  Several kinds of qubit-string-based(QS-based) bit commitment protocols are
presented, and a definition of information-theoretic concealing is given. All
the protocols presented here are proved to be secure under this definition. We
suggest an attack algorithm to obtain the local unitary transformation
introduced in no-go theorem, which is used to attack the binding condition,
then study the security of our QS-based bit commitment protocols under this
attack via introducing a new concept "physical security of protocol". At last
we present a practical QS-based bit commitment scheme against channel loss and
error.
