Forwarded from comp.dcom.telecom:
Subject: Re: Truly Amazing, Truly Amazing ... Feds
Reply-To: Michael_Lyman@sat.mot.com
Message-ID:
I wonder, is the signal only digitally encoded or digitally encoded and *additionally* encrypted?
Yes on both counts. On the air interface between the base station ( BSS ) and mobile station ( MS ) the signals are digitally encoded. Traffic channel rate is 13 Kb/s. In addition to this, ciphering is done to protect the signalling channel such that user data privacy is provided then, encryption is provided for all voice traffic. As an aside, the GSM system also assigns "alias" subscriber numbers which are changed automatically with ( usually ) every call -- the subscribers real phone number is never ( well, almost never ) used over the air.
I'm getting a little paranoid over this, but in Germany when you buy an approved wireless phone you are told that it is impossible to listen in to it. Bullsh*t. In fact it's only impossible to listen in with another (unmodified) wireless phone -- just get a scanner and you're in.
For GSM, the level of privacy for both signalling and voice is considerable. Just to give you an idea, encryption keys change for each call made by the subscriber and the encryption algorithms use the changing physical properties of the radio channel. As a matter of fact, GSM is SO secure that several European governments including Britain are insisting that the scrambling algorithm ( called A5 in GSM ) be modified to allow at least government operatives ( read "undercover eavesdroppers" ) to listen in on suspected criminal activities. Agencies such as GCHQ, the British government's listening post near Cheltenham and the FBI in America are concerned that the A5 scrambling algorithm provided with the GSM Mobile Stations is equivalent to many military systems and in fact when exported may be adapted for military applications. Vendors of GSM equipment are starting to run into export problems due to the nature of the encryption / ciphering. Although there are some industrious "scanners" out there, I dare say that listening in on a GSM conversation will be a bit of a job. Michael Lyman Motorola - Iridium Phoenix, Arizona