Abstract: Access to protected resources across client devices having different authentication systems is provided. An authorization provider creates cross-certificates to trusted public keys provided by resource protection providers. Each resource protection provider installs a digital certificate on a device. This digital certificate is signed by the resource protection provider, and includes on the device a private key which are used for client authentication. Client authentication is performed by a security module on the client device digitally signing an authentication request to the authorization provider using a protection provider module-specific digital signature algorithm and providing the signed request with the device's provider-digital certificate to the authorization provider. If the authorization provider verifies the signed request and that the digital certificate is part of the authentication PKI, then the client device will be authenticated.