toCbor method
Converts the object to a CBOR object.
Implementation
@override
CborObject toCbor() {
  return serializationConfig.toCbor([
    headers.protected.toCbor(),
    headers.unprotected.toCbor(),
    if (ciphertext == null)
      const CborNullValue()
    else
      CborBytesValue(ciphertext!),
    recipients.toCbor(),
  ]);
}