The problem I see is that this is going to be seen as such a fun challenge to hackers and crackers that an actual boarding pass generator program will be created and passed around in no time. A small amount of known data being encrypted with constant keys with loads of samples available is a small task to crack in today's world.

To make this pretty much secure you need:
- Passes checked against airlines' systems in real-time.
- Encryption keys updated often (weekly or sooner).
- Use stronger encryption or encrypt lots of extra "junk" data. This is probably not possible while keeping the decryption time low and the barcode within the size requirements.

And there is always the possibility of something like the TSA's copy of all the airline keys getting loose some day.