[SDK] Treat messages/files for encryption/decryption as binaries not strings

[Dzeranov]

Description
Need to change the way of handling messages/files for encryption/decryption. Currently it's a string but for a better interoperability it should be a buffer.

Motivation
Current implementation won't work properly in case message to encrypt/decrypt is not a string (.zip for example).

Implementation details
TBD by developer.