If I have an Arduino Zero (with two USB ports), can I make something like this:
-to one USB port I connect pendrive
-the second USB port I connect with PC
-when I move file from PC to pendrive, the file will be encrypted "inside" Arduino(I want to create my own algorithm to this)
-only way to decrypt file it will be connect pendrive to USB port in Arduino and move back to PC.
Is this possible?
Why?
You can write encryption program using VB.NET or C#. Pretty sure Python can do it.
The first step would be to get file transfer code working, in both directions, without encryption.
Let us know how that goes!
robert16444:
I want to create my own algorithm to this
I doubt you are trained in comprehending encryption.
So creating your own algorithm is considered not to constitute genuine encryption. 
robert16444:
-only way to decrypt file it will be connect pendrive to USB port in Arduino and move back to PC.
Is this possible?
no.
encryption is a software routine. there is no software you can put on an Arduino that you cannot put on a PC. in fact, you write the software on the PC, then move that to the Arduino.
so, any device that has that software can encrypt and decrypt the file.
if you scramble the data and have a key to unscramble, that key can be used on any platform
the Arduino is not a secure device so anyone who has access to it could copy the program, then take it apart and figure it out. Bletchly Park took a year with dozens of top people, so we know that eventually it could be done.
but this is true for every encryption ever made. the 'no' goes to your use of the word "ONLY"