Blob Fundamentals

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Run Blob examples in the interactive playground

Conceptual Guide - For API reference and method documentation, see Blob API.

Blobs (Binary Large Objects) are temporary data availability storage introduced in EIP-4844. They enable L2 rollups to post data to Ethereum at significantly lower costs compared to calldata, accelerating Ethereum’s scaling roadmap.

What Are Blobs?

Blobs are fixed-size data containers (exactly 131,072 bytes = 128 KB) attached to transactions. Unlike calldata:

Temporary - Stored for ~18 days (~4096 epochs), then pruned
Cheaper - Use separate blob gas market with lower base fees
Not executable - EVM cannot directly access blob data
Verified - Use KZG commitments to prove data integrity

Key Properties

import { Blob } from 'tevm';

console.log(Blob.SIZE);                    // 131,072 bytes (128 KB)
console.log(Blob.FIELD_ELEMENTS_PER_BLOB); // 4,096 field elements
console.log(Blob.BYTES_PER_FIELD_ELEMENT); // 32 bytes per element
console.log(Blob.MAX_PER_TRANSACTION);     // 6 blobs maximum

Why Blobs Exist (EIP-4844)

Before EIP-4844, L2 rollups posted transaction data as calldata:

Expensive - 16 gas per byte (~2M gas for 128 KB)
Permanent - Stored forever in blockchain state
Inefficient - Paying for permanent storage when temporary availability suffices

EIP-4844 (proto-danksharding) introduces blobs:

Cheaper - Separate blob gas market (~0.5M gas for 128 KB)
Temporary - Pruned after ~18 days
Sufficient - L2s only need data available long enough for fraud/validity proofs

Blob Structure

A blob contains 4,096 field elements, each 32 bytes, organized for BLS12-381 scalar field operations:

Blob (131,072 bytes)
├─ Field Element 0  (32 bytes) [0:32]
├─ Field Element 1  (32 bytes) [32:64]
├─ Field Element 2  (32 bytes) [64:96]
│  ...
└─ Field Element 4095 (32 bytes) [131040:131072]

Creating Blobs

From Data (Standard)
From Raw Bytes

import { Blob } from 'tevm';

// Encode arbitrary data with length prefix
const data = new TextEncoder().encode("L2 batch transactions");
const blob = Blob.fromData(data);

console.log(`Original: ${data.length} bytes`);
console.log(`Blob: ${blob.length} bytes`); // Always 131,072

// Extract data
const decoded = Blob.toData(blob);
console.log(new TextDecoder().decode(decoded));
// "L2 batch transactions"

Standard encoding format:

Bytes 0-7: Data length (8-byte little-endian uint64)
Bytes 8+: Actual data
Remaining: Zero padding

import { Blob } from 'tevm';

// Pre-formatted blob data (exactly 131,072 bytes)
const rawBlob = new Uint8Array(131072);
// ... fill with your data format ...

const blob = Blob(rawBlob);

// Validates size
try {
  const invalid = Blob(new Uint8Array(1000)); // Too small
} catch (e) {
  console.error(e); // Error: Invalid blob size
}

KZG Commitments

KZG (Kate-Zaverucha-Goldberg) commitments prove blob data integrity without revealing contents. Each blob has:

Commitment (48 bytes) - Cryptographic binding to blob data
Proof (48 bytes) - Proves commitment matches blob
Versioned Hash (32 bytes) - Transaction reference (version byte + SHA256 of commitment)

Computing Commitments

TypeScript

import { Blob } from 'tevm';

const blob = Blob.fromData(data);

// Compute KZG commitment (requires trusted setup)
const commitment = Blob.toCommitment(blob);
console.log(`Commitment: ${commitment.length} bytes`); // 48 bytes

// Generate proof
const proof = Blob.toProof(blob);
console.log(`Proof: ${proof.length} bytes`); // 48 bytes

// Verify proof
const isValid = Blob.verify(blob, commitment, proof);
console.log(`Valid: ${isValid}`); // true

Versioned Hashes

Transactions reference blobs using versioned hashes, not raw commitments:

import { Blob } from 'tevm';

const blob = Blob.fromData(data);
const commitment = Blob.toCommitment(blob);

