I think that who is interested in crypto analysis like me, should try simple approaches like this one to learn and be aware of the rules of cryptoanalysis.<p>Arduino has 16mhz. On your phone you could have enough computational power to break every type of encryption made by an AVR in a timeframe near to a second.
I you want to crypt something with Arduino really in a difficult way the procedure could last hours if not days (something can be made in less then a second on a smartphone).<p>So in any case an Arduino compatible system is not secure because of its lack of computational power.