  With the help of a simple quantum key distribution (QKD) scheme, we discuss
the relation between BB84-type protocols and two-step-type ones. It is shown
that they have the same essence, i.e., information-splitting. More
specifically, the similarity between them includes (1) the carrier state is
split into two parts which will be sent one by one; (2) the possible states of
each quantum part are indistinguishable; (3) anyone who obtains both parts can
recover the initial carrier state and then distinguish it from several possible
states. This result is useful for related scheme designing and security
analyzing.
