In the case of professional edition there's a windows tools used to encrypt and decrypt contents. Also could be done some customization to fit exactly your requirements ( not all the possible implementation will fit with customer requirement that's why do not exist a repository encryption feature from OpenKM server side only a client in the customer side for a manual control).
About compression I suggest discard this kind of ideas, nowadays disk space is cheap and cost in terms of compress and decompress in time and hardware is huge. Should be a great reason behind for it. Maybe the best approach is apply at OS level ( for the OpenKM application assigned to store data in the datastore folder the compress and decompress will be transparent, but in terms of performance you will be penalized, how much? it depends of hardware you should test scenario with and without compression.
