According to an example aspect of the present invention, there is provided a method comprising storing, by an apparatus, one or more key information elements, wherein each key information element comprises a stored key identifier and a stored key or a certificate, receiving by the apparatus, from a requesting network function, a request message comprising a first field and a second field, wherein the first field comprises an instance identity of the requesting network function and the second field comprises the instance identity of the requesting network function, and the first field is unprotected and the second field is protected with the key or the certificate, determining by the apparatus the key or the certificate used for protecting the second field, validating by the apparatus the second field using the key or the certificate, validating by the apparatus the request message when the instance identity of the requesting network function in the first field matches with the instance identity of the requesting network function in the validated second field and transmitting by the apparatus, after a successful validation of the second field and the request message, a response to the requesting network function.