I can see why this might be tricky, even after consulting a dictionary. The OED lists 11 meanings for the word sphere, but the one that applies here is Meaning #7:
The whole province, domain, or range of some quality, thing, etc., esp. of action, activity, operation, etc.
When the word sphere is used in this way, probably the most common usage is in the phrase sphere of influence. As Darryl Davis1 wrote:
It's called a sphere of influence because it consists of a circle of people you know, who know other people, who in turn know other people, and so on.
I'm assuming your journal article had something to do with computer security. In this case,
the system, its firewalls, and its users might all have certain roles and responsibilities for maintaining security and ensuring privacy. Conceptually, a customer's sphere of privacy might consist of all the data we would expect to be protected in such a system, and security sphere of implementation might be all the components that protect the system from a security standpoint, but it's hard to say for sure without seeing how the authors2? used (and perhaps defined) those terms.
The word sphere have have been used simply because a security model used circles to depict the boundaries of such system roles and responsibilities, as was done in one tome3, or it could be because system security – much like people who know people who know people – can be modeled as concentric circles4:
As a side note, these are not common terms. I ran a few Ngrams, just to illustrate how predominant the phrase sphere of influence is, compared to, say, sphere of privacy. Notice how the results for sphere of privacy, sphere of security, and sphere of implementation all flatline, as sphere of authority and sphere of influence are added to the query: