  An elementary derivation of best eavesdropping strategies for the 4 state
BB84 quantum cryptography protocol is presented, for both incoherent and
two--qubit coherent attacks. While coherent attacks do not help Eve to obtain
more information, they are more powerful to reveal the whole message sent by
Alice. Our results are based on symmetric eavesdropping strategies, which we
show to be sufficient to analyze these kind of problems.
