An access control system applies contents-based policies to data that is being transferred. This transfer can be between different computers, different operating systems on a single computer, different applications within on the same operating system, or different parts of the same operating system, for example. Data is scanned at a scan engine associated with a security reference monitor (SRM) as the data is being transmitted, on-the-fly. The data is forwarded directly to the recipient, so the data is not stored at the SRM. The data is encrypted at the SRM as it comes by, and the key is revealed to the recipient if it is subsequently determined that the transfer is allowed.