalar, GProof);

GRawScalar: The 256-bit cell value (U256)
GProof: 48-byte KZG proof

Used for: Point queries, light client sampling

GMultiProof: Proves multiple cells with a single proof

runtime/src/kate/mod.rs:25

pub type GMultiProof = (Vec<GRawScalar>, GProof);

Vec<GRawScalar>: Multiple cell values (16×64 = 1024 cells)
GProof: Single 48-byte proof for all cells
GCellBlock: Defines the rectangular block of cells

Used for: Efficient bulk verification, downloading app-specific data

Header Extension

The Kate commitment is stored in the block header extension (avail_core::header::HeaderExtension).

Extension Builder

The runtime API builds header extensions (runtime/src/apis.rs:49-60):

runtime/src/apis.rs

pub trait ExtensionBuilder {
    fn build_extension(
        extrinsics: Vec<OpaqueExtrinsic>,
        data_root: H256,
        block_length: BlockLength,
        block_number: u32,
    ) -> HeaderExtension;
    
    fn build_data_root(
        block: u32,
        extrinsics: Vec<OpaqueExtrinsic>
    ) -> H256;
}

Extension Contents

The HeaderExtension contains:

Commitments: Kate commitments for each row
Data Root: Merkle root of all extrinsics
Dimensions: Matrix rows and columns
App Lookup: Mapping of AppId to row ranges

The header extension is verified during block import (node/src/da_block_import.rs:81-111), ensuring blocks without valid commitments are rejected.

Data Availability Sampling

Light clients perform Data Availability Sampling (DAS) to gain confidence that data is available:

Sampling Process

Random Cell Selection

Light client randomly selects N cells from the extended matrix (e.g., 16 cells).

Request Cells + Proofs

Queries full nodes for the selected cells and their Kate proofs.

Verify Proofs

Verifies each proof against the commitment in the block header:

verify(commitment, cell_position, cell_value, proof) == true

Calculate Confidence

With k successful samples, confidence that >50% of data is available:

confidence = 1 - (0.5)^k

16 samples: 99.998% confidence
20 samples: 99.9999% confidence

Reconstruction Guarantee

Due to 2x erasure coding:

If >50% of cells are available, the full block can be reconstructed
Light clients gain high confidence with just a few samples
No need to download the entire block

Example: A 256×512 matrix has 131,072 cells. A light client only needs to sample ~20 cells (0.015% of the block) to achieve 99.9999% confidence that the block is available.

Application-Specific Data Retrieval

Avail supports multiple applications submitting data to the same block, each identified by an AppId.

AppId System

Applications register keys through the da_control pallet (pallets/dactr/src/lib.rs:158-178):

pub fn create_application_key(
    origin: OriginFor<T>,
    key: AppKeyFor<T>,
) -> DispatchResultWithPostInfo {
    let owner = ensure_signed(origin)?;
    ensure!(!key.is_empty(), Error::<T>::AppKeyCannotBeEmpty);
    
    let id = AppKeys::<T>::try_mutate(&key, |key_info| {
        ensure!(key_info.is_none(), Error::<T>::AppKeyAlreadyExists);
        
        let id = Self::next_application_id()?;
        *key_info = Some(AppKeyInfo { id, owner: owner.clone() });
        Ok(id)
    })?;
    
    Self::deposit_event(Event::ApplicationKeyCreated { key, owner, id });
    Ok(().into())
}

Data Submission

Applications submit data with an implicit or explicit AppId:

pallets/dactr/src/lib.rs:186-200

pub fn submit_data(
    origin: OriginFor<T>,
    data: AppDataFor<T>,  // Up to 1 MB
) -> DispatchResultWithPostInfo {
    let who = ensure_signed(origin)?;
    ensure!(!data.is_empty(), Error::<T>::DataCannotBeEmpty);
    
    let data_hash = blake2_256(&data);
    Self::deposit_event(Event::DataSubmitted {
        who,
        data_hash: H256(data_hash),
    });
    
    Ok(().into())
}

The CheckAppId extension extracts the AppId from the transaction for matrix placement.

App Data Retrieval

Clients can retrieve only their application’s data using the Kate API (runtime/src/kate/native.rs:141-168):

fn app_data(
    submitted: Vec<AppExtrinsic>,
    block_length: BlockLength,
    seed: Seed,
    app_id: u32,
) -> Result<Vec<Option<GRow>>, Error> {
    let grid = EGrid::from_extrinsics(submitted, MIN_WIDTH, max_width, max_height, seed)?;
    
    let dims = grid.dims();
    let Some(rows) = grid.app_rows(AppId(app_id), Some(dims))? else {
        return Err(Error::AppRow);
    };
    
    // Return only rows containing app_id data
    let mut all_rows = vec![None; dims.height()];
    for (row_y, row) in rows {
        let g_row = row.into_par_iter()
            .map(|s| s.to_bytes().map(GRawScalar::from))
            .collect()?;
        all_rows[row_y] = Some(g_row);
    }
    
    Ok(all_rows)
}

This allows applications to:

Download only their own data rows
Avoid downloading unrelated application data
Verify data integrity with Kate proofs

Block Length Dynamics

The matrix dimensions can be adjusted via governance (pallets/dactr/src/lib.rs:204-254):

pub fn submit_block_length_proposal(
    origin: OriginFor<T>,
    rows: u32,
    cols: u32,
) -> DispatchResultWithPostInfo {
    ensure_root(origin)?;
    
    // Validate dimensions
    ensure!(
        rows <= MaxBlockRows && cols <= MaxBlockCols,
        Error::<T>::BlockDimensionsOutOfBounds
    );
    ensure!(
        rows >= MinBlockRows && cols >= MinBlockCols,
        Error::<T>::BlockDimensionsTooSmall
    );
    
