const IV_BYTES: &[u8; IV_LEN] = b"LDK Invoice ~~~~";
let secp_ctx = Secp256k1::new();
let hmac = Hmac::from_engine(expanded_key.hmac_for_offer(nonce, IV_BYTES));
const IV_BYTES: &[u8; IV_LEN] = b"LDK Invoice ~~~~";
let secp_ctx = Secp256k1::new();
let hmac = Hmac::from_engine(expanded_key.hmac_for_offer(nonce, IV_BYTES));