Application programming interface (API) hooks are injected into an application program executing at a client during run-time. Responsive to these hooks, data intended for encryption prior to transmission from the client is diverted, for example for content filtering, compression, etc., prior to being encrypted. In the case of encrypted data received at the client, the data is decrypted but before being passed to the application it is diverted, under control of the API hooks, for content filtering, decompression, etc.