  We present an algorithm to identify the types of supernova spectra, and
determine their redshift and phase. This algorithm, based on the correlation
techniques of Tonry & Davis, is implemented in the SuperNova IDentification
code (SNID). It is used by members of the ESSENCE project to determine whether
a noisy spectrum of a high-redshift supernova is indeed of type Ia, as opposed
to, e.g., type Ib/c. Furthermore, by comparing the correlation redshifts
obtained using SNID with those determined from narrow lines in the supernova
host galaxy spectrum, we show that accurate redshifts (with a typical error <
0.01) can be determined for SNe Ia for which a spectrum of the host galaxy is
unavailable. Last, the phase of an input spectrum is determined with a typical
accuracy of ~3 days.
