[gxrimdue] Lightly encrypted content

The server provides content which has been encrypted with a short key; the client is expected to brute force the key as a normal operation.  The goal is to make it only slightly inconvenient for a human browsing one item at a time but make it prohibitively expensive for some bot to access many items simultaneously, a similar goal as a CAPTCHA or HUPTCHA previously.

An alternative is that the client's original request to the server must be accompanied with a Hashcash stamp (proof of work).