The ``impossibility proof'' on unconditionally secure quantum bit commitment
is critically analyzed. Many possibilities for obtaining a secure bit
commitment protocol are indicated, purely on the basis of two-way quantum
communications, which are not covered by the impossibility proof formulation.
They are classified under six new types of protocols, with security proofs for
specific examples on four types. Reasons for some previously failed attempts at
obtaining secure protocols are also indicated.Comment: 10 pages, revtex4; significant changes (see footnote