A method and system for providing an electronic shopping service integrated into a software application, and for controlling the set of web sites that are reachable from within the shopping service. Each web site is represented by data which is signed by a private key, and the data together with this signature is delivered to a plurality of computing devices that provide the shopping service. Each of the computing devices has access to the public key that corresponds to the private key, and uses the public key to verify the signature. The computing device displays links to those web sites whose representative data validates against the signature.