Since you're working with a low possibility space, the absolutely critical thing is that you use Salting and/or Initialization Vectors (IV) - otherwise, what you're doing isn't secure at all.
Here's what I mean, based on various attack vectors:
Person gets access to your app code. You're hosed - I'm assuming you've got a hard-coded encryption key residing in your app. If that's the case, getting the app-code is game-over no matter what.
Person gets access to your back-end data, but doesn't have front-end access. In this case, if you're not using Salt or IV, all the '123's are going to encrypt to the same value. The attacker won't bother trying to 'crack' your encryption key - they're going to try to gather from the data based on the counts/distribution of the encrypted values. Depending on what the data represents, it might be scarily easy to make educated guesses on what each encrypted value represents simply based on its occurrence distribution.
Person gets access to your back-end data, and also has access to use your program. Now you're definitely not okay if you're not using Salt or IV. Because, in this case, the attacker can arrange circumstances to get your app to write a specific value to the database - and then simply look up what the output value is. No guessing - they just cracked all the matching entries without the key. Repeat as many times as needed to get as many values as you need.
TL;DR - You need a Salt/IV for this. Once you do that, you're good to go, and it's not going to matter that you're encrypting a small amount of bits.