6. The method of claim 1 , wherein the search query comprises search keywords, each keyword being associated with one negative or one positive operator, the step of transforming the search query in the query matrix comprising several rows comprises the following steps: expressing the search query in disjunctive normal form, as disjunction of conjunctive clauses, separating in each conjunctive clause the keywords associated with a positive operator from the keywords associated with a negative operator, transforming in each conjunctive clause: each search keyword associated with a positive operator in a vector, called positive vector, in the span of the orthonormal basis; each search keyword associated with a negative operator in a vector, called negative vector, in the span of the orthonormal basis; and determining the search query as a matrix where each row of the query matrix corresponds to one conjunctive clause and each row is a vector based on the positive and negative vectors, each row comprising a first sum in which the positive vectors are gathered and a second sum in which the negative vectors are gathered.
6. The method of claim 1 , wherein the search query comprises search keywords, each keyword being associated with one negative or one positive operator, the step of transforming the search query in the query matrix comprising several rows comprises the following steps: expressing the search query in disjunctive normal form, as disjunction of conjunctive clauses, separating in each conjunctive clause the keywords associated with a positive operator from the keywords associated with a negative operator, transforming in each conjunctive clause: each search keyword associated with a positive operator in a vector, called positive vector, in the span of the orthonormal basis; each search keyword associated with a negative operator in a vector, called negative vector, in the span of the orthonormal basis; and determining the search query as a matrix where each row of the query matrix corresponds to one conjunctive clause and each row is a vector based on the positive and negative vectors, each row comprising a first sum in which the positive vectors are gathered and a second sum in which the negative vectors are gathered. 7. The method of claim 6 , wherein correspondences between each keyword and each vector of the orthonormal basis are stored in a first secret conversion table.