Data reduction is a process where a body of data is reduced to a smaller body of data. We distinguish among three modes for data reduction Perfect Recovery Effective Recovery No Recovery In the "no recovery" mode it is impossible to regenerate the large data from the small one Using a proper algorithm (hash) it is possible to reduce the large data to small data such that given the small data it would be expectedly difficult to find a large data that would be associated with the small data, using the same hash algorithm. This property is the basis for authentication and non-repudiation crypto protocols.