Decrypting AES encrypted file I wasn't sure where else to ask this so I will ask it here. 

I work for a company that uses a rather old piece of tooling and we use customized software to control it.  We hired a guy to code the controller for us, and it works fine, however, we caught him stealing and fired him.  After we fired him, we found out that he's coded in a check that is comparing an encrypted file to the serial number of the machine so we can't use the code on any other machine on the floor(we have ten of these things).  I looked at the code in hopper, and it appears that this is simply the serial number AES encrypted somehow.  Knowing this, would it be possible to reverse engineer the file so that we can create this file for each of the machines?
AES is a symmetric cipher &mdash; it needs the same key for encryption and decryption. Thus, in order for the code to encrypt anything with AES, it must have the key stored somewhere. Find that key, and you will have completely broken the encryption.

(It's possible that the key isn't stored directly, but is derived from some other data stored in the program code. It's even possible that the key depends on the machine number in some way; this would make it impossible to decrypt the encrypted number without already knowing the real machine number, but for just verifying that the numbers match this would be fine. In any case, as long as you know what inputs the key is calculated from and how, you can repeat the same calculation yourself.)

All that said, I'd really recommend tossing out the code and getting someone more trustworthy to rewrite it for you. You already know that the original author cheated you, obfuscated their code and included at least one hidden booby-trap in it. Do you really want to take the risk of bypassing that trap and using the code, only to find out that the next trap will physically damage your machines? Besides, with only an obfuscated binary, you won't be able to fix the code when (not if!) you find a bug in it, anyway.