git - How to keep/maintain public and private code in the same repository? (at repository hostings) -

i'm working on own project has 2 parts:

a. kernel/generic code (public part)

b. code works proprietary protocol etc (private part, available me , few authorized persons)

i want repository hosting (maybe github, assembla, ...) allows working in public , private branches in same repository.

i don't want 2 repositories because i'm actively working on both parts , want avoid diverged repositories.

which solution available me?

which allows working in public , private branches in same repository.

that doesn't seem compatible how git works: if have access repo, can clone all content (including branches).

a git hosting service bitbucket or gitlab allows protect branch (meaning cannot push back). still able see content.
gitolite doesn't prevent read-access @ branch level.

so 2 separate repos still best approach, repo (kernel) declared submodule of repo b.