Random seed

[tracker1]

When you encrypt multiple objects with the same key, the resulting base-64 string will start the same, this could be a potential vector for attack, because it's predictable across different instances. By having a random (even single-byte) seed at the front, it will make it less so.

I would suggest added a random seed to the begining of your unencrypted string.

The effect this would have is you'll need to check to see if the first character that is "{" as part of the decrypt, (while preventing the seed from containing that character).