    // Must be powers of 2
    ensure!(rows.is_power_of_two(), Error::<T>::NotPowerOfTwo);
    ensure!(cols.is_power_of_two(), Error::<T>::NotPowerOfTwo);
    
    // Update block dimensions
    let block_length = BlockLength::with_normal_ratio(
        BlockLengthRows(rows),
        BlockLengthColumns(cols),
        BLOCK_CHUNK_SIZE,
        DA_DISPATCH_RATIO_PERBILL,
    )?;
    
    DynamicBlockLength::<T>::put(block_length);
    
    Self::deposit_event(Event::BlockLengthProposalSubmitted { rows, cols });
    Ok(().into())
}

Dimension Constraints

MinBlockRows = 32
MinBlockCols = 32
// Minimum block: 32 KB (before erasure coding)

Dimensions must be powers of 2 for efficient FFT (Fast Fourier Transform) operations during polynomial interpolation.

Data Root Calculation

In addition to Kate commitments, Avail computes a data root - a Merkle root of all extrinsics. This provides:

Fast inclusion proofs for specific transactions
Compatibility with standard Merkle proof systems
Additional data integrity verification

fn build_data_root(
    block: u32,
    extrinsics: Vec<OpaqueExtrinsic>
) -> H256;

Security Guarantees

Data Availability Guarantee

Honest Majority

As long as >50% of validators are honest, data availability is guaranteed

Erasure Coding

50% of cells are sufficient to reconstruct the full block

Cryptographic Proofs

Kate proofs are cryptographically binding and verifiable

Sampling Confidence

Light clients achieve >99.99% confidence with minimal samples

Attack Resistance

Block Withholding Attack

Attack: Block producer creates invalid erasure coding or withholds dataDefense:

Header extension verification during block import ensures valid commitments
Light clients detect unavailable data through sampling failures
Validators download and verify random cells before voting
Invalid blocks are rejected by GRANDPA consensus

Data Withholding After Finality

Attack: Validators finalize a block, then all collude to delete dataDefense:

Honest minority (even 1 node) can store and redistribute full block
Economic incentives: archival nodes earn fees for serving data
Redundancy: 2x erasure coding means multiple copies exist
Slashing: Provable data unavailability results in validator slashing

Performance Characteristics

Proof Generation

Single Proof: ~1-5ms per cell (parallel generation)
Multiproof (1024 cells): ~50-100ms
Full Block Commitment: ~100-500ms depending on size

Proof Size

Single Proof: 48 bytes (constant)
Multiproof: 48 bytes + (32 bytes × number of cells)
Header Extension: ~10-50 KB depending on block size

Verification

Single Proof Verification: ~2-10ms
Multiproof Verification: ~5-15ms (amortized cost)
Light Client Sampling (20 cells): ~50-200ms total

RPC API for Data Availability

The Kate RPC provides access to DA proofs (runtime/src/apis.rs:68-73):

pub trait KateApi {
    // Get proof for specific transaction
    fn data_proof(
        block_number: u32,
        extrinsics: Vec<OpaqueExtrinsic>,
        tx_idx: u32
    ) -> Option<ProofResponse>;
    
    // Get specific rows
    fn rows(
        block_number: u32,
        extrinsics: Vec<OpaqueExtrinsic>,
        block_len: BlockLength,
        rows: Vec<u32>
    ) -> Result<Vec<GRow>, Error>;
    
    // Get proofs for specific cells
    fn proof(
        block_number: u32,
        extrinsics: Vec<OpaqueExtrinsic>,
        block_len: BlockLength,
        cells: Vec<(u32, u32)>
    ) -> Result<Vec<GDataProof>, Error>;
    
    // Get multiproofs for cell blocks
    fn multiproof(
        block_number: u32,
        extrinsics: Vec<OpaqueExtrinsic>,
        block_len: BlockLength,
        cells: Vec<(u32, u32)>
    ) -> Result<Vec<(GMultiProof, GCellBlock)>, Error>;
}

Enable Kate RPC with the --enable-kate-rpc flag.

Next Steps

Architecture Overview

Understanding the overall system architecture

Runtime Architecture

Deep dive into runtime pallets

Consensus

Learn about BABE and GRANDPA

Getting Started

Running a Node

Architecture

Pallets

Operations

Development

Documentation Index

​Overview

​Data Matrix Construction

​Extrinsic to Matrix Transformation

​Cell Structure

​Kate Polynomial Commitments

​How It Works

​Implementation

​Proof Types

​Header Extension

​Extension Builder

​Extension Contents

​Data Availability Sampling

​Sampling Process

​Reconstruction Guarantee

​Application-Specific Data Retrieval

​AppId System

​Data Submission

​App Data Retrieval

​Block Length Dynamics

​Dimension Constraints

​Data Root Calculation

​Security Guarantees

​Data Availability Guarantee

Honest Majority

Erasure Coding

Cryptographic Proofs

Sampling Confidence

​Attack Resistance

​Performance Characteristics

​Proof Generation

​Proof Size

​Verification

​RPC API for Data Availability

​Next Steps

Architecture Overview

Runtime Architecture

Consensus

Build docs developers (and LLMs) love

Overview

Data Matrix Construction

Extrinsic to Matrix Transformation

Cell Structure

Kate Polynomial Commitments

How It Works

Implementation

Proof Types

Header Extension

Extension Builder

Extension Contents

Data Availability Sampling

Sampling Process

Reconstruction Guarantee

Application-Specific Data Retrieval

AppId System

Data Submission

App Data Retrieval

Block Length Dynamics

Dimension Constraints

Data Root Calculation

Security Guarantees

Data Availability Guarantee

Attack Resistance

Performance Characteristics

Proof Generation

Proof Size

Verification

RPC API for Data Availability

Next Steps