1. A method for fingerprinting a content item, the method comprising: providing the content item, the content item comprising a set of content item elements, the set of content item elements denoted E, such that E={E 1 , E 2 ,. .. E i ,. .. E m }; providing information uniquely associated with a single user, the information comprising a string of bits, hereinafter denoted S 0 ; parsing S 0 into a plurality of subsequences of strings of bits, hereinafter denoted S 1 ,. .. , S n , the parsing being performed such that S 0 equals a function of S 1 ,. .. , S n ; providing a matrix of content item replacement elements, the matrix denoted R, each row of matrix R comprising, for at least each one of n members of set E, an array of content item replacement elements for E i denoted R i , such that R i ={R i1 ,. .. R ij }; uniquely associating, by a processor, each one of the possible combinations of subsequences for each of S 1 ,. .. , S n with one matrix element of matrix R, so that for every one of the possible combination of subsequences of bits and for each S 1 ,. .. , S n there exists a corresponding element of E; for every one of S 1 ,. .. , S n replacing at least one instance of the corresponding element E in the content item with the associated one matrix element of matrix R for the corresponding one of S 1 ,. .. , S n ; and outputting a replacement content item comprising the result of the replacing, wherein the members of R i for each E i are chosen according to at least one criterion of similarity to the content item E i to be replaced.