  In a realistic situation, it is very difficult to communicate securely
between two distant parties without introducing any disturbances. These
disturbances might occur either due to external noise or may be due to the
interference of an eavesdropper sitting in between the sender and the receiver.
In this work, we probe here the existence of the possibility of the situation
of generation of a secret key even if the eavesdropper is able to construct an
entangled ancilla state in such a way that she can extract information from the
intercepted qubit. To achieve this task, we consider and modify the six-state
QKD protocol in which Eve can construct the unitary transformation that may
make all ancilla components entangled at the output. Then, we calculate the
mutual information between Alice and Bob and Alice and Eve, and identify the
region where the secret key is generated even in the presence of Eve. We find
that, in general, the mutual information of Alice and Eve depends not only on
the disturbance D, but here we have shown that it also depends on the
concurrence of the ancilla component states. We have further shown that it is
possible to derive the disturbance-free mutual information of Alice and Eve, if
Eve manipulates her entangled ancilla state in a particular manner. Thus, in
this way, we are able to show that a secret key can be generated between Alice
and Bob even if the disturbance is large enough. Moreover, we show that Bruss's
six state QKD protocol failed to generate the secret key in the region where
the modified six-state QKD protocol can generate the secret key.
