Phone Call Security

Matthew S. Hamrick mhamrick at
Sun Jun 3 18:03:15 CEST 2007

Encrypted voice calls is a question that's been around for a while.  
When I worked for RSA and later Certicom, we had frequent discussions  
about the strength (or lack thereof) of the LFSR-based encryption  
that was then in frequent use in GSM phones.

I should probably mention that GSM and CDMA provide over the air  
authentication and confidentiality services. So if you're worried  
about bad guys cloning or eavesdropping on your phone calls (or text  
messages), you're safe already.

Now... if your definition of "bad guys" includes the people who have  
operational control over the GSM network, then the story gets a  
little more complicated. Now, before you start saying, "oh, Matt's  
just being paranoid" or "oh, Matt's going to say something that will  
help the terrorists," let me just remind you that outside the US,  
there's some pretty clear evidence that national governments are  
eavesdropping on the conversations of traveling tech company  
executives and passing economic intelligence along to competing  
companies in their own nations. So end-to-end encryption is an issue  
that's near and dear not only to the hearts of the bleeding heart  
liberals at the EFF, but also the uber-industrialists of the far right.

"End to end" security is the term used to describe confidentiality  
and origin integrity services that provide assurance that the two  
endpoints in a communication are a) really talking to the person they  
think they're talking to, and b) the content of the call is not being  
intercepted by a malicious eavesdropper. This is distinct from the  
existing GSM security services which protect only the over the air  
portion of the comm link.

Approaches to end to end security on GSM phones started with layering  
voice data over the GSM data channel. There are some significant  
issues with this approach. First is obviously that you've got to have  
a phone that can be programmed to channel encrypted voice data across  
the GSM data channel. But, this message is going out to a community  
that groks this concept, so the only thing I'll say is... if we do  
something like this, let's fully specify what we do so people working  
on other programmable phones can interoperate with us.

Next is the issue of carrier support. I don't know if it's still an  
issue, but in the olden days it seemed that Cingular required you to  
call them up and explicitly activate your GSM data line. Then at the  
end of the month, they would turn it off requiring you to call up and  
get it activated again. But that's less of an issue these days as we  
move into an era where we have EDGE now and HSDPA on the horizon.

But... the issue of latency is important. The GSM data channel has  
terrible latency characteristics. Products like the CryptoPhone  
( ) suffer from this. If your latency is too  
high, the delay makes a normal conversation virtually impossible. You  
wind up having to say "over" after each thing you say to tell the  
other person it's okay for them to speak. This is okay if you're a  
whacked out cypherpunk who gets off on acting like a spy, but having  
been in the military already, it's just annoying for me to have a  
half duplex channel.

EDGE latency characteristics can be better than GSM data, but there's  
a fair amount of variability in EDGE latency. Sometimes it's high,  
sometimes it's low. Ditto for EVDO.

A couple years ago Nick Lane-Smith gave a presentation at DefCon  
about doing putting encrypted voice over "data over GSM voice." More  
info at:

The last I heard about this project, they discovered they couldn't  
get enough throughput to maintain a voice channel, encrypted or  

All this and we haven't even talked about SIP over DTLS and SRTP...

-Matt H.

On Jun 3, 2007, at 6:08 AM, Mikko Rauhala wrote:

> su, 2007-06-03 kello 07:08 +0200, Ortwin Regel kirjoitti:
>> IIRC there has been lots of discussion about this a few months back.
>> Take a look at the mailing list archives or the wiki if you can find
>> it!
> Spesifically, see subjects "Voice over GPRS?" and "Encrypting
> voice communications" in the February archive page at
> thread.html
> Summary: Possible to code support for this using GSM data calls  
> (not GSM
> voice calls due to GSM chip restrictions), which come with the quality
> of service GPRS lacks. May cost a bit extra depending on your  
> provider.
> Compatibility with probably possible, since their
> protocol seems to be up for reimplementation. Either way, at least
> Moko-to-Moko encrypted calls are quite possible to implement, just  
> that
> somebody (TM) needs to do the work.
> -- 
> Mikko Rauhala <mjrauhal at>
> University of Helsinki
> _______________________________________________
> OpenMoko community mailing list
> community at

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