Blobchan

Demo

About

Blobchan is a fully decentralized image board. You connect with WalletConnect, generate a burner posting key, fund it with testnet ETH, and start posting. Every thread and reply is a 128 KB blob transaction on Sepolia — the on-chain imageboard. Posts are publicly readable by anyone.

How it's made

### Posting (browser → blob) 1. User connects their wallet, generates a dedicated burner posting key, and funds it with ETH. This key is required because no browser wallets support signing EIP-4844 (blob) transactions directly. 2. User writes a post (text + optional WebP image, compressed to ~60 KB) 3. Browser computes a **KZG commitment and proof** over the 128 KB blob using `kzg-wasm` (C-KZG compiled to WebAssembly) 4. Viem constructs and signs a **Type 3 (EIP-4844) transaction** with the blob sidecar using the burner key, then sends it to a fixed marker address (`0x...b10b`) 5. Post structure: first 2048 bytes = JSON metadata (board, threadId, name, content, timestamp), remaining bytes = hex-encoded image 6. Gas cost: ~0.0001 ETH per post on Sepolia ### Reading (blockchain → browser) 1. **Etherscan V2 API** lists all transactions sent to the marker address 2. For each uncached tx, **eth_getBlockByHash** retrieves the `parentBeaconBlockRoot` 3. **Beacon API** (`ethereum-sepolia-beacon-api.publicnode.com`) looks up the beacon header by root, gets the slot, fetches blob sidecars 4. Blob hex is decoded: null-terminated JSON header at offset 0, hex image at offset 2048 5. Posts are cached in IndexedDB — cached tx hashes are skipped on subsequent refreshes to save API credits 6. Polls every 30 seconds for new content ### Stack - **SvelteKit** + static adapter → Cloudflare Pages - **WalletConnect** via @reown/appkit + wagmi adapter - **Viem** for all chain interactions (send, sign, query) - **kzg-wasm** for KZG commitments (required by EIP-4844) - **Public RPC & Beacon API** (publicnode.com) — no paid infrastructure - **Etherscan V2 API** (free tier) for transaction discovery