// Compute versioned hash (used in transaction)
const versionedHash = Blob.Commitment.toVersionedHash(commitment);

console.log(versionedHash[0]); // 0x01 (version byte)
console.log(versionedHash.length); // 32 bytes

// Validate versioned hash
console.log(Blob.isValidVersion(versionedHash)); // true
console.log(Blob.VersionedHash.getVersion(versionedHash)); // 1

Format: 0x01 (KZG version) + SHA256(commitment)[1:32]

Complete Example: L2 Data Posting

L2 rollup posting batch data to Ethereum:

TypeScript
Verification

import { Blob } from 'tevm';

// Step 1: L2 creates transaction batch
const batchData = new TextEncoder().encode(JSON.stringify({
  transactions: [
    { from: '0x...', to: '0x...', value: '1000' },
    { from: '0x...', to: '0x...', value: '2000' },
    // ... more transactions
  ],
  blockNumber: 12345,
  timestamp: Date.now(),
}));

console.log(`Batch size: ${batchData.length} bytes`);

// Step 2: Encode into blob
const blob = Blob.fromData(batchData);
console.log(`Blob size: ${blob.length} bytes`); // 131,072

// Step 3: Generate KZG commitment and proof
const commitment = Blob.toCommitment(blob);
const proof = Blob.toProof(blob);

// Step 4: Compute versioned hash for transaction
const versionedHash = Blob.Commitment.toVersionedHash(commitment);

console.log(`Commitment: ${commitment.length} bytes`);
console.log(`Proof: ${proof.length} bytes`);
console.log(`Versioned hash: ${versionedHash.length} bytes`);

// Step 5: Create EIP-4844 transaction
const tx = {
  type: '0x03', // EIP-4844 blob transaction
  chainId: 1,
  nonce: 42,
  maxFeePerGas: 30_000_000_000n,       // 30 gwei
  maxPriorityFeePerGas: 1_000_000_000n, // 1 gwei
  maxFeePerBlobGas: 50_000_000n,       // 50 gwei blob gas
  gas: 21000n,
  to: '0xYourL2Contract',
  value: 0n,
  data: '0x', // Empty calldata
  blobVersionedHashes: [versionedHash], // Reference to blob
  // Blob sidecar (included in mempool, not on-chain):
  blobs: [blob],
  commitments: [commitment],
  proofs: [proof],
};

// Step 6: Verify before sending
const isValid = Blob.verify(blob, commitment, proof);
if (!isValid) {
  throw new Error('Invalid blob proof');
}

console.log('Transaction ready to send');
console.log(`Versioned hashes: ${tx.blobVersionedHashes.length}`);

// Step 7: Network validators verify
const receivedBlob = tx.blobs[0];
const receivedCommitment = tx.commitments[0];
const receivedProof = tx.proofs[0];
const receivedHash = tx.blobVersionedHashes[0];

// Verify versioned hash matches commitment
const computedHash = Blob.Commitment.toVersionedHash(receivedCommitment);
const hashMatches = Blob.equals(computedHash, receivedHash);

// Verify KZG proof
const proofValid = Blob.verify(receivedBlob, receivedCommitment, receivedProof);

if (!hashMatches || !proofValid) {
  throw new Error('Blob verification failed');
}

console.log('Blob verified successfully');

// Step 8: L2 can reconstruct data later (within ~18 days)
const reconstructedData = Blob.toData(receivedBlob);
const batch = JSON.parse(new TextDecoder().decode(reconstructedData));
console.log(`Recovered batch #${batch.blockNumber}`);
console.log(`Transactions: ${batch.transactions.length}`);

Handling Multiple Blobs

Large data requires multiple blobs (max 6 per transaction):

TypeScript

import { Blob } from 'tevm';

// Large L2 batch (300 KB)
const largeBatch = new Uint8Array(300_000);
// ... fill with transaction data ...

