Preparing a document for transmission over a network in a manner that increases the security of URLs included in the document. A server accesses a document that contains a number of URLs. For one or more of those URLs, the server calculates a digital signature that is unique to the intended recipient client, or unique to the user of the client. The server then modifies each of those URLs by including the digital signature in the URL. The document with the modified URLs in then transmitted over the network to the client. Thus, even if a sensitive URL is discovered, that modified URL is unique to the client only or to the user of the client only. Therefore, the damage that may be caused by an unauthorized user is limited to just that client or user.