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/**
 * Envelope v1: [format][tagLen][tag][nonce(24)][xchacha20-poly1305 ciphertext].
 * The header and caller-supplied associated data are authenticated, so a ciphertext
 * can't be reinterpreted as another type or moved to another node.
 */
export function seal(key: Bytes, typeTag: string, plaintext: Bytes, ad: Bytes = EMPTY): Bytes {
  const tag = utf8(typeTag);
  const header = concatBytes(Uint8Array.of(FORMAT, tag.length), tag);
  const nonce = randomBytes(NONCE_BYTES);
  const ct = sodium().crypto_aead_xchacha20poly1305_ietf_encrypt(
    plaintext, concatBytes(header, ad), null, nonce, key,
  );
  return concatBytes(header, nonce, ct);
}