Problems with A5/3 encryption

Tomcsányi Domonkos domi at tomcsanyi.net
Tue Mar 6 11:14:27 UTC 2018


Hi Jan,

Do you have traces of the attach/authentication? It seems like for some reason your MS don't like A5/3, because they are supposed to reply with Cipher Mode Complete. So top of my head I had the thought there might be something wrong with the initial establishment of the keys.

Cheers,
Domi

> 2018. márc. 6. dátummal, 10:58 időpontban Bruckner Jan (ETAS-SEC/ECT-Mu) <Jan.Bruckner at escrypt.com> írta:
> 
> Dear list,
>  
> I’m having trouble using the A5/3 encryption in my setup. A5/1 works perfectly fine [attachment a5_1.pcapng]. As soon as I switch to A5/3 and e.g. send an SMS, the last valid message I see in the Wireshark traces of the GSMTAP of osmo-bts-trx is the Ciphering Mode Command requesting A5/3. After that, several messages arrive at the bts, but it seems like it can’t make any sense of them. The MS repeatedly tries to send the SMS but never succeeds  [attachment a5_3.pcapng]. Both MSs are connected to the same bts.
>  
> According to the Classmarks of all MSs, A5/1 as well as A5/3 are supported.
> This is my Setup:
> -       USRP N210
> -       osmo-trx
> -       osmo-bts-trx
> -       osmo-nitb
> -       osmo-pcu
> -       osmo-sgsn
> -       osmo-ggsn
> I’m using a Debian 9 VM and tried both the packages from osmocom-latest as well as osmocom-nightly.
> The MSs I’ve tested are two Nexus 6 and one Samsung Galaxy S I9000. All three with sysmocom nano USIMs.
>  
> Could the decryption at the bts be incorrect? Has anyone tested/used it recently?
> I’ll be happy to provide additional information if needed.
>  
> Thanks,
> Jan
> <a5_3.pcapng><a5_1.pcapng>

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