[2414] in cryptography@c2.net mail archive
Re: Existing digital phone encryption
daemon@ATHENA.MIT.EDU (Greg Rose)
Tue Mar 31 09:16:19 1998
To: Jeff Simmons <jsimmons@goblin.punk.net>
cc: shamrock@netcom.com (Lucky Green), cryptography@c2.net
In-reply-to: Your message of Mon, 30 Mar 1998 21:19:06 PST.
<199803310519.VAA16706@goblin.punk.net>
Date: Tue, 31 Mar 1998 19:26:55 +1000
From: Greg Rose <ggr@qualcomm.com>
Jeff Simmons writes:
>> >I've just gotten a digital cellular phone. One of many options is to
>> >turn on digital encryption that supposedly kicks in whenever I talk to
>> >another digital cell phone.
There is a misapprehension here. Cell phones only
ever talk to base stations; if the base station is
prepared to use crypto on its link to the phone it
will be turned on, whether the recipient of the
call is fixed or mobile, encrypted or not.
>> >Can anyone point me to info on the crypto type and implementation?
>> >I've checked many places, but haven't found anything yet, and I
>> >have trouble imagining that it's worth a damn.
>This one is TDMA. I'd be interested in the types of encryption
>that are being used by the other systems, also.
TDMA uses a 520 bit fixed Voice Privacy Mask,
xored into every frame. This is calculated once at
the beginning of the call as a side effect of the
authentication signature generation.
CDMA uses an offset on the spreading code;
cryptographically weak (not as weak as the VPM)
but it is hard to get the signal unless you
already know it.
GSM uses A5, a hardware shift register based
algorithm; Golic has a paper in Eurocrypt last
year showing it to be about 42 bits worth.
However, you have no way of knowing whether it is
turned on or not...
After the breaking of CMEA and ORYX (used for
signalling and data encryption over wireless
phones, resp.) last year, the TIA standards
committee working on this stuff is initiating a
public process to look for successors to the
entire current panoply.
Greg.
Greg Rose INTERNET: ggr@qualcomm.com
QUALCOMM Australia VOICE: +61-2-9743 4646 FAX: +61-2-9736 3262
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