Encrypting your content store is a feature that - without looking at any 3rd-party addons in the Alfresco community - is only available in Alfresco Enterprise Edition. Since Alfresco is Open Source and encrypting file-content is a rather trivial exercise in Java-code, it is quite easy for any member of the community to implement such a feature without having to spent the tens of thousands of dollars (or more) that an Alfresco subscription would cost (especially if encryption is the only feature you may care about). I have implemented an encrypting content store as part of one of my addons, and I have seen other addons doing similar things in the past (mostly in hack-a-thon events).
