HOST Primer

The Chain of Trust

HOST implements a strict security model to ensure that off-chain resources are accessed securely and only by authorized smart contracts. This prevents spam and ensures that Validator Nodes only expend resources on legitimate, approved requests.

The security model consists of three distinct layers of authorization:

HOST Protocol Encryption

HOST allows smart contracts to trigger HTTP webhooks with encrypted sensitive data (API keys, tokens, secrets). The encryption ensures that only the designated validator can decrypt the values when executing the webhook.

Ciphertext Format

| + [ephemeralPubKey (65 bytes)] + [nonce (12 bytes)] + [ciphertext (variable)] + [authTag (16 bytes)]

|               - Magic prefix indicating encrypted value
ephemeralPubKey - Uncompressed secp256k1 public key (0x04 prefix + 64 bytes)
nonce           - Random 12-byte IV for AES-GCM
ciphertext      - Encrypted data
authTag         - 16-byte GCM authentication tag

Client-Side Encryption (JavaScript)

const secp256k1 = require('secp256k1');  // v4.0.3 - CRITICAL: Must use v4.x
const crypto = require('crypto');

function encrypt(validatorPubkeyHex, plaintext) {
    const validatorPubkey = Buffer.from(validatorPubkeyHex, 'hex');

    // 1. Generate ephemeral keypair
    let ephemeralPriv;
    do {
        ephemeralPriv = crypto.randomBytes(32);
    } while (!secp256k1.privateKeyVerify(ephemeralPriv));
    const ephemeralPub = Buffer.from(secp256k1.publicKeyCreate(ephemeralPriv, false));

    // 2. ECDH - derive shared secret (raw x-coordinate)
    const shared = Buffer.from(
        secp256k1.ecdh(validatorPubkey, ephemeralPriv, { hashfn: (x, y) => x }, Buffer.alloc(32))
    );

    // 3. AES-256-GCM encrypt
    const nonce = crypto.randomBytes(12);
    const cipher = crypto.createCipheriv('aes-256-gcm', shared, nonce);
    const encrypted = Buffer.concat([cipher.update(plaintext, 'utf8'), cipher.final()]);
    const tag = cipher.getAuthTag();

    // 4. Output format: | + hex(ephemeralPub || nonce || ciphertext || tag)
    return '|' + Buffer.concat([ephemeralPub, nonce, encrypted, tag]).toString('hex');
}

Usage in Smart Contracts

// Plaintext values - no encryption
string[] memory headerValues = new string[](2);
headerValues[0] = "application/json";

// Encrypted value - prefix with |
headerValues[1] = "|04abc123...def456";  // Encrypted API key

registry.addRequest(
    "https://api.example.com/webhook",
    '{"Content-Type":"$1","Authorization":"Bearer $2"}',
    headerValues,
    ...
);