  Sealing information means making it publicly available, but with the
possibility of knowing if it has been read. Commenting on [1], we will show
that perfect quantum sealing is not possible for perfectly retrievable
information, due to the possibility of performing a perfect measurement without
disturbance, even on unknown states. The measurement is a collective one, and
this makes the protocol of quantum sealing very interesting as the only example
of the power of collective measurements in breaking security.
