As stated, the only real issue you'd have with the hardware of the calculator is keeping track of time.  TI89 has a real-time clock, and the backup battery (beneath the AAAs) will keep this running.  The main concern is that you likely do not have efficient means of keeping the clock synchronized to a network time.  Google's newest release of the app now even contains its own NTP task to fetch a time rather than using the phones. I haven't looked into it, but given the app, and the pseudo-code you posted, your grace period is exactly 30 seconds.  How long you have remaining (the countdown in the app) is controled by the modulo 30 of the time seed used.  I would expect that there are open implementations of SHA1 and the necessary hash functions you need in TI-BASIC89.  Really should be quite simple to implement, just need to make sure you check that your clock is accurate fairly regularly.