DE-crypting calls

Al Johnson openmoko at
Wed May 21 01:52:37 CEST 2008

On Tuesday 20 May 2008, joerg at wrote:
> Am Fr  16. Mai 2008 schrieb Al Johnson:
> > On Friday 16 May 2008, ramsesoriginal wrote:
> > > I was just wondering: we have seen the problems of encryptingcalls.
> > > But what about decrypting them? Let*s say someone hasa method to call
> > > you with some sort of encryption.. would it be possible to write a
> > > decryption for it? Maybe public key?
> >
> > Depends what sort of calls you mean. If it's GSM voice calls then the
> > same problems apply as already discussed. If you're talking about a data
> > link encrypted end to end and passing voice data then it's possible,
> > probably using established standards. You could make an OpenVPN or IPsec
> > tunnel end to
> > end, then use SIP, h323 or IAX2 over it. You could use one of the secured
> > versions of SIP directly, at least one of which uses public keys, but
> > none of them are universally accepted.
> What about ZRTP? (Zimmerman, PGP inventor). Have a look at!

That's one of them. There's also MIKEY and SDES. What I meant by not being 
universally accepted is that none of them is implemented by all SIP clients, 
making interoperability somewhat hit and miss. I hope this will change soon!

> Via 
> GSM you can setup not only voice calls, but also fax and *DATA* calls with
> up to 14400bps. Tunneling SIP and low bandwidth RTP/ZRTP through this link
> should be feasible with low effort

Not only that but it has lower and less variable latency than the 'faster' 
connection methods. You should always get 9600bps so that's the bandwidth to 
aim for. There's a video of a presentation that covers most of this somewhere 
in the list archive. 

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