Method of detecting an attack against a function on a client computer including generating a first hash value having a weak collision resistance; sending the first hash value to a server computer for storing to a database of known hash value pairs, a hash value pair including the first hash value and a second hash value calculated for the entity, the second hash value having a strong collision resistance, receiving a request for the entity with an object including a first hash value and a second hash value; accepting the received object and transmitting data relating to the received object to the server computer for a validity check when the first hash value of the received object is identical with the first hash value stored in the local database, and detecting a hash collision attempt when the hash value pairs do not match.