Active attacks, involving a device called an “IMSI catcher,” may still be able to eavesdrop on individual calls by manipulating a phone’s security settings directly, without having to crack the encryption.
So, just hardens against passive eavesdropping (and only by upgrading to the latest standard, not by any specially devised method).
Also, I think a tower (real or bogus) can instruct your phone to downgrade to no-encryption, in which case the cipher won't matter.
If they really wanted to be "progressive" they would allow the phone to display a cipher icon for proper encryption with the tower, which was always part of the GSM spec, but was abandoned very early on. I think your SIM card needs to support that as well, IIRC ...
With 2G the network verifies the handset and the handset blindly trusts the network. It's not technically a downgrade, A5/0 null cypher is a perfectly valid choice of the 4 available. Sure it screws you as a user but it's not going to cost the network so that's fine.
Clarification: Notification to null encryption still exists, and iirc then it's actually mandatory. It's just that you can disable the warning by setting a bit on the simcard which it seems nearly every operator in the world does. As it was so unused the majority of even vaguely modern phones don't seem to have bothered writing the code to handle it anyway.
iirc, India _only_ uses A5/0 as it's illegal for them to use crypto [someone please clarify and educate me].
To be fair, it's a lot easier to harden the equipment they own vs the equipment your customers own. I'm not sure such a cipher icon is even possible in iOS without Apple's help. It certainly wouldn't be easy.
It's been a while since I went down this rabbithole, but I think it is required via spec, but only if your SIM card has that feature enabled ... and no carriers anywhere (globally) enable that feature.
So I would be interested to see what happens if you insert a SIM card with security checking turned on, into an iphone...
Active attacks, involving a device called an “IMSI catcher,” may still be able to eavesdrop on individual calls by manipulating a phone’s security settings directly, without having to crack the encryption.
So, just hardens against passive eavesdropping (and only by upgrading to the latest standard, not by any specially devised method).