Problems with A5/3 encryption

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Neels Hofmeyr nhofmeyr at sysmocom.de
Tue Mar 6 16:10:03 UTC 2018


Hi Jan,

A5/1 works, which means that your auth keys in the SIM and subscriber
database match.

In the CM Service Request, your MS indicates that it is capable of A5/3.
(In the Classmark 2)

I see that you only have one Location Updating with A5/1. It should work
to switch to A5/3 on-the-fly, but just for curiosity, you could try to
detach and re-attach the phone after switching to A5/3.

You write that you are using osmo-nitb. Does the problem persist if you
use osmo-bsc + osmo-msc + osmo-hlr instead? See:
https://osmocom.org/projects/cellular-infrastructure/wiki/Osmocom_Network_In_The_Box

Other than that I can't see anything wrong with anything.

If you switch back and forth between A5/3 and /1, do the results remain
stable? So it's not your SDR coincidentally clock-unsyncing in the wrong
moment by coincidence?

Seems curious that recently there have been other questions about auth /
ciph not working for specific osmo-bts-trx variants. There is a faint
possibility that one thing or other doesn't get encoded/decoded right??

~N

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