Available Encryption algorithms

Onno van Eijk onno at holmes.nl
Thu May 29 07:10:34 CEST 2008

> > I am a little confused about the available Encryption algorithms for the
> > Neo1973. When I enter the ATcommand "AT%EM=2,7" in libgsmd-tool (in
> > atcmd mode) it get the response:
> >
> > STR=`AT%EM=2,7'
> > RSTR=`%EM: 1,1,0,3,0,1,1,0,0,1,1,0
> > 6,0,0,0,0,1,1,1,4,0,0,0,0,1,0,0,0,0'
> >
> > According to
> > http://wiki.openmoko.org/wiki/GTA01_gsm_modem#Power_Parameters_.282.2C7.29
> > this should mean that A5/1 and A5/3 Encryption algorithms should be
> > available. But all the information I found on the TI Calypso chipset state
> > the ciphering processor implements both A5/1 and A5/2 algorithms.
> >
> > Can somebody confirm whether A5/2 of A5/3 is supported by the neo1973?
> The A5/* cyphers are used to encrypt voice data between the phone and the
> base station. What cyphers you can use depends on what your phone and
> network support. (In some countries consumer crypto is forbidden so all
> GSM voice calls are in the clear).
> I don't actually know, but I would assume that both Neo 1973 and
> Freerunner support whatever the GSM module supports. Remember that when
> you make a voice call the analogue audio is routed from the mic directly
> to the GSM module where it is digitized and sent over the network. The
> Freerunner does not get involved.
> If there is a difference between what the spec sheet for the GSM modem
> supports and what the AT command says is supported, then my guess would be
> that the GSM modem is also considering what is permitted by the network &
> SIM card. Alternatively there may be a bug in the OpenMoko software that
> runs and interprets that AT command.

I assume the available Encryption algorithms are those that are
supported by both the GSM Modem and the network. If the (un-mangled as
jOERG pointed out) AT command says it is supported, the modes should be
supported by the network and the GSM modem.

Unfortunately, this brings me back to the original confusion. The
documentation I found on the Calypso chip says A5/1 and A5/2 but
interpreting the response of the AT command using the information in the
wiki leads me to believe A5/1 and A5/3 is supported. 
Either the wiki or de documentation on the used Calypso chip is wrong.
Since interpretation of the output on the wiki is based on "educated
guesses and should not be relied on" I tent to believe the Calypso chip
But maybe there is somebody with access to the closed (NDA)
documentation, who can reliably confirm one or the other?


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