As already mentioned, GE wireless sensors and fobs send messages in plaintext. If one desires to do so, it is much easier and less expensive to spoof a wireless GE/Interlogix sensor than any of the modern gadgets that implement encryption, even with all their flaws, like the notorious Phillips zigbee lamps from a parallel thread.
Here's an example of a GE wireless motion sensor that I put in my basement years ago just to see how long the battery would last (it's been 4 years and sitll going strong). It is sending a heartbeat every hour + some minutes by blindly shooting three 61-bit packets :
2016-11-07 16:51:40 0001 be4482 c 115 20 Basement Motion
2016-11-07 16:51:40 0001 be4482 c 115 20 Basement Motion
2016-11-07 16:51:41 0001 be4482 c 115 20 Basement Motion
...and now that motion is detected transmitting 8 packets:
2016-11-07 20:23:20 0001 be4482 0 914 3c Basement Motion
2016-11-07 20:23:20 0001 be4482 0 914 3c Basement Motion
2016-11-07 20:23:20 0001 be4482 0 914 3c Basement Motion
2016-11-07 20:23:20 0001 be4482 0 914 3c Basement Motion
2016-11-07 20:23:20 0001 be4482 0 914 3c Basement Motion
2016-11-07 20:23:21 0001 be4482 0 914 3c Basement Motion
2016-11-07 20:23:21 0001 be4482 0 914 3c Basement Motion
2016-11-07 20:23:21 0001 be4482 0 914 3c Basement Motion
In the above, the timestamp is mine, 0001 is the two start bytes, followed by 3 sensor id bytes, followed by a nibble that contains a state transition count and battery low indication, followed by the actual state message (five inputs, three nibbles), out which the first is violated (observe 9 instead of 1 in the three nibble group), followed by stop bits and a check parity/oddity bits.
https://www.google.com/patents/US5761206
The messages are received by a cheap ($15) rtl sdr receiver tuned to 319.5Mhz..
Presumably, the new elk two way sensor protocol is encrypted, but I do not know how well. However, all this fancy two way/encrypted stuff does not come for free: messages are bigger, frequency is higher (902-928 Mhz vs 319.5Mhz) to accommodate more bits thus battery life and the range is lower.