  We study the duality between the two dimensional black hole and the
sine-Liouville conformal field theories via exact operator quantization of a
classical scattering problem. The ideas are first illustrated in Liouville
theory, which is dual to itself under the interchange of the Liouville
parameter b by 1/b. In both cases, a classical scattering problem does not
determine uniquely the quantum reflection coefficient. The latter is only fixed
by assuming that the dual scattering problem has the same reflection
coefficient. We also discuss the relation of this approach to the method that
exploits the parafermionic symmetry of the model to compute the reflection
coefficient.
