Re: [Openbts-discuss] encryption questions

[Sylvain M. <24...@gm...>]

Hi,

> I think it's better to use boolen flags o indicate that encryption should be
> enabled/disabled for a given channel.

IMHO You'd  both the Kc _and_ the current algorith (0=no cipher,
1=A5/1 2=A5/2 3=A5/3 ...)

Ok, A5/2 should not be used but A5/3 should when possible (phone
support) and in an open source project I think it's important to show
the right way to do it.

> Just to summarize:
> - TCH+FACCH and SACCH could enable encryption (if appear layers set appropriate flags)
> - BCCH and CCCH have encryption always disabled.
>
> What about SDCCH, NDCCH, FCCH and SCH?

SDCCH yes
NDCCH ... are we talking about GSM ? I have _no_ idea what NDCCH is ...
FCCH & SCH ... that doesn't even make sense ...


> This means that if MS missed the ciphering command than it'll be effectively
> shut-off. After some timeout it'll probably try to recamp to BTS and will trigger
> authentication procedure again. Is that how it supposed to work?

No. If the MS missed the ciphering command, the BTS will never see the
LAPDm ACK for that message (since it started deciphering as soon as it
processed the cipher mode command) and will retransmit the ciphering
command (in clear because it hasn't seen any ciphered frame from the
mobile yet).

This is exactly why the BTS starts ciphering / deciphering at
different point in time.

Cheers,

    Sylvain