Use random IV

[aidin36]

Using a constant IV with CBC mode is vulnerable to some attacks. On the other hand, IV can be public.

I changed the app to generate a random IV, and stores it at the beginning of the ciphered text. It makes the generated cipher a little more secure.

I also suggest using the random IV as the key's salt too. It makes the key unique in every encryption, making the cipher a little bit more secure.