Abstract: The invention provides a computer-implemented method for validating the respective identities of co-operating entities on a computer network, and comprises generating, transmitting or exchanging a signed digital certificate. The certificate includes a public key associated with an entity on the network; and an arbitrary identifier associated with the public key. The identifier is arbitrary such that the identity of the entity cannot be, or is unlikely to be, discerned from the identifier alone; and/or its generation is random or pseudo-random; and/or selection of the identifier is not related to the identity of the entity or the public key. The certificate is generated and issued by a Certificate Authority in response to a Certificate Signing Request from the entity. In order to validate each other's identities, entities on a network exchange their arbitrary identifiers.