fhEVM Encrypted Counter (Sepolia)

A minimal confidential counter dApp built on Zama fhEVM. The contract stores a euint32, supports homomorphic addition of encrypted inputs, and demonstrates public reveal via the Decryption Oracle. The frontend (single HTML file) encrypts inputs in the browser via the Relayer SDK and calls the contract on Sepolia.

• Contract: 0xC5B8f66e56D41d067D88C06413AbCe7b99727E44 (Sepolia)
• Stack: Solidity 0.8.25, @fhevm/solidity v0.8.x, ethers v6, Relayer SDK (CDN)

Smart contract

See EncryptedCounter.sol. It:

• stores an euint32 counter;
• add(externalEuint32, bytes inputProof) validates and adds encrypted input;
• getEncryptedHandle() returns a ciphertext handle for public decrypt (HTTP);
• requestReveal() / onReveal() performs on-chain reveal with KMS signatures.

The contract imports FHE.sol and SepoliaConfig from @fhevm/solidity, which provides the required addresses for Sepolia. See docs (Relayer SDK & Solidity guides).

Docs: Web apps CDN + init flow; Sepolia config and decryption patterns. :contentReference[oaicite:3]{index=3}

Frontend

Open index.html in a static server (recommended) and:

1. Click Connect MetaMask (network: Sepolia).
2. Enter a number and click Add (encrypted) — SDK encrypts input and calls add(...)
3. Click Show via HTTP Public Decrypt — SDK returns plaintext via relayer HTTP public decryption.
4. Optionally click On-chain Reveal (Oracle), then read lastPlain after the oracle callback.

Serve locally

Any static server works, e.g.:

npx http-server . -p 8080
# or
python -m http.server 8080