  The breakout of a supernova shock wave through the progenitor star's outer
envelope is expected to appear as an X-ray flash. However, if the supernova
explodes inside an optically-thick wind, the breakout flash is delayed. We
present a simple model for estimating the conditions at shock breakout in a
wind based on the general observable quantities in the X-ray flash lightcurve:
the total energy E_X, and the diffusion time after the peak, t_diff. We base
the derivation on the self-similar solution for the forward-reverse shock
structure expected for an ejecta plowing through a pre-existing wind at large
distances from the progenitor's surface. We find simple quantitative relations
for the shock radius and velocity at breakout. By relating the ejecta density
profile to the pre-explosion structure of the progenitor, the model can also be
extended to constrain the combination of explosion energy and ejecta mass. For
the observed case of XRO08109/SN2008D, our model provides reasonable
constraints on the breakout radius, explosion energy, and ejecta mass, and
predicts a high shock velocity which naturally accounts for the observed
non-thermal spectrum.
