[Openbts-discuss] When is my voice or SMS traffic encrypted during BTS and MS communication?

[Danny C. <tra...@gm...>]

Hi,

I have the OpenBTS set up. Voice and SMS are working perfectly. But I was
confused about one thing. In GSM, BTS encrypts voice (and SMS?) traffic
using A5. The key used in A5 is derived from a random number and the shared
key stored in the sim card and the network database using A8. This shared
key is looked up in the network database by the IMSI or TMSI value.

In my set up, I purchase a sim from a commercial carrier. That means my
OpenBTS does not have a shared key with the sim card. But I can still do
the following:

1. Make echo phone call from MS to OpenBTS
2. send sms from OpenBTS to MS
3. send sms to 411 from MS to OpenBTS
4. send sms between two MS.

Moreover, I can see my SMS messages in plaintext using wireshark.

Is there any encryption happening in the four scenarios I mentioned?
If yes, why I can still see the plaintext through wireshark and how can the
OpenBTS decrypt it.
If no, which protocol specification does it follow?


Thanks,