One example discloses a system for filtering digital certificates within a communications network, comprising: a first set of network-nodes, having a first attribute and a respective first set of digital certificates; a second set of network-nodes, having a second attribute and a respective second set of digital certificates; and a digital certificate authority, having a digital certificate validity list which includes the first and second sets of digital certificates; wherein the certificate authority filters the validity list based on the first attribute and transmits the filtered validity list to the first set of network nodes. Another example discloses a method for filtering digital certificates, comprising: maintaining a digital certificate validity list; identifying a set of network-nodes, having an attribute; filtering the validity list based on the attribute; and transmitting the filtered validity list to the set of network-nodes.