// Check how many blobs needed
const blobCount = Blob.estimateBlobCount(largeBatch);
console.log(`Requires ${blobCount} blobs`); // 3 blobs

if (blobCount > Blob.MAX_PER_TRANSACTION) {
  throw new Error(`Too many blobs: ${blobCount} (max ${Blob.MAX_PER_TRANSACTION})`);
}

// Split into blobs
const blobs = Blob.splitData(largeBatch);
console.log(`Created ${blobs.length} blobs`);

// Generate commitments and proofs for each
const commitments = blobs.map(b => Blob.toCommitment(b));
const proofs = blobs.map(b => Blob.toProof(b));
const versionedHashes = commitments.map(c => Blob.Commitment.toVersionedHash(c));

// Batch verify (more efficient than individual verification)
const allValid = Blob.verifyBatch(blobs, commitments, proofs);
console.log(`All valid: ${allValid}`);

// Create transaction with multiple blobs
const tx = {
  type: '0x03',
  blobVersionedHashes: versionedHashes, // Array of 3 hashes
  maxFeePerBlobGas: 50_000_000n,
  // ... other fields
  blobs: blobs,
  commitments: commitments,
  proofs: proofs,
};

// Later: reconstruct original data
const reconstructed = Blob.joinData(blobs);
console.log(`Reconstructed ${reconstructed.length} bytes`);
console.log(`Matches original: ${Blob.equals(reconstructed, largeBatch)}`);

Blob Gas Market

Blobs use a separate gas market from regular transactions:

Gas Pricing

import { Blob } from 'tevm';

// Gas per blob (fixed)
console.log(Blob.GAS_PER_BLOB); // 131,072 (2^17)

// Target per block (3 blobs)
console.log(Blob.TARGET_GAS_PER_BLOCK); // 393,216

// Calculate cost
const blobBaseFee = 50_000_000n; // 50 gwei (example, varies by demand)
const blobCount = 2;

const blobGasCost = Blob.calculateGas(blobCount, blobBaseFee);
console.log(`Cost: ${blobGasCost / 10n**9n} gwei`);

// Compare with calldata
const calldataBytes = Blob.SIZE * blobCount;
const calldataGas = BigInt(calldataBytes) * 16n; // 16 gas per byte
const calldataCost = calldataGas * 30_000_000_000n; // 30 gwei base fee

const savings = ((1 - Number(blobGasCost) / Number(calldataCost)) * 100).toFixed(1);
console.log(`Savings vs calldata: ${savings}%`);

Fee Market Dynamics

Blob base fee adjusts based on usage:

Target: 3 blobs per block (393,216 blob gas)
Max: 6 blobs per block (786,432 blob gas)
Adjustment: Exponential (similar to EIP-1559)

// Fee increases if usage > target
if (blobGasUsed > Blob.TARGET_GAS_PER_BLOCK) {
  // Base fee increases
}

// Fee decreases if usage < target
if (blobGasUsed < Blob.TARGET_GAS_PER_BLOCK) {
  // Base fee decreases
}

Field Element Constraints

Each 32-byte field element must be < BLS12-381 scalar field modulus:

Modulus: 0x73eda753299d7d483339d80809a1d80553bda402fffe5bfeffffffff00000001

Invalid field elements cause transaction rejection. Tevm handles this automatically in fromData():

import { Blob } from 'tevm';

// Data encoding ensures field element validity
const blob = Blob.fromData(data);

// Manual blob creation requires validation
const rawBlob = new Uint8Array(131072);
// ... set bytes ...

// Check validity
if (!Blob.isValid(rawBlob)) {
  throw new Error('Invalid field elements');
}

Data Availability Window

Blobs are temporary (~18 days = 4096 epochs):

import { Blob } from 'tevm';

const BLOB_RETENTION_EPOCHS = 4096;
const SECONDS_PER_EPOCH = 384; // 32 slots * 12 seconds
const RETENTION_SECONDS = BLOB_RETENTION_EPOCHS * SECONDS_PER_EPOCH;
const RETENTION_DAYS = RETENTION_SECONDS / 86400;

console.log(`Retention: ~${RETENTION_DAYS} days`); // ~18 days

// L2s must:
// 1. Download blob data within 18 days
// 2. Store data locally for fraud/validity proofs
// 3. Make data available to users (RPC nodes)

After pruning, only the versioned hash remains on-chain. L2s cannot reconstruct data from hash alone.

Security Considerations

Commitment Binding

KZG commitments cryptographically bind blob data:

// Cannot create two different blobs with same commitment
const blob1 = Blob.fromData(data1);
const commitment1 = Blob.toCommitment(blob1);

const blob2 = Blob.fromData(data2); // Different data
const commitment2 = Blob.toCommitment(blob2);

// Commitments differ
console.log(Blob.equals(commitment1, commitment2)); // false

// Verification enforces binding
console.log(Blob.verify(blob1, commitment1, proof1)); // true
console.log(Blob.verify(blob1, commitment2, proof2)); // false (wrong commitment)

Trusted Setup

KZG requires trusted setup ceremony:

Powers of Tau - 4096 participants contributed randomness
Security - Safe if at least 1 participant honest
Transparency - All contributions public and verifiable

See KZG Ceremony for details.

Common Patterns

Single Blob Transaction

import { Blob } from 'tevm';

const data = new TextEncoder().encode("Rollup batch");
const blob = Blob.fromData(data);
const commitment = Blob.toCommitment(blob);
const proof = Blob.toProof(blob);
const versionedHash = Blob.Commitment.toVersionedHash(commitment);

const tx = {
  type: '0x03',
  blobVersionedHashes: [versionedHash],
  maxFeePerBlobGas: 100_000_000n, // 100 gwei
  blobs: [blob],
  commitments: [commitment],
  proofs: [proof],
};

Multi-Blob Transaction

import { Blob } from 'tevm';

const blobs = Blob.splitData(largeData);
const commitments = blobs.map(b => Blob.toCommitment(b));
const proofs = blobs.map(b => Blob.toProof(b));
const versionedHashes = commitments.map(c => Blob.Commitment.toVersionedHash(c));

const tx = {
  type: '0x03',
  blobVersionedHashes: versionedHashes,
  maxFeePerBlobGas: 100_000_000n,
  blobs: blobs,
  commitments: commitments,
  proofs: proofs,
};

Gas Estimation

import { Blob } from 'tevm';

async function estimateBlobCost(data, provider) {
  const blobCount = Blob.estimateBlobCount(data);
  const blobBaseFee = await provider.getBlobBaseFee();

  const blobGasCost = Blob.calculateGas(blobCount, blobBaseFee);
  const executionGas = 21000n; // Base transaction cost
  const executionBaseFee = (await provider.getFeeData()).gasPrice;

  const totalCost = blobGasCost + (executionGas * executionBaseFee);

  return {
    blobCount,
    blobGasCost,
    executionCost: executionGas * executionBaseFee,
    totalCost,
  };
}

Resources

EIP-4844 - Proto-danksharding specification
KZG Ceremony - Trusted setup details
Danksharding Roadmap - Ethereum scaling plan
c-kzg-4844 - Reference KZG implementation
Blob Transaction Format - EIP-4844 transaction structure

Next Steps

Overview - Type definition and API reference
EIP-4844 - Detailed specification coverage
KZG - KZG commitment scheme deep dive
Usage Patterns - Real-world examples

Try it Live

​What Are Blobs?

​Key Properties

​Why Blobs Exist (EIP-4844)

​Blob Structure

​Creating Blobs

​KZG Commitments

​Computing Commitments

​Versioned Hashes

​Complete Example: L2 Data Posting

​Handling Multiple Blobs

​Blob Gas Market

​Gas Pricing

​Fee Market Dynamics

​Field Element Constraints

​Data Availability Window

​Security Considerations

​Commitment Binding

​Trusted Setup

​Common Patterns

​Single Blob Transaction

​Multi-Blob Transaction

​Gas Estimation

​Resources

​Next Steps

What Are Blobs?

Key Properties

Why Blobs Exist (EIP-4844)

Blob Structure

Creating Blobs

KZG Commitments

Computing Commitments

Versioned Hashes

Complete Example: L2 Data Posting

Handling Multiple Blobs

Blob Gas Market

Gas Pricing

Fee Market Dynamics

Field Element Constraints

Data Availability Window

Security Considerations

Commitment Binding

Trusted Setup

Common Patterns

Single Blob Transaction

Multi-Blob Transaction

Gas Estimation

Resources

Next Steps