In an implementation of a data escrow agent using a MAC, the user provides the data escrow agent with demographic data indexed by a session identifier keyed to the agent's realm. When contacting a content provider the client constructs a session identifier using a MAC of the session identifier keyed by the provider's realm. The escrow agent may construct a linkage between the provider's logfiles and entries in the escrowed database by calculating a MAC for every entry in the database. Although this technique involves a larger number of operations that the public key based scheme, these operations are approximately four orders of magnitude faster. Interaction with Proxies and Caches.
Many Web users browse the Web through a caching proxy.
In many countries this mode of operation is essential due to saturation of international network connections. When a proxy serves a user from a local cache the originating server has no knowledge of the transaction. Consequently logfiles may be incomplete. This problem is most serious for commercial sites which use hit counts as a measure of readership. A number of techniques may be used to prevent proxies from caching data.