Design a Disk based Cache for client (iOS,Android,Windows,Linux)

Platform independent 
Key will be 32 bytes
Value can be anything but in Byte array
Cache should be persistent
Cache should max hold 100k+ Objects
Cache Max size should be configurable ( Max Size : 10 MB upto 1GB)
Cache should be opaque
Cache should be Secure

Language does not matter (assume that it will be translated to C++ later to be distributed on all clients)

Not much pointers provided, it seems interviewer wanted to rebuild the cache same way they have built.