  We investigate a general class of quantum key distribution (QKD) protocols
using one-way classical communication. We show that full security can be proven
by considering only collective attacks. We derive computable lower and upper
bounds on the secret key rate of those QKD protocol involving only entropies of
two--qubit density operators. As an illustration of our results, we determine
new bounds for the BB84, the six-state, and the B92 protocol. We show that in
all these cases the first classical processing that the legitimate partners
should apply consists in adding noise. This is precisely why any entanglement
based proof would generally fail here.
