**group‑box** is a tiny JavaScript library that lets a sender encrypt a message once and make it readable by many recipients without creating separate ciphertexts for each. It takes a deterministic external nonce and an array of 32‑byte symmetric keys (derived from Curve25519 exchanges or shared group keys) and produces a single ciphertext that any holder of one of the keys can decrypt after a bounded number of key‑guess attempts. The tool is aimed at developers building peer‑to‑peer or decentralized messaging systems that need efficient “multi‑recipient” encryption for direct messages, broadcast groups, shared groups, and administrated groups.