"The IMSI catcher masquerades as a base station and logs the IMSI numbers of all the mobile stations in the area, as they attempt to attach to the IMSI-catcher. It allows forcing the mobile phone connected to it to use no call encryption (i.e., it is forced into A5/0 mode), making the call data easy to intercept and convert to audio."
If it is possible for an Android/iOS app to detect when a GSM call is initiated without encryption then it should be possible to warn the user. Does anyone know if the encryption level is available to the OS, or is it restricted to the baseband processor?
As usual, everything interesting happens on the baseband processor. It would be trivial to detect that an IMSI catcher is used, even when it has support for encryption (unlikely given the age of the Stingray system and the need to communicate back with the mobile provider).
But alas, all the baseband processors are propietary software designed by companies that will happily compromise your privacy.
I think it's restricted to the baseband, but I'm not an expert. Fun fact: SIM cards actually have a flag which tells the handset to display a warning to the user if they are using an unencrypted connection. Basically no commercial SIM cards have this flag enabled.
I believe you have to enable a bit to _disable_ the warning (eg, it should be there as default). On my own network with my own sim cards, I've yet to find a modern handset that actually alerts me, though :(
"The IMSI catcher masquerades as a base station and logs the IMSI numbers of all the mobile stations in the area, as they attempt to attach to the IMSI-catcher. It allows forcing the mobile phone connected to it to use no call encryption (i.e., it is forced into A5/0 mode), making the call data easy to intercept and convert to audio."
If it is possible for an Android/iOS app to detect when a GSM call is initiated without encryption then it should be possible to warn the user. Does anyone know if the encryption level is available to the OS, or is it restricted to the baseband processor?