1. A computer implemented method of retrieving candidate sets of digital data (SDD) stored in a database comprising: a) receiving, into a buffer by a processor, one or more vectors of equivalence signatures of target SDD, each said one or more vectors comprises: i) a list of numbers, and ii) the number of elements of the vector, b) calculating the lengths of said one or more vectors using a isometric distance calculator, c) creating a record for each section of each target SDD, said record comprises a key, wherein the most significant half of the key value, is the calculated length of the equivalence signature vector of said target SDD, d) eliminating potential candidate SDD by comparing the most significant halves of the keys of the target SDD to those of the stored candidate SDD, and e) performing further processing on the remaining candidate SDD in order to return a final list of candidate SDD.