  We consider the collective eavesdropping of the BB84 and six-state protocols.
Since these protocols are symmetric in the eigenstates of conjugate bases, we
consider collective attacks having the same kind of symmetry. We then show how
these symmetric collective attacks are sufficiently strong in order to minimize
the Devetak-Winter rates. In fact, it is quite easy to construct simple
examples able to reach the unconditionally-secure key-rates of these protocols.
