Extend interface for elementary file operations with encryption-decryption layer by Obj-C style private interface for encryption end decryption

Summary: Define additional private methods to encrypt and decrypt messages before the are written to or read from flat file storage

Test Plan: No test plan

Reviewers: karol-bisztyga, palys-swm, varun

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4383