1. A computer-implemented method, implemented by hardware in combination with software, the method comprising: (A) obtaining a first data item signature for a first data item, the first data item signature comprising an association between a plurality of synch points in the first data item and a corresponding plurality of block signatures, and associating at least one action with said first data item; (B) attempting to find a synch point of the plurality of synch points in a second data item; and (C) if a synch point of the plurality of synch points is found in the second data item, then: (C)( 1 ) determining a block signature of a corresponding block of bits in the second data item, wherein the block signature is determined by applying a hash function or message digest function to the corresponding block of bits in the second data item; and (C)( 2 ) ascertaining whether or not the synch point and the corresponding block signature from the second data item correspond to a synch point and block signature in the first data item signature; and (C)( 3 ) when the synch point and the corresponding block signature from the second data item correspond to a synch point and block signature in the first data item signature, maintaining information indicative of the correspondence; (D) repeating steps (B) and (C) while at least some of the second data item remains unprocessed, until a predetermined number of synch points and corresponding block signatures from the second data item match synch points and corresponding block signatures in the first data item signature; (E) determining a match between the first data item and the second data item when the predetermined number of synch points and corresponding block signatures from the second data item match synch points and corresponding block signatures in the first data item signature; and (F) in response to said determining a match in (E), performing one or more actions of said at least one action associated with the first data item.