The invention relates to a method of processing data, the method comprising steps of enabling to (210) encrypt first data for a first source, and encrypt second data for a second source, (220) provide the encrypted first and second data to a server that is precluded from decrypting the encrypted first and second data, and from revealing identities of the first and second sources to each other, (230) perform a computation on the encrypted first and second data to obtain a similarity value between the first and second data so that the first and second data is anonymous to the second and first sources respectively, the similarity value. providing an indication of a similarity between the first and second data. The method may further comprise a step (240) of using the similarity value to obtain a recommendation of a content item for the first or second source. The first or second data may comprises a user profile or user ratings of content items. One of the applications of the method may be in collaborative filtering